Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A FULL F~AME FLASH ILLUMINATION SYSTEM
UTILIZING A DIFFUSE INTE~RATING OPTICA~ CAVITY
BACKGROUND OF T~IE INVENTION
The present invention relates generally to
an illumination system in a document copying machine
and more particularly, to a full rame flash illumination
system utilizing an integrating cavity as the light hous-
ing.
As demands for faster copying and duplicating
10 have increased, conventional machines which scan documents
in incremental fashion to provide a flowing image on
a xerographic drum have proved inadequate. New high
speed techniques have evolved which utilize flash ex-
posure of an entire document (~ull-frame) and the arrange-
15 ment of a moving photoconductor in a flat condition atthe instant of exposure.
PRIOR ART STATEMENT
In prior art flash illumination systems, the
illumination at the document plane can be generally
20 characterized as either wholly specular, or partially
specular, partially diffuse. U. S~ Patents 3,777,135;
3,586,849 and 3,669,538 are typical of optical systems
of the former type. In each of the disclosed systems, ~
light from a plurality of flash lamps is directly reflec-
25 ted, by means of reflective elements partially enclos-
ing the lamps, toward the object to be illuminated,
i. e. a document lying on a platen. Light reflected
from the docum~nt passes through a lens to be focused
onto the image plane. However, a significant portion
30 of the light generated by the light source is absorbed
by the housing walls or passes out of the illumination
area, prior to, and following incidence on, the platen.
These illumination losses reduce the amount of light
reaching the lens and, therefore, to achieve adequat~
35 exposure, a plurality of high intensity light sources
utilizing large and costly power supplies are required.
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Significant gains in illumination efficiency
are obtained if the light sources are enclosed within
an integratin~ cavity whose walls are made of, or coated
with, a diffuse reflecting material. The theoretical
implications of diffusely illuminating an object by
causing multiple reflections within an "integrating
sphere" is set forth in Princi~le of Optics, pp. 278-
283, (McGraw Hill, 1932). It is therein disclosed that,
if the interior wall of a sphere is coated with a white,
10 diffusely reflecting material with high reflectance,
multiple reflections within the sphere produce an almost
uniform diffuse illumination of an object plane. Copier
illumination systems disclosed in U. S. Patents 3,197,177
(Booth); 3,498,715 (Gold) and 3,428,397 (Elmer) have
15 attempted to utilize this principle with varying degrees
of success. A diffusely reflecting integrating cavity
has also been used to achieve uniform fixing of a develop-
ed xerographic image as disclosed in U. S. Pa~ent 3,52g,129
(Rees).
The Gold and Elmer references are representa-
tive of optical systems which illuminate an object plane
with light that is both diffuse and specular. In Gold, a
pair of lamps are partially enclosed by baf1es with
reflective interiors. The lamps directly illu~inate
25 the object plane with increased illumination directed
towards the end of the support by the baffles and by
diffuse reflecting suraces located above the lamps.
The housing, however, is not a true integrating cavity
since the chamber is not wholly enclosed and many areas
30 are blackened to achieve the specific purposes set forth
in the disclosure.
Elmer also uses a pair of lamps partially en-
closed by specular reflective elements. A pair of side
walls opposite the reflectors provide a first primary
35 diffuse reflection of light from the opposing liyht source.
The end walls provide a second specular reflection of
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the diffuse illumination incident thereon. This arrangement
provides illumination at the object plane which is substantial-
ly diffuse and with greater intensity at the edges.
The Booth reference has a pair of light sources which
are contained in the upper compartment of a two chamber cavity.
Direct rays from the light sources are prevented from going
through the lens by making the bottom chamber of narrower
cross-section so the top edges intercept the direct rays.
The interior housing walls are coated with a diffuse reflect-
ing material except for a darkened area on one of the walls.The object plane is illuminated by direct light from the
lamps and diffuse illumination after multiple reflections
from the cavity walls.
In U. S. Patent 4,250,538, issued February 10, 1981,
Durbin et al, a single flash lamp is disposed wi~hin an inte-
grating cavity whose interior surfaces are entirely coated
with a diffuse reflective material. In one embodiment, the
lamp is partially enclosed by a blocker element which acts
to prevent direct light flashes from being viewed from a
position above the platen. The present invention discloses
additional blocker element configurations which improve the
distribution of light in areas of the platen overlying the
lamp while maintaining the desired exposure levels at the
image plane. None of the prior art references disclose these
blocker configurations.
SUMMARY OF THE INVENTION
It is therefore an object of an aspect of this inven-
tion to provide improved platen illumination in optical sys-
tems by using a single flash lamp source within a diffusely
reflecting integrating cavity chamber.
It is an object of an aspect of the invention to
provide improved platen illumination while maintaining
desired exposure levels at an image plane.
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According to one aspect of this invention there is
provided a full frame, flash illumination system or a docu-
ment copying machine comprising: a housing having a top
surface defining an object plane and containing a platen for
supporting a document to be copied, a bottom sur~ace accom-
modating a lens for focusing an image of a document onto an
image plane, said housing completely enclosed by side walls
joined to said top and bottom surfaces, said housing further
characterized by having its interior surfaces completely
covered with a high reflectivity material such that said
surfaces are highly diffusely reflective to light incident
thereon, a single flash illumination source positioned
within said housing between said top and bottom surfaces,
a blocking element positioned between the lamp and the
platen of sufficient length and width to prevent direct
illumination from the lamp from reaching any portion of
the platen, said element including an aperture therein, and
means to intermittently pulse said illumination source to
produce light flashes of pre-defined direction and intensity
a portion of said light passing through said aperture r
said flashes undergoing multiple reflections from said
coated surfaces to achieve a uniformly diffuse irradiation
at the platen.
DESCRIPTION OF THE DRAWINGS
Figure 1 is a partial perspective view of the illu-
mination housing of the present invention showing a first
embodiment of a lamp and blocker arrangement.
Figure 2 is a sectional view taken along line 2-2
of Figure 1.
26 Figure 3 is another perspective view showing an
alternate blocker construction.
DESCRIPTION
.
The blocker element configurations of the present
invention are especially suited for use in a system as des-
cribed in the above-noted Durbin et al. patent and there-
fore, the Durbin et al system and the use of the present
invention within that system will firstly be described below.
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Referring to the drawings, Figures 1 and 2 illustrate
a first embodiment of an integrating optical cavity contain-
ing a single flash lamp having a blocking element partially
enclosing the lamp according to the invention.
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A completely enclosed housing 10 is generally
rectangular in shape having a first pair of opposing
side walls 12, 14 and a second pair o opposing side
walls 16, 18. An upper, or top wall 20 includes a rec-
tangular aperture 22 which, at the center thereofy accommo-
dates a glass platen 24 forming the object plane. The
platen may have a hinged cover (not shown) which may
be pivoted upward to permit open platen copy;ng of mater-
ials such as books.
The lower, or bottom, wall 30 has an aperture
32 therein which accommodates a circular lens housing
34. An image plane 36, which may be, for example, a
photoconductive sheet to which a charge has been applied,
is positioned for exposure to light reflected from a
document placed on platen 24 and transmitted by lens
34. The projected light image selectively discharges
the photoconductive sheet resulting in formation of a
latent electrostatic image thereon.
Mounted in the lower half of side wall 16
by bracket 40 is flash illumination source 42, which
is of relatively short length compared to the platen
length in the same directionO Source ~2 in a pre~erred
embodiment is a lamp consisting of an envelope contain-
ing Xenon gas and a pair of electrodes at each end which
are not electrically connected to each other. The lamp
is connected to pulsing triggering circuitry (not shown)
which may consist of a DC power supply connected across
a storage capacitor which in turn is connected to the
flash lamp through an inductor. When the pulsing cir-
cuit is activated, the lamp undergoes a gas breakdown
which, in turn, pulses the lamp resulting in a flash
of appropriate duration. In operation, the lamp is
periodically energized in timed relation to the move~
ment of the image plane past the lens.
The interior walls of the cavity (12, 14,
16, 18, and 30) and barrel 34a of lens housing 34 have
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substa~tially diffuse reflecting surfaces resulting from
coating the interior wall surfaces with a high reflec-
tivity (90%) material. Paints such as a Celanese poly-
ester thermal setting paint #741.13 or Glidden Enamel
~424-W-02100 in cellosolve acetate thinner have been
found to be acceptable materials.
A first thin blocker element 44 is secured
to bottom wall 30 and pro~ects upwardly and laterally
a sufficient distance to prevent direct light from lamp
10 42 from reaching lens 34. Element 44 is of generally
rectangular shape except for the omission of the upper
corners.
The surfaces of element 44 may be coated with
the same reflecting material which comprises the coating
for the internal cavity walls.
According to the present invention a second
blocker element 46 is mounted above source 42 and secured
to wall 16 by screws 50 fastened through end portion
4~a. Located at the end of the blocker closest to the
20 wall is aperture 52. Blocker 46 projects at a downward
angle relative to wall 16 and is of sufficient width
and length to prevent direct light from source 42 from
being viewed from any portions of platen 24.
Blocker element 46 can be constructed of
any lightweight opaque material; the purpose of the
blocker is to protect an operator from direct flash
illumination in situations where the job requirements
call for an open platen. The blocker should therefore
be of sufficient length and width to completely block
a view of the lamp from any point on the platen. Blocker
44 can also be constructed of a lightweight opaque material.
Its function is two-fold; to prevent direct light from
source 42 from entering the lens and for preventing a
"hot spot" of intense illumination from forming. The
"hot spot", in the absence of the blocker would result
from the direct light striking in front of the lens
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surface by light reflecting downward from the bottom
of blocker 46, and directly from the lamp.
Because of the relatively close positioning
of blockers 44 and 46 to the lamp, the diffusely re~lec-
ting surfaces facing the lamp may, for some systems,inefficiently direct light to the interior walls. It
may, therefore, be desirable to form one or both surfaces
facing the lamp with a polished aluminum surface to pro-
vide specular instead of diffusely reflection a~ that
surface.
As shown in Figures 1 and 2, aperture 52 is
located at the extreme end of blocker ~6. The purpose
of aperture 52 is to permit entry of a portion of direct
light from lamp 42 into the area above the blocker.
It has been found that without the presence of aperture
52, illumination of the platen area generally located
above blocker 46 may be lower than the remainder of the
platen. The irradiance level is increased by permitting
direct light to exit through aperture 52, reflect of~
wall 16 and onto the platen. The location of aperture
52 may be varied somewhat provided light is not directly
visible through the aperture from any point in the platen.
Figure 3 shows another optical blocking con-
figuration according to the present invention. A block-
er assembly 60 is positioned above the lamp. Assembly60 consists of a bottom plate 62 having an aperture 64
in a generally central location. Plate 62 projects at
a downward angle relative to wall 16 and is secured to
the wall by screws 63. A generally U-shaped member 65
is secured to the surface of plate 62 along edges S6a,
66b~ Member 65 straddles aperture 64 and prevents any
direct light exiting from the aperture from reaching
the platen. The dimensions of plate 62 are otherwise
sufficient to prevent any direct light from lamp 42 from
reaching the platen.
It is noted that with this Figure 3 configura-
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tion, the aperture can be made more centrally located
than the previously described embodiment. The light
exiting from aperture 64 is now reflectecl from the bottom
surEace of member 65 and, following an additional reElec-
tion from the top surface of plate 62, proceeds upwardly
in a more evenly distributed fashion.
As in the blo~ker embodiments of Figu~es 1
and 2, various combinations oE diffusely or specularly
reflective coatings may be applied to the surfaces of
the plate 62 and member 65.
Referring again to Figures 1 and 2, an efficient
flash illumination system is reali~ed when utilizing
as the illumination source, a Xenon lamp with an overall
length of between 12.7 and 17.8 cm and producing a nominal
energy of 40 joules at a wavelength of 400 to 700 nm.
The lamp, as shown, is mounted approximately 1/3 the
height of wall 16 but other locations are possible dependent
upon system requirements (so long as the lamp remains
outside of the lens field of view).
For the Figure 3 embodiment, blocker element
44 and blocker assembly 60 can be constructed of any
lightweight opaque material such as aluminum. The purpose
of blocker assembly 60 is to protect an operator from
direct flash illumination in situations where the job
re~uirements call for an open platen while at the same
time permitting passage of sufficient illumination from
lamp 4~ to provide a platen illumination above the assembly
uniform with that of non-overlying areas. The function
of blocker 44 is as previously described.
It may be desirable, for certain applications,
that the blockers 44, 46 or the component parts of blocker
assembly 60 be translucent. For example, when documents
having a relatively large amount of grey background area
are to be copied, the light reflecting from these dark
areas only partially discharge the charge on the photoreceptor
image plane. The image of blocker assembly 46 or 60
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reflected from the partially reflecting platen,
is in these circumstances, visible as a super-imposition
on the partially discharged image plane. Making the
assembly translucent greatly reduces the unwanted blocker
image.
Because of the re]atively close positioning
of blockers 46, 62 and 44 to the lamp, the diffusely
reflecting surfaces facing the lamp may, for some sys-
tems, inefficiently direct light to the interior walls.
It may, therefore, be desirable to make the blocker sur-
faces facing the lamp of a specular reflecting material
such as polished aluminum.
From the foregoing, it will be appreciated
that integrating cavity 10 is constructed to approximate
an integrating sphere and hence is beneficiary to the
increase in irradiance which is characteristic of such
a construction.
It will be appreciated that when source 42
is pulsed and caused to flash, light is directed against
the cavity walls, and after undergoing multiple reflec-
tions from all interior surfaces a near-uniform diffuse
illumination is produced over the platen. Additional
direct light passing through the blocker aperture in
the various embodiments adds to the illumination of the
overlying platen area. Significantly, light reflected
from the document itself and not directly entering the
lens will undergo additional reflections and contribute
further to platen irradiance. The underside and topside
of the platen may be coated, if desired with one or two
coatings o~ an anti-reflection material such as Mgf2
or multilayer of anti-reflective material. As is well
known in the art, this material will prevent any platen-
derived specular reflection from entering the lens.
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or multilayer o~ anti-reflective material. As is well
known in the art, this material will prevent any platen-
derived specular reflection from entering the lens~
