Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FLUORESCENT ACTIVATED, SPATIALLY QUANTITATI~E LIGHT
.
DETECTOR
This invention relates to a light detector for
detecting light scattered from an information-bearing
surface and, more particularly, to a spatial,
quantitative light detector comprising a
fluorescent-activated collector member and a
photosensing element.
There are a number of present systems in the
telecommunication, facsimile and xerographic areas
wherein information content on a document is scanned by
a flying spot beam; and the light reflections therefrom
converted to analog imaye signals representative of the
information being scanned. Typically, a cylindrical
light collector is disposed adjacent the document to
gather or collect the scattered and reflected light
emanating from the document surface and channel the
light to a photosensing element. A similar technique
is used to scan and detect latent images on a
photoconductive surface as disclosed in EPO Publication
0002,102 assigned to IBM Corporation or to scan and
detect developed toner images as disclosed in U.S.
Patent No. 4,345,835, issued August 24, 1982 and
assigned to the same assignee as the present invention.
While light detectors may take various forms, a
preferred type, for many of the applications listed
above, is an elongated cylindrical rod having at least
a portion thereof transparent so as to admit reflected
light into the rod. One or more photosensors are
placed within the rod (generally at one or both ends)
so as to detect light collected by the rod and convert
the light into an analog signal representative of the
scanned and collected information. Most conventional
detectors operate at a low level of efficiency,
generally in the order of 0.2~. The reason for this
low level of efficiency is that much of the light that
enters the collector is lost either by direct
transmission through the rod or by being scattered out
of the rod after striking a scattering strip generated
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on a surface of the rod. Only a small portion of the
incident light is scattered at a large enough angle to
be trapped within the rod and propagate along the rod
to the pho-tosensors through the action of total
internal reflection. Attempts have been made to
improve detector eff.iciency by applying various
reflective or opaque coatings to the exteriors of the
rod or by placing the rod in an integrating cav.ity, as
disclosed in
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U.S. Patent No. 4,321,630, issued March 23, 19~0. These
expedients ha~e resulted in some improvements but still
render the collector rods ine~icient for many purposes.
From what has been said above, it can be observed
that a more efficient collector must be able to redirect
incident light at a fairly large angle by some scattering
process so as to guide as much of the incident light as
possible to the photosensor. And, no less important, the
light intensity at the p~otosensor should not vary strongly
as a function of the location of the incident light on the
collector rod. In other words, the detected si.gnal should
be fairly uniform as the incident beam is scanned along the
collector rod.
A first attempt at redirecting incident light so
that a greater portion of light enters a guided mode within
the collector is disclosed in U.S. Patent No. 4,314,283,
issued February 2, 1982. In that patent, the reflected
light is incident on a variable period diffraction grating
formed on the surface of a collector rod. A portion of
the light incident on the grating is diffracted at angles
sufficiently large so as to propagate within the rod by
total internal re~lections, to the photosensor. Problems
still remain with this approach since high efficiency
gratings are difficult to fabricate and grating efficiency,
in terms of mode coupling, is dependent on the angular
spectrum of the incident light. Therefore, there is still
room for improvement in both detector efficiency and
uniformity.
Applicant, iII the present invention, has disclosed
an improved, highly efficient, spatial qualitative light
detector, the detector, in one embodiment, comprising a
collector rod formed of a fluorescent material with at
least one photosensor at one end thereof. Depending upon
the fluorescent dye selected, up to 100% of incident light
is absorbed and re-radiated (scattered) as fluorescent
light. This re-radiated light has two important character-
istics. Firstly, the scattering is isotropic, i.e. uniform
in all directions, resulting in a large percentage of light
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entering the guided mode so as to be directly coupled to
- the photosensor. Secondly, the waveleng~h of the incident
light is shifted so that most of the scattered light is
not reabsorbed and rescattered by the fluorescent dye.
A fluorescent light collector thus serves to both increase
the amount of incident light which is scattered at the
desired large angles corresponding to guided mode of the
rod and to confine more of the scattered light within the
rod. Both of these efforts contribute to the greater
efficiency of the detector.
Various aspects of t~e invention are as ~ollows:
A fluorescent-activated spatial qualitative light
detector comprising
an elongated rod-like member, said member contain-
ing a fluorescent dye molecularly dispersed throughout a
generally transparent medium, and
at least one photosensor in operative association
with said member to detect fluorescent light emitted by
said member upon exposure to incident light.
In a flying spot scanning system adapted to raster
scan on information bearing surface, a fluorescent-acti-
vated light detector for sensing light reflected from said
scanned surface, said light detector comprising
a generally cylindrical rod containing a fluores-
cent dye dispersed throughout a generally transparent
medium and a photosensor at one end of, and in operative
association therewith,
whereby said reflected light incident on said
light detector is absorbed ~y said dye and reradiated at
the fluorescent wa~elength, a portion of this reradiated
light reaching the photosensor to generate an output signal
the~efrom.
In a flying spot scanning system adapted to raster
scan an information bear~ng surface, a light detector
assembly for collecting light reflected from sa~d scanned
surface, saicl light detector assembly comprising
a generally cylindrical, hollow integrating cavity
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having a first entrance slit aligned with a second exit,
re-entry slit disposed ad~acent said scanned surface where-
in a scanning beam enters t~e cavity through said first
slit, passes through said second slit and is scattered from
said scanned surface bac~ into said cavity,
a fluorescent-activated light detector placed with-
in said cavity, said detector comprising an elongated
rod-like member containing a fluorescent dye molecularly
dispersed throughout a generally transparent medium, said
collector further comprising a photodetector optically
coupled to said rod-like member,
whereby sald light entering said cavity, following
multiple diffuse reflections from the interior surfaces of
said cavity is absorbed by said dye and scattered thxough-
out said rod-like member, a portion of said light being
coupled to said photosensor to provide an output signal
representative of said scanned surface.
A light detector apparatus comprising at least two
fluorescent-activated, spatial qualitative light detectors,
each detector comprising an elongated rod-like member
containing a fluorescent dye molecularly dispersed through-
out a generally transparent medium each of said medium
fluorescing in the principle wavelengths of a primary color
media each medium provided wit~ a filter medium to permit
a narrow band of light to impinge upon said fluorescent
medium, each medium emitting fluorescent light upon
exposure to light of different wavelengths.
In a copying apparatus having a photoreceptor,
means to charge said photoreceptor in preparation for
imaging, exposure means for exposing said charged photo-
receptor to produce a latent electrostatic image,
developing means ~or developiny the latent electrostatic
image, transfer means to remove said developed image from
said photoreceptor and read means to scan said developed
image on said photoreceptor to provide electrical image
signals representative of said developed image, said means
including an integrated light collector, the improvement
comprising an integrated light collector comprising a
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fluorescent medium in operative association with a light
detector.
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The use of fluorescent materials as planar concentrators is known
in the art and is described in an article by V. Wittwer et al., "Efficiency and
Stability of E2~perimental Fluorescent Planar Concentrators", published in 'ITheConference Record of the Fourteenth IEEE Photovoltic Specialist Conference
1980", pages 760 - 764. See also the article by J.S. Batchilder et al. in the
Journal of Applied Optics, Vol. 18, No. 18, September 15,1979, pages 3090 -3110.A fluorescent radiation converter is also described in an article published in
the Journal of Applied Optics, Vol. 20, No. 6, 15 March 1981, page A52.
However, the use of a -fluorescent activated material in a spatial
quantitative light detector is not known in the art. The following description
describes various embodiments of a fluorescent activated collector in com-
bination with coatings and structure to improve uniformity. These embodi-
ments are described in conjunction with the following figures.
Figure 1 is a schematic view showing utilization of a fluorescent-
activated, spatial quantitative light detector in an apparatus for carrying out
multiple function image processing.
Figure 2 is a cross-sectional view showing details of the light
detector of this invention used in the apparatus shown in Figure 1.
Figure 3 is a graphical representation of the absorption spectrum
and fluorescent emission spectrum of a representative fluorescent dye.
Figure 4 is a graphical representation of data obtained with a
fluorescent light detector of this invention indicating the degree of signal
uniformity as modified by the use of spectrum filters.
Figures 5 (a), (b) and (c) are a cross-sectional view of the Figure 2
embodiment showing alternate photosensor locations.
Figures 6 (a) - (d) are optional configurations in cross-section of
the light detector of this invention.
Figure 7 is a graphical representation of the relationship between
collector efficiency and dye concentration in a light detector of this invention.
Figure 8 is a graphical representation of data obtained in use of a
preferred embodiment of this invention.
Figure 9 is a cross-sectional view of a fluorescent collector having
multiple dye eoncentration segments.
Figure 10 is a cross-seetional view of a tapered fluorescent
collector rod.
Figure 11 is view of another alternative embodiment of this
invention.
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Figure 12 is a cross-sectional view of a light detector of this
invention capable of converting optical, polychromatic images into digital
form.
Figure 13 is the apparatus of Figure 1 modified to include the use of
5 a light collector as part of a read clock.
Figure 14 shows yet another alternative embodiment of the inven-
tion wherein reflectors are used to more efficiently direct light to the
fluorescent activated detector.
Referring now particularly to Figure 1 of the drawings, there is
10 shown a xerographic type reproduction apparatus 10 incorporating a fluo-
rescent-activated light detector. Xerographic reproduction apparatus 10
includes a viewing station or platen 12 wherein document originals 13 to be
reproduced or copied are placed. For operation in the COPY mode, a
light/lens imaging system 11 is provided, the light/lens system including a light
source assembly 15 for illuminating the original 13 at platen 12 and a lens 16 for
transmitting image rays reflected from the original 13 to the photoconductive
surface 19 of drum 18 at exposure station ~1.
Charging, developing, transfer and cleaning stations 20, 22, 26 and
32, respectively, are disposed about drum 18 in operative relation thereto.
Charging station 20 includes a corona charging means 23 for depositing a
uniform electrostatic charge on photoconductive surface 19 in preparation for
imaging. A suitable ~eveloping mechanism, which may, for example, comprise
a magnetic brush 25, is provided at developing station 22 for developing the
latent electrostatic images created on surface 19.
At transfer station 26, corona transfer means 27 effects transfer
of the developed image to a suitable COw substrate material 28. A suitable
drum cleaning device such as a rotating cleaning brush 33 is provided at
cleaning station 32 for removing any remaining developer material from
surface 19. Brush 33 may be disposed in an evacuated housing through which
developer materials removed from the drum surface by the cleaning brush are
exhausted.
In the example shown, photoconductive surface 19 comprises a
uniform layer of photoconductive material such as amorphous selenium on the
surface of drum 18. Drum 18 is supported for rotation by suitable bearing
means (not shown). A suitable drive motor (not shown) is drivingly coupled to
drum 18 and rotates drum 18 in the direction shown by the solid line arrow
when proeessing copies.
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When operating in the COPY mode, the photoconductive surface 19
of drum 20 is charged to a uniforrn level by corona chflrging means 23. Platen
12 and the original document 13 thereon is irradiated by light assembly 15, the
light reflected from document 13 being focused onto the photoconduetive
surface 19 by lens 16 at exposure station 21. Platen 12 and the document 13
thereon are at the same time movecl in synchronism with rotation of the drum
18. The light reflected from the original 13 selectively discharges the charged
photoconductive surface in a pattern corresponding to the image that com-
prises the original document.
The latent electrostatic image created on surface 19 is developed
by magnetic brush 25 and transferred to copy substrate material 28 through
the action of transfer corona means 27. Following transfer, the photocon-
ductive surface 19 is cleaned by cleaning brush 33 to remo~e leftover
developer material. A suitable fuser or fixing device (not shown) fixes the
image transferred to copy substrate material 28 to render the copy permanent.
While a drum type photoconductor is illustrated, other photocon-
ductor types such as a belt, web, etc. may be envisioned. Photoconductive
materials other than selenium, as for e~ample, organic photoconductors may
also be used. And, while a scan type imaging system is illustrated, other types
of imaging systems such as full frame flash, may be used.
The photoconductor may be opaque, that is, impervious to light, or
wholly or partially transparent. The exemplary drum 18 typically has an
aluminum substrate which renders the drum opaque. lIowever, other substrate
materials such as glass may be used3 which would render drum 18 wholly or
partially transparent. One material consists of an aluminized Mylar substrate
having a layer of selenium dispersed in poly-N-vinyl carbazole with a trans-
parent polymer overcoating containing a charge transport compound such as
pyrene.
Xerographic reproduction apparatus 10 includes a flying spot
scanner 59. Scanner 59 has a suitable flux source of electromagnetic radiation
such as laser 60. The collimated beam 61 of monochromatic radiation
generated by laser 60 is refleeted by mirror 62 to a modulator 65, which for
operation in the WRITE mode, modifies the beam 61 in conformance with
information contained in image signals input thereto, as will appear. Modu-
lator 65 may comprise any suitable modulator, such as acousto-optic or
electro-optic type modulators for imparting the informational content of the
imSlge signals input thereto to beam 61.
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Beam 61 is diffracted by disc deflector 68 of a holographic
deflector unit 70. Deflector 68 comprises a substantially flat disc-like
element having a plurality of grating faces or facets 71 forming the outer
periphery thereof. Deflector 68, which is preferably glass, is driven by motor
72. Preferably, deflector 68 is disposed so that light beam 68 is incident to
the facets 71 thereof at an angle of substantially 45. The diffracted scanning
beam 61' output exits at a complementary angle.
irhe scanning beam 61' passes to an imaging lens 75. As shown, lens
75 is located in the optical path between deflector 68 and mirror 77, lens 75
having a diameter suitable to receive and focus the scanning light beam
diffracted by facets 71 of deflector 68 to a selected spot in the focal plane
proximate the surface 19 of drum 18, as will appear.
The scanning beam 61' from lens 75 is reflected by mirror 77 to
read/write control mirror 78. Mirror 78, when in the solid line position ~hown
in the drawings, reflects beam 61' to mirror 80 which, in turn, reflects the
beam to a location on the surface 19 of drum 18 downstream of developer 22.
In the case where the photoconductive material is opaque, light
impinging on the surface 19 of drum 18 is scattered. In the case where the
photoconductive material is transparent, the light is transmitted, depending on
the degree OI transparency of the photoconductive material, through the
photoconductive material to the drum interior. As will be understood,
scattered light is composed of both specular and diffuse reflected light while
transmitted light is composed of specular and di-ffuse transmitted light. The
scattered or transmitted light from the photoconductive surface 19 and the
developed image thereon is collected in lluorescent-activated light detector
10~, and there converted to image signals when operating in the READ mode,
as will appear.
Read/write control mirror 78 is supported for limited movement
between a read position (shown in solid line in the drawing) anc3 a write
3~ position (shown in dotted line in the drawing). A suitable driving mechanism
such as solenoid 81 is provided to selectively move the mirror 78 from one
position to the other. Return spring means ~not shown) may be provided to
return mirror 78 to the original position upon deenergization of solenoid 81.
When in the WRITE position (the dotted line position), the scanning
beam 61' is reflected by mirrors 78, 85 to a location on the surface of drum 18
upstream of developer 22.
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While F:igure 1 illustrates one type of apparatus
utilizing the light detector 100 of this invention many
other embodimen-ts are contemplat:ed. Thus, other
apparatus disclosed in aforementioned U.SO Patent No.
4,345,835 may be used.
Referring to Figure 2, fluorescent activated light
detector 100, shown in cross-sectional view, comprises
an elongated cylindrical collector rod 102 containing a
fluorescent medium 104. Medium 10~ is a liquid or
solid containing dissolved fluorescent dye 106. Dye
106 is represented herein as dots throughout medium 104
but, in actuality, said dye is molecularly dispersed in
the medium. In the embodiment wherein the medium 104
is a solid, rod 102 is integral with the medium or, in
other words, the medium is self-supporting. In such
embodiment, a transparent cladding 107 protects the rod
from contamination. Incident light beam 61', at a
wavelength a enters rod 102, is absorbed by the
fluorescent dye and is reradiated almost
instantaneously at wavelength b. This spontaneous
reradiation or scattering is isotropic in nature and,
as shown, a large portion of the scattered light,
typically 50%, becomes trapped within guided modes of
the rod and proceeds along the rod by being totally
internally reflected from the rod surface. For a
cylindrical shaped rod, the ratio of trapped light Pt
to the total reradiated light field P, is given by: -
C = P /P = 1 - 2 sin n (1)
t
~r
where n is the ratio of the reflective index of the
medium surrounding the rod to the index of the
fluorescent medium.
As the light propagates along the tube, a portion
proceeds directly to photosensor 108 while another
portion reaches the photosensor via reflection from
reflective surface 110. Since most of this light is at
a different wavelength then the incident light, it is
not reabsorbed and rescattered by the fluorescent light
during its propagation. ( A small portion is
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reabsorbed and this sub~ect is addressed in further
detail as will be seen).
Photosensor 108, for greatest efficiency, is
preferably in contact with medium 107 and has a hiyh
sensitivity at the fluorescent wavelengths. While any
suitable reflector material may be used to form surface
110, copper, silver or gold are preferred because of
-their relatively higher reflectance at the fluorescent
wavelengths generated by a He - Ne laser. Filter 111
is used to block certain portions of the fluorescent
spectrum for purposes discussed in more detail below.
The electrical signals produced by the photosensor
are transmitted to an electronic data
processing/storage means represented in Figure 2 by box
EDP/SM. Such means are well known in the art and are
part of a system which can store, convert, transmit or
otherwise utilize the electronic information created by
scanning ~raphic information as described above.
Fluorescent materials suitable for use in the
collector rod shown in Figure 2, and in the other
embodiments to be described below, are known in the art
and are typically dyes which are dissolved in a liquid
or solid medium. For example, typical fluorescent dyes
include Rhodamine 6G, Cresyl Violet, Nile Blue A
perchlorate, oxazine perchlorate, fluorescein, l,2-d-
l-naphthyl-ethylene, 1,4-bis[2-~-methyl-5-phenylox-
azolyl~] benzene, amino G acid, anthracenes such as
9,10-diphenylanthracene, 9,chloroanthracene, Perylene
Coronene, 7-hydroxycoumarin and acridine yellow. Many
other fluorescent dyes may be utilized as are listed in
various publications such as Eastman Kodak Company Data
Service Publication JJ-169 (1979) entitled, "Eastman
Laser Dyes" and Vol. 1 of Topics in Applied Physics,
entitled "Dye Lasers" edited by E.P. Schafer and
published by Springer-Verlay, New York, New York
(1973).
It is typical to include the fluorescent dyes of
this invention in a li~uid or solid medium. For
purposes of this invention, solid media is preferred
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because oE its durability and ease of shipping and
handling. However, this invention is not to be limited
by the media ln which the dye is dissolved. Typical
liquid media are those utilized in dye lasers and
include organic solvents such as chloroform, benzene,
toluene, ethanol, ethylene glycol, glycerl, heptane,
and many others as are listed in the above-mentioned
Vol. 1 of Topics in Applied Physics. Commonly
available dye solutions are published in Eastman Kodak
company Data Service Publication JJ-169 (1979) entitled
"Eastman Laser Dyes".
Preferred solids are transparent polymers having
dimensional stability under the conditions of use
herein. Typically, suitable polymers and copolymers
include polycarbonates such as Lexan*, commercially
available from the 5eneral Electric Company, and
Merlon*, commercially available from the Mobay Chemical
Company, polystyrenes, polyesters such as polyarylates
and polysulfonates~ acrylates such as polymethyl
methacrylate, commercially available under the
tradename Lucite* from E.I. duPont de Nemours &
Company, Inc., and Plexiglass* from the Rohm & Haas
Company, acrylonitrile
* trade marks
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methacrylate copolymers such as is available under the tradename Barex from
Vistron Corporation, and condensation resins such as arylsulfonamide/formal-
dehyde resin available under the tradename Sanfolite~MHP from Monsanto
Company. Any other suitable, relatively transparent material can be utilized
as the choice of supportive medium.
The detector embodiment shown in ~igure 2 serves to illustrate the
operative principles upon which a fluorescent-activated spatial quantitative
light detector may be constructed. This detector may be improved and
modified in ways which enhance efficiency and/or improve the uniformity of
the collected signal. These improvements may be classified as follows:
1. Elimination of non-uniformity effects caused by the small
amount of light that is reabsorbed and rescattered by the fluorescent medium
after the initial absorption and reradiation.
2. Elimination or reduction of the non-uniformity effects caused
by loss, or leakage of light which is scattered at less then the critical guidedmode angles ("leaky" modes).
3. Elimination of the effects of surface scattering by conta-
minants or scratches on the exterior surfaces of the rod.
a~. Selection of a fluorescent dye solution having an optimum
quantum efficiency.
5. Modification of fluorescence along the length of the collector
rod (Improvement of uniformity.).
These improvements are considered separately below.
Reducing Non-Uniformity Attributable to Reabsorption
The reabsorption phenomenon is illustrated by reference to Figure
3. In this figure, curve A represents the absorption spectrum of a typical
fluorescent dye, oxazine 1, perchlorate in a glycerine medium. Curve B of
Figure 3 is the speetrum of the emitted fluorescent light by the dye upon
absorption of incident light. (In general, the peak and shape of the absorption
and fluorescent spectrum of a dye is dependent on the solvent used and the
spectral shift is to larger wavelength when either a larger molecular or higher
viscosity solvent is used. And, although there is some energy loss during the
wavelength shift, the aforementioned sensitivity as matching of photodetector
108 to the higher wavelength minimizes this loss).
As shown in Figure 3, there is a small area, C where the curves
overlap. It is in this area C that reabsorption and hence reradiation of the
e ~ks
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fluorescent light takes place. Frorn Equation (1), about half of this reradiatedlight is lost to the collector. Thus, collector efficiency is unavoidably
proportionately reduced. Collector uniformity is also affected since the
effects of reabsorption vary depending upon the point at which the incident
5 beam strikes the collector rod, i.e. the greater the distance from the
photosensor, the greater the chances that the trapped light will be reabsorbed.
These signal non-uniformity effects can be reduced by the filtering techniques
described below.
Referring again to ~igure 2, filter 111 is inserted in front of
10 photosensor 108 to block the shorter wavelengths of the fluorescent spectrum
and thereby smooth, or make more uniform9 the detected signal. To illustrate
the effect of incorporating filter 111 into the detector of this invention, there
is illustrated in Figure 4 a graphical presentation of data obtained with various
filters. A fluorescent medium was constructed by dissolving Rhodamine 6G in
15 a solvent composed of 70 percent ethylene glycol and 30 percent ethanol by
volume. The concentration of the dye resulted in an absorption factor of 95
percent/cm. That is, 95 percent of the light transmitted through the solution
is absorbed within 1 cm. of travel. The solution was contained in a tube 28.2
cm. in length and 9.6 cm. in diameter. The input radiation is 514 nm and the
20 fluorescence was in the range of about 590 nm. Three different filters were
inserted between the collector and the photosensor and the length of the
collector then scanned by the radiation. The relative strength of the detected
signal is plotted in relationship to the distance of incident light from the
photosensor in Figure 4. In ~igure 4, curve A indicates the results obtained
.~ 25 utilizing a Corning clear filter 5850; curve B utilizing a Corning orange filter
2434 and curve C utilizing a Corning red filter 2418. ~s is seen in Fi~ure 4,
the end-to-end variation in relative strength of the detected signal is greatly
improved utilizing the red filter of curve C. Such filter eliminates the short
wavelengths of the fluorescent spectrum produced by the Rhodamine dye. The
30 uniformity variation for C~urve A is + 14.5 percent while the variation for curve
B is ~ 7.4 percent. Curve B offers the best tradeoff providing a response
almost as flat as curve C but with a lower energy loss. Curve C utili~ing the
deep red filter, has a variation of ~ 5.2 percent while achieving 42 percent of
the unfiltered detector signal. This data indicates that at least half of the
35 end-t~end collector uniformity variation for this dye is due to self absorption
and that spectrum filtering techniques are a valuable tool for improving
detected signal uniformity.
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"Leaky'' Modes
~ s previously mentioned, approximately 5û% of the light radiated
by the fluorescent medium is coupled to a guided mode. Of the remaining non-
coupled fraction, a portion, perhaps 10%, is transmitted instantly out of the
5 rod while the remaining 40% is radiated at a~gles less then the critical totalinternal reflection (TIF~) or coupling angle. These radiated light rays propa-
gate down the rod towards the photosensor and, depending upon point of origin
and intern~l reflection angle, some may inpinge on the photosensor while
others may not. With each reflection, part of the non-TIP ray energy "leaks"
10 out of the rod, the total effect contributing to the non-uniformity of the
detected signal.
The light intensity Id for a leaky mode at photosensor 108 is
represented by the formula
Id = IoRm (2)
lS where Io is the initial intensity of the mode, R is the reflectivity from theinner surface of the rod and m is the number of reflections that the mode
undergoes before reaching the photodetector. From this equation, it is evident
that the further the incident beam is from the photosensor, the larger will the
value of m be and the less of a factor a leaky mode will be to the overall
20 signal level at the photosensor. In other words, at a certain distance from th~
photosensor only light rays travelling in the guided mode will influence the
output signal. From this observation7 it would be beneficial if all lealcy modeseffectively originate at some significant distance from the photodetector so
that all of the leaky mode energy is dissipated before arrival. As a practical
25 matter, only the leaky modes propagated so as to arrive at the photodetector
after only a few reflections will seriously distort uniformity of the output
signal. According to another aspect of the present invention, the photosensor
is spaced from the end of the rod to reduce the leaky mode contribution to
signal non-uniformity as will be shown in the following description of Figures
30 5a, 5b and 5c.
- Referring to Figure 5a, an air gap 114 has been introduced between
the end of collector rod 102 and photosensor 108. As incident light beams 61'
generates beams 62, 62' after absorption. Beam 62 is totally internally
reflected from the sides of the rod and therefore enters a guided mode. Beam
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62' strikes the sides of the rod at less than the critical 'rI~ angle and therefore
constitutes a leaky mode. As shown, the end of rod 102 is essentially
perpendieular to the sides of the rod and, under this condition part of guided
mode ray 62 is transmitted by the rod end interface to the photosensor. Leaky
mode ray 62' however, is reflected at the end surface back into the rod and is
thus prevented from reaching the photosensors. A spacing d of only a few
wavelengths of light would suffice to perform the desired blocking function.
Figure 5b illustrates a second variation wherein the photosensor 108
is optically coupled to collector rod 102 via optical fiber bundle 116. In this
embodiment, some of the leaky energJ mode will arrive at the detector but at
a reduced level since some energy will lea~ out through the fiber bundle.
Figure 5c is a variation of Figure 5b where the Eiber bundle 116' is
tapered. In this embodiment, an added benefit is the use of a small area
photosensor 108' thereby reducing capacitance and improving system
bandwidth.
Surface Scattering
Impurities and imperfections on the surface of the collector rod,
such as dust and dye spills, and scratches produce undesirable scattering
effects both on the incident light beam and on the trapped light as it
propagates by TIR along the length of the collector. Both detector efficiency
and uniformity are adversely affected. Figure 6 illustrates several collector
embodiments designed to reduce surface scattering effects. The embodiments
also serve to suggest other configurations that the detector may assume
consistent with the principles of the invention.
For each embodiment, the surface of the fluorescent medium, if in
solid rolled form, or the encapsulating cover, if in the liquid form, should be
thoroughly cleaned. From Equation (1), it is seen that the material surrounding
the collector rod should have as low an index of refraction as possible. Air
(index of 1.0) is therefore an ideal cladding material and is so used in the
Figures 6a, 6c and 6d.
Referring to Figure 6a, detector 120 comprises a cylindrical
fluorescent rod 122 containing dissolved fluorescent dye 12~. Rod 122 is
encased within glass tube 126 leaving an entrapped air or gas space 128 to serveas the cladding for the rod.
In Figure 6b~ detector 130 compises cylindrical rod 132 comprising a
liquid fluorescent medium 134 contained within glass cladding 135. Medium 13~1
'
.'~ ' `' ' ', ',
.
~ ~ 7 3 ~
--13--
contains dissolved fluorescent dye 136. A highly specular reflective coating
138 covers a large portion of the collector surface. The incident light enters
the rod through the uncoated portion, or entrance window, which can be kept
clean by maintaining an air flow over its surface. This embodiment increases
5 the efficiency of the collector by trapping more of the scattered light withinthe rod. The output signal uniformity can be maintained by modifying the
shape and/or density of coating 138 as described in further detail in the
description of Figure 11 or by varying the spatial transmission coefficient of
the entrance window.
Referring to Figure 6c, dletector 140 is a solid, rectangular fluo-
rescent member 142 containing fluorescent dye 144. Member 142 is encased
within glass tube 146 leaving an encapsulated air space 148 to serve as the
cladding for the rod. l~eflective coating 149, which can be specular or diffuse,serves the same purpose as coating 138 described above in connection with
15 Figure 6b.
In Figure ~d, detector 150 is a cylindrical fluorescent rod 152
containing dissolved fluorescent dye 154. Rod 152 is contained within inte-
grating cavity 156. The concept of using an integrating cavity, per se, as a
light detector is disclosed in my co-pending application Serial No. 183,134
20 whose contents are hereby incorporated by reference. In the present embodi-
a ment, the interior surface of cavity 156 is coated with a highly reQective- material such as Celanese polyester thermal setting paint #741-13. Incident
light beam 61' enters the cavity through entrance slit 157, passes through exit
slit 15~ and is reflected from surface 19 back into cavity 156 striking surface
25 strip 159 on the interior wall of cavity 156. Light is then reflected from the
cavity wall to provide multiple diffuse reflections of the light, much of which
is eventually incident on rod 152 to be reradiated at the fluorescent wave-
length. The rod, in this embodiment, may be kept clean by pressurizing the
cavity to maintain a positive pressure barrier. The effectiveness of this
30 embodiment is described in connection with the discussion of Figure 7 covered in the following discussion of quantum efficiency.
Optimum Quantum Efficiency
The quantum efficiency OI the dye solution of the fluorescent
collector rod is related to dye concentrations as illustrated by the graph plots35 of Figure 7. In the figure, curve A represents data obained by plotting lightabsorption vs quantum efficiency for a Nile Blue A perchlorate in chloroform.
~ t-r~de mRrk
73(-~9~
Curve B represents the data for Cresyl Violet perchlorate in isopropyl alcohol
solution.
Fluorescent quantum efficiency is defined as the ratio of the
number of quanta emitted to the number of quanta absorbed. The quantum
5 efficiency data was obtained by simultaneously measuring the dye cell
transmission and fluorescent output. Quantum efficiency fall off with dye
concentration (greater light absorption) as shown in Figure 7 is due to
quenching effects. The basic equation defining the relationship of fluore-
scence F to dye concentration is:
F = ~ I (1 - e ~bc ), (3)
where ~ and C are, respectively, the quantum efficiency, molar
absorptivity, and molar concentration of the dye; I is the incident radiant
power, and b is the path length of the dye cell. For very dilute solutions this
equation reduces to a linear relationship between fluorescence and concentra-
15 tion. For a linear response to be obtained the solution must absorb less than5% of the exciting radiation. The relationship between cell transmittance, IT,
and dye concentration is given by Beer's law:
- ~cb
The product of ~ and c is termed the absorption coefficient, O~, of the
~o solution. With the aid of Bouguer's law,
IT = Ie-oLb~ (5)
the absorption factors presented in Figure 7 can be related to the absorption
coefficient and the transmittance of any thickness eell can be calculated.
Several observations and deductions can be made from the results
25 of Figure 7 and the relationships set forth above. The first, restated, is that
the collector quantum efficiency is inversely related to the dye concentration.
But, total coUector efficiency is also dependent on the total amount of light
which is absorbed and reradiated. If the dye concentration in a rod of fixed
diameter is simply diluted, less light is being absorbed and the total efficiency
30 may decrease. One obvious way to increase the total efficiency would be to
increase the rod diameter thereby increasing the total amount of light
reradiated. There are practical limits, however, to the rod size since it must
bear some fixed relationship to the photosensor si~e, typically 1 cm. The rod
thickness can be effectively increased, however, without physically increasing
35 the rod thickness. This can be done by placing a reflective coating around the
collector as shown in the Figures 6b, 6c embodiments. These configurations
34.~
--15--
provide a second fluorescence component Vifl the reflection from the coatings
thereby effectively doubling the amount of reradiated light.
The detector size is also effectively increased by placing a rod
within the integrating cavity configuration shown in Figure 6d. Figure 8 shows
5 the data obtained by placing a rod containing the Cresyl Violet solution
described above having an absorption factor of 80%/cm.. Measurements were
made of relative detector signals at different distance from the detector for a
bare rod (curve A) and for the same rod placed in the Figure 6d configuration
(curve B). From the results shown in Figure 7, enclosing the rod in the
10 integrating cavity increases its mean signal level by 47%. About 40% of this
total gain is due to the larger absorption associated with the '~increase" of the
effective rod diameter with the remainder of the gain attributed to the
integrating cavity contribution. This particular configuration of a rod within
an integrating cavity is thus a very desirable embodiment of the present
15 invention. rhere is a remaining problem with signal non-uniformity, however,
since the random nature of the re-reflections within the cavity produce a
similarly random result along the surface of the rod. The following section
discusses some methods of improving uniformity of a collector in this (and
other) environments.
Improving Collector Uniformity
~actors contributing to non-uniform photosensor signals have been
discussed in the immediately preceding sections as well as the section relating
to reabsorption effects. The kernel of the problem is that the photosensor is
"seeing" the sum of a plurality of fluorescent light levels differing each from
25 the other in relation to their distance from the photosensor at their incidence
point. A light beam incident at a point in close proximity to the photosensor
provides a higher signal, all other things being equal, then a signal incident at
the center ~or other end) of the rod. The remedy~ in general, for this problem
is to smooth out the response so that the photosensor is receiving a uniform
30 level regardless of the incidence distance of the information beam. There areseveral ways of accomplishing this. These are listed below and each is
discussed in further detail:
1. The dye concentration can be varied along the length of the
collector rod by impregnation, or other techniques. Figure 9 illustrates an
35 embodiment wherein the rod of Figure 2, now labeled 160, has been divided
into four solid fluorescent segments each segment optically joined to the
73A9~
-16-
other. Segment 160a has been selected to have the highest dye concentration
relative to segments 160b, 160c, 160d, each of which has a progressively lower
dye concentration. The concentration can be selected so that the percentage
of incident light absorbed in each segment produce the same fluorescent level
5 signal at photosensor 162.
2. Another techni~ue for adjusting the incident light intensity
along the length of the rod is shown in Figure 10 wherein detector 170
comprises a tapered fluorescent rod 172 containing a fluorescent dye 174 with
relatively high transmissivity. With this configuration the absorption volume
10 for the incident light becomes relatively larger with increasing distance from
photosensor 176 permitting more light to enter, be absorbed and, therefore,
reradiated. The proportional diameter decrease needed to obtain a uniform
signal can be determined with minimum experimentation.
Figure 11 is a second embodiment wherein, instead of the rod being
15 tapered, a tapered coating is applied to the protective cover surrounding rodsurface (as in the Figure 6c embodiment). Referring to Figure 11, detector 180
is a cylindrical fluorescent rod 182 contAining fluorescent dye 184 with
relatively high transmissivity. The rod has a tapered reflective coating 186
placed on the back surface of the protective cover, i.e. opposite the light
20 entrance aperture. The proportions of coating 186 are selected so maximum
amounts of scattered and transmitted light are reflected back into rod 182 and
reradiated at the end opposite the detector with decreasing amounts of
reflection and reradiation occuring with decreasing distance to the photo-
sensor. The result again is to smooth out the signal level impinging at the
25 photosensor.
The same effect can be obtained by placing a mask in the path of
the incident light, the mask having a tapered aperture thereon which controls
the effective light entrance aperture. Detector 180 can be modified in several
additional ways to obtain the same end result. For example~ coating 186 can
30 be a series of dots instead of a solid coating. And, the coating can be replaced
by a specular reflecting surface placed on the rod in the manner shown in
Figure 6b.
A third method of adjusting incident light intensity is to vary the
density of a reflective or absorption coating applied to the entrance aperture
35 of the rod surface. This process is referred to as apodization. As an example,
~ ~73~9:1
-17-
the Figure ~c embodiment can be modified by altering the transmission of
entrance window 1~6. The areas of the window furthest from the photosensor
are made to have the highest transmission so that more of the incident light is
delivered to rod 1~2. Areas of the coating window nearer the photosensor are
of progressively lesser transmissivity. The net effect, again, is to smooth out
the detector response. The apodization can be achieved by a variety of
techniques. One method is to produce a desired density variation pattern
photographically and then attach the strip to the rod entrance surface. The
photographic technique includes a positive or a negative transparency. The
desired apodization could also be achieved by placing a variable transmission
filter in front of the reflector in the ~igure ~c embodiment to modify the
transmittal light reflected from the reflector back into the rod. For this case,the rod must transmit a relatively high percentage of the incident light. These
techniques lend themselves to other applications such as tailoring the entrance
aperture to conform to specific illumination requirements, i.e. as illumination
slits for photocopier applications.
3. Another way of adjusting the incident light intensity is to
modify the scanning electronics. For example, in ~igure 1, scanner 59 can be
modified to alter the intensity of the READ beam 63 with time so that a
uniform signal level is achieved at the detector when a uniform reflecting
surface is scanned.
Example
An exemplary fluorescent-activated detector with high efficiency,
good end-to-end uniformity and high signal to noise ratio was achieved with
the following collector constructed according to the embodiment shown in
Figure 6b.
A 10.5 inch rod of 0.315" diameter having as the fluorescent dye,
oxazine 170 perchlorate dissolved in a solid acrylic medium. A gold mirror was
placed at one end of the rod and an EC&G DT-110 pin photodiode was optically
coupled to the collector end with an epoxy potting cement. A visible spectrum
blocking filter Corning CS2-6~ was placed in front of the photodiode. A
reflective coating on the back side of the outer jacket was apodized. This
collector was used as the collec$or in the laser scanning system of Figure 1 in
conjunction with a HeNe laser light source of wavelength 63~.8 nm.
Collector performance measurements were made at each step of
the assembly. With the filter in place, collector end-to-end uniformity
3 ~ 9 :~.
--18-
improved from + 2~% to ~ 20%. Apodization of the jacket improved the end-
to-end uniformity to ~ 10%.
This embodiment achieved a collector efficiency of 3%, a 50:1
signal to noise ratio (~NR) at 6 MHZ bandwidth and a 30:1 SNR at 20 MHZ
5 bandwidth.
The end-to-end uniformity bandwidth and signal to noise charac-
teristic of this embodiment illustrate the potential use of this type of detector
in laser RIS systems, as pixel clocks for laser ROS and RIS systems and large
area detectors for laser communication systems.
Polychromatic Fluorescent Activated Detectors
The specification thus far has considered detectors of monochro-
matic light. In another aspect of this invention, a broad band of illumination
can be utilized by employing a series of light detectors~ each containing a
fluorescent dye having an excitation band matched to one of the major
15 components eomprising the illumination source. For example, a polychromatic
image reflection can be collected by a minimum of at least two light
detectors. In the preferred mode, an integrating cavity in conjunction with
the light detector of this invention as illustrated in Figure 6d, is utilized.
There is thus provided, an integrating cavity having mounted therein at least
20 two separate light detectors, each detector comprising a collector rod
containing a dye which absorbs in the principle wavelength of a primary color.
Further selectivity may be achieved by filtering the light each collector rod
receives. Since the excitation and fluorescence spectra are different, the
absorption of the filtration layer must be separated from the fluorescent
25 medium. In addition, each detector must be shielded from light emitted or
leaked from the other detectors so as to prevent fluorescence in the adjacent
detectors due to the leaked light. Each detector thereby provides input data
or digital recording of the image scanned including the color information
which can be utilized to reconstruct the scanned polychromatic image.
30 Utilizing digitally stored image information to reconstruct the original image
is well known in the art. Of course, full, three color original image
information is converted to digital information in accordance with this
invention by utilizing at least three light detectors, each absorbing ligh~ in one
of the principle wavelengths of the primary colors, red, green and blue.
~urther, the amount of each color of light is also converted to
digital information when means are provided in conjunction with the photo-
~ :l 7 3 '~
sensor to measure the intensity of the signals. Thus, for example, when an
original document containing a print of yellow color is scanned by a flying spotscanner composed of red, green and blue radiation, the reflections of the scan
are absorbed by the detectors, collector rods which iluoresce in response to
the principle wavelength of the red color as well as the light collector
responding to the principle wavelength of the green color. If the original is a
perfect yellow color, then the red and green lights will be of equal intensity
and so detected by the photosensor. In this embodiment of the invention, it is
preferred to utilize the means described above with respect to Figures 6a - d,
10 and 11 or a combination thereof to proportionately modify the amount of
fluorescence obtained such that the amount of fluorescence detected is
substantially the same throughout the body of detector 100. By this means, the
intensity of fluorescence is directly related to the fraction of light of each
individual color absorbed by each of the light detector rods. Also, in this
embodiment of the invention wherein it is desired to digitally store a
polychromatic image, a combination of a three color laser lines is utilized to
scan the original image.
A typical light detector suitable for collecting light reflected from
a polychromatic image is provided by an apparatus such as described in Figure
12. In Figure 12, light beam 200 from a suitable light source such as a flying
spot scanner strikes substrate 202 bearing image 204. Reflected light beam
200' enters integrating cavity 206 which is coated with a highly reflective
surface as described above. The reflected light from the walls of integrating
cavity 206 enter light detectors 210, 210' and 210ll which may be, as for
example, light collectors as described with regard to Figure 6a. In the
embodiment of Figure 12, however, transparent protective covers 212, 212l and
212" additionally contain a filter which permits only the principle wavelengths
of one primary color to be transmitted therethrough to strike light detectors
210, 210' and 210". Such filter material may be applied to the exterior of
transparent protective covers 212, 212' and 212". Another function of the filtermedia on each of the protective covers is to block the fluorescent emission
from the other light detectors~ Thus, the respective red, green and blue filterspermit, as nearly as possible only the principle wavelengths of each of the
primary colors to reach the respeetive light collectors within. Thus, for
example, image portions 204 may be of different colors while the flying spot
beam 200 comprises at least two different colors, and, for three color imaging
1 ~734~ 1
-20~
at least three colors. A component of light beam 200 would then be reflected
from one portion of image 2n4 while another component of light beam 200
would be reflected from the other portion of image 204.
There has thus been described a novel, light detector useful in
many applications to collect light reflected from an image on an im~ge
bearing substrate and to convert the ligm so collected into electronic
information. Such information can be stored, transmitted or utili~ed in any
manner as is well known in the art. For purposes of illustration, the light
detector of this invention has been described in Figure 1 with relationship to amulti-function copying apparatus. Obviously, the light collector of this
invention may be utilized in conjunction with any scheme~ process, or
apparatus wherein the image to be scanned resides on a document substrate.
It is well known that light reflected from a paper substrate is highly diffused
and thus difficult to collect. However, the light detector of the present
invention has been found to be highly suitable for collecting light reflected
from images residing on paper because of the increased efficiency brought
about by incorporation of a fluorescent medium in the collector rod.
It is to be understood that the above-described method and
arrangements are simply illustrative of the application of the principles of theinvention and that many modifications may be made without department from
the spirit and scope thereof.
For example, the detector lends itself to other light-monitoring
functions such as a read clock to measure the scanning movement of a laser
beam. Figure 13 represents the system of Figure 1 with the addition of two
components, beamsplitter 220 placed in the path of the read beam and a
second fluorescent-activated light detector 222. As the write beam is incident
on beamsplitter 220, a fraction of the beam is diverted to detector 222. The
photosensor within one end of detector 222 will produce a train of electrical
signals, i.e. clocking pulses which are a direct measure of the scanning
movement of laser beam 61', if a suitable encoding strip (Ronchi ruling) is
placed between detector 222 and beamsplitter 220.
Figure 14 illustrates another detector embodiment in which effi-
ciency is increased by placing reflective elements in beam-focusing relation-
ship to the detector. As shown, beam 230 is reflected from information
bearing surfaee 232. Curved reflectors 234, 235 are designed and located so as
to focus incident rays onto the surface of fluorescent activated detector 240.
,
,
;;~ 7 3 ~
-21--
Reflectors 234, 235 may assume a parabolic or reflective configuration or may
be a facetted reflector of the type described in U. S. Patent No. 4,190,355.
As another example of an additional function of the detector of the
present invention, the light output of a tubulsr light source such as a
fluorescent lamp can be monitored. In applications ~rhere it is important to
detect total average output, a conventional detector may give erroneous
readings due to the movement of the "hot spot" within the tube. A
fluorescent-activated light detector placed adjacent the lamp will average in
the "hot spot" output, regardless of its position and detect the true average
output over time. For example, in those systems wherein a diffusely
reflective surface such as a paper document is being scanned, it may be
desirable to place the detector a greater distance from the paper than the
distance used to collect the specularly reflected light from a photorecptor.
As an example of a still further modification, the collector rod can
be so formed that the solid medium has a parabolic refractive index distri-
bution which is highest in the center and decreases with increasing distance
from the center. A fluorescent-activated detector having a solid collector
with these characteristics would have less surface scatter and an increased
capture ratio. Technigues for fabricating such a rod are described in an
article by Yosup Othsuka et al. in Applied Optics, Vol. 20, No. 2, 15 Jan. 1981.
.
