Note: Descriptions are shown in the official language in which they were submitted.
PCT~JS9o/026 2 ~
0830c-32
- DUAL AR9A~ OPTICAh 8~0RAGE DI8X ~IT~
ERA8ABL~ AND Ro~ R~GION8
BACKGROUND OF THE INVENTION
Field of the Inventio~
The present invention relates to the field of
recording media. In particular, one embodiment of the
invention provides an optical disk upon which pre-
recorded data or information and user-supplied data or
information may be stored on the same optical disk.
~escription of Related Art
Optical data storage media in the form of
compact disks are well known as an alternative to long-
playing records and magnetic tape cassettes. The disks
with which consumers are familiar are optical read-only
disks and the common disk player is designe~ specifically
for this type of disk. These disks havo a reflective
surface containing pits which represent data in binary
form. A description of these pits and how they function
is provided by Watkinson, "The Art of Digital Audio,"
Focal Press, Chapter 13.
Compact disks are currently produced by a
pressing process similar to tho process used to produce
conventional long playing records. The process, referred
to herein as the Umastering" process, starts by first
polishing a plain glass optical disk. This disk has an
out~ide diameter from 200 to 240 mm, a thickness of 6 mm
and undergoes various cleaning and washing steps. The
di~ then coated with a thin chrome film or coupling
agent, a step taken to produce adhesion between the glas~
disk and a lay-r oS photo-resist, which is a photosensi-
tiv- material. Data on a compact disk master tape are
then transferred to the glass disk by a laser beam
cutting method.
The glass disk is still completely flat after
it is written on by the laser boam because pits are not
formed until the glass is photographically devoloped.
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WO90/14663 ~ PCT/US90/02627
" 1262l
Media on which data can be recorded directly on
and read directly from have a different configuration and
operate under a somewhat different principle. One exam-
ple is described in U.S. Patent No. 4,719,615 (Feyrer et
al.). The medium disclosed in Feyrer et al. includes a
lower expansion layer of a rubbery material which expands
when heated. The expansion layer is coupled to an upper
retention layer which is glassy at ambient temperature
and becomes rubbery when heated. Both layers are suppor-
ted on a rigid substrate. The expansion and retentionlayers each contain dyes for absorption of liqht at dif-
ferent wavelengths. Data are recorded by heating the
expansion layer by absorption of light from a laser beam
at a "record" wavelength to cause the expansion layer to
expand away from the substrate and form a protrusion or
"bump" extending into the retention layer. While this is
occurring, the retention layer rises in temperature above
its glass transition temperature so that it can deform to
; accommodate the bump. The beam is then turned off and
the retention layer cools quickly to its glassy state
before the bump levels out, thereby fixing the bump.
Reading or playback of the data is then achieved by a low
intensity "read" beam which is focused on the partially
reflecting interface between the retention layer and air.
When the read beam encounters the bump, some of the re-
flected light is scattered, while other portions of the
reflected light destructively interfere with reflected
light from non-bump areas. The resulting drop in inten-
sity is registered by the detector. Removal of the bump
to erase the data is achieved by a second laser beam at
an "erase" wavelength which is absorbed by the retention
layer and not the expansion layer. This beam heats the
retention layer alone to a rubbery state where its visco-
elastic forces and those of the expansion layer return it
to its original flat configuration. The write, read and
erase beams all enter the medium on the retention layer
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side, passing through retention layer before reaching the
expansion layer
Copending application Serial Nos 294,723, and
357,377 assigned to the assignee of the present
application and incorporated herein by reference for all
purposes, disclose a plurality of improved optical data
storage media In one embodiment the invention described
therein includes an expansion layer, a retention layer,
and a reflective layer Th- re~lectiv layer provides a
double pass through the media ~or writing and erasing
thereon
It would be desirable to provide an optical
StOragQ media that would include both permanent data
storage via pre-recording or write-oncQ recording by a
user as well as the ability to store user-supplied or
erasable data
SUMMA~X OF TNE INvENT~QN
An i~proved optical data storagQ method and
apparatu~ is disclosed The method and apparatus permit
storage o~ write protected data and user-supplied data on
th- sa~e di~k
In one mbodiment an optical Jtorag- disk is
providod with two ar-al r-gion~ Th~ ~irst ar-al region
iJ uJed ~or p-r~anent or J-~i-perman nt r-cord-d in~orma-
tion me p-r anent in~ormation ~ay b- applied by way of
a pr-~ing proc-ss or the like during the manu~acture o~
tho dl~k The second areal region includes a material
which ~ay written upon by a user The socond areal re-
gion i~ pr-~-rably an erasabl- media including, ~or
exampl-, a dy loadQd xpanJion lay-r, and a retention
lay-r ~or holding the ~xpansion lay r in an xpanded
~tat- Alternativ ly, th- ~-cond region i~ ~ writ--onc-
read-many (WORM) media
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W O 90/14663 PC~r/US90/02627
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In another embodiment the invention provides
an optical media which includes a first layer used to im-
press pre-recorded information such that it is permanent
or write protected. A second layer of the media is
provided which can be recorded upon by a user. In a pre-
ferred embodiment the second layer includes an expansion
- region and a retention region for holding the expansion
region in an expanded state.
Information is read from the first layer and
the second layer by separate mechanisms. For example
information may be read from the first layer by differ-
ential absorption of a laser beam, while information may
be read from the second layer by phase cancellation or
absence of a refl'ected beam. Pre-recorded data and user-
supplied data may be-obtained substantially simultane-
ously and are automatically synchronized.
Accordingly, in one embodiment the invention
comprises a first region, the first region comprising
; non-erasable, optically-detectable information; and a
second region, the second region adapted to record user-
supplied optically-detectable information. The storage
; medium in one embodiment is a substantially planar disk
wherein the first region is horizontally displaced on
said disk from said second region. Alternatively, the
first region is a first layer of a substantially planar
disk and the second region is a second, vertically
displaced layer of the disk.
A method of fabricating a data storage medium
is also provided. In one embodiment the method includes
the steps of providing a substrate; providing vertically
displaced regions on a surface of the substrate, the
vertically displaced regions representing permanent
optically-detectable information: and applying a user-
recordable region on the substrate, the user-recordable
region responsive to light of a first wavelength so as to
represent optically-detectable user-supplied information.
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WO90/14663 ~ ,5 ~ PCT/US90/02627
In a preferred embodiment a method of reading
user-supplied and permanently-stored information from an
optical disk comprises the steps of directing a first
read beam of a first wavelength to a location on an opti-
cal disk, the optical disk comprising a substrate havingbumps representing the permanently-stored information in
binary form; an expansion layer, the expansion layer
absorptive of light of a wavelength of the first read
beam; and a retention layer for hold ng the expansion
layer in an expanded state, the expansion layer absorp-
tive of a second read beam of a second wavelength;
directing the second read beam to the location, the
second read beam having the second wavelength; and based
on an amount of the first read beam and the second read
beam absorbed at the first location, determining if the
user-supplied information and the permanently stored
information are present at the location.
3RIEF DESCRIPTION OF THE FIGU~ES
Figure l illustrates a first embodiment of an
interactive compact disk disclosed herein.
Figure 2 illustrates the compact disk shown in
Figure l in cross-section.
Figure 3 illustrates an alternative embodiment
of the invention in cross-section.
Pigure 4 qualitatively illustrates the light
intensity of two reflected beams for the media illustra-
ted in Figure 3.
DET~ILED DESCRIPTION OF T~E PREFERRED E~MBODIMENTS
An optical data storage medium and method of
recording and reading information thereon is provided
herein. The method and apparatus will find application
in a wide variety of fields. Merely by way of example,
the method and apparatus could be used for permanent,
write-protected storage of software on an optical disk
which is also used for permanent or temporary storage of
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W O 90/14663 V ~ PC~r/US90/02627
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user-supplied data or information. Alternatively, the
method and apparatus could be used in the educational
field. For example, the method and apparatus could be
used for permanently storing recordings for use in a
language laboratory and temporarily storing responses of
a student. Other possible fields of use include the
music industry, Japanese Karaoke, and the like. Further
fields of use will be apparent to those of skill in the
art upon review of the disclosure.
Figures 1 and 2 illustrate one embodiment of
an optical storage media which may be used in accord with
the invention. In general the media includes a first -
areal region 2 and a second areal region 4. The first
areal region 2 is~used for temporary or permanent stor-
age of user-supplied information such as software, data,
voice recordings, or music recordings. The second region
4 is used for permanent storage of pre-recorded software,
data, voice recordings, music recordings, or other
information.
In a preferred embodiment the first region is
an erasable media chosen from the media disclosed in co-
pending application Serial No. 294,723 (assigned to the
assignee of the present invention and previously incor-
porated herein by reference). In a preferred embodiment
the second region is of the type readily known to those
of skill in the art and disclosed in, for example,
Watkinson (cited above), which is incorporated herein by
reference.
More particularly, as shown in Figures 1 and 2,
the first region and the second region are both mounted
on a substrate 6 which may be a transparent material such
as, for example, glass, polycarbonate, or the like. The
second region 4 includes a filler 8 or the like over the
polycarbonate substrate. The region 8 may be, for
example, only the retention layer, only the expansion
material, combinations of the two, or another material,
preferably dye loaded. Region 8 could, alternatively,
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- PCTlUS9o/0262,
comprise the material of the reflective region, which
would enable the second region to be CD compatible.
The first region includes an expansion layer 10
on the substrate. Above the expansion layer, a retention
layer 12 is provided. Above both the first region 2 and
the second region 4 a reflective layer 14 and a protec-
tive layer 16 are provided. Sealing regions 18 and 20
are provided at the inside and outside edges of the disk.
The second region of the media is impressed
with permanent information by a pres~ing process like the
one described in Watkinson et al. Thereafter, the outer,
first region of the disk is coated with the expansion and
retention layers and the inner, second region of the disk
is coated with the expansion, retention, and/or reflec-
tive materials. The reflective layer 14 i~ then provided
over both the first and second regions. Similarly, the
protective layer 16 is provided over the entire reflec-
tive layer. In preferred embodiments the expansion,
retention, and reflective layers are selected from the
materials disclosed in copending application Serial No.
357,377, entitled "Recording Media," filed on the same
date as the present application and incorporated herein
by reference ~or all purposes.
In alternative embodiments the second region i9
a write-once read-many region. Such media include abla-
tive materials and the like.
A disk playing and recording unit such as de-
scrib-d in copending application Serial No. 294,723 is
then used to play and record information on the disk.
Specifically, the unit will identify information in the
second region in the same manner as it does with a stan-
dard compact disk. At one or more selected locations in
tho pre-recorded information, a unique code is provided
to permit the player to address any in~ormation that is
recorded in the first region.
Su8srlTU~E S~l-.El'
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'`!'^` 0/14663 2 ~ ~ 3 ~ 5 ~ PCT/US90/02627
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Of course, the invention has been described
above primarily with reference to the use of erasable
media using expansion/retention layers for recordation of
user-supplied information, but it will be apparent to
those of skill in the art that other media could readily
be utilized. For example, magneto-optic media may be
utilized in some embodiments. A separate recording head
may be necessary if the two regions operate on different
principles and, therefore, the embodiment shown in
Figures 1 and 2 is preferred.
An alternative preferred em~odiment of the
invention is illustrated in Figure 3, which shows a
portion of recording media in cross-section. The
em~odiment shown in Figure 3 provides a permanent data
record on a first layer and coatings on the same areal
region which are "active", i.e., layers on which a user
may record and erase at will.
In general the disk includes a substrate or
similar layer into which data bumps or marks 102a and
102b are impressed. The data marks 102 in the substrate
may be created by conventional means known to those of
skill in the art such as the pressing process or via a
write-once read-many process. In a preferred embodiment
the data bumps on the substrate are significantly deeper
than on prior art compact disks for easier data proces-
sing. In a most preferred embodiment, the data bumps
102a and 102b are up to 6000 A deep. In addition to
providing a source of permanent data the pits may serve
as a tracking guide for a recorder used in conjunction
with the disk.
The substrate is preferably a clear or nearly
clear material which provides protection to the media
from outside forces and provides structural rigidity.
Merely by way of example the substrate may be made of
glass, polycarbonate, or the like. Other materials of
construction will be apparent to those of skill in the
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W O 90/14663 ~ ~ j V ~ J ~ P(~r/US90/02627 ..
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art. In a preferred embodiment the substrate is poly-
carbonate and is about 1 mm thick.
Adjacent the substrate, an expansion layer 104
is provided. The expansion layer is formed of a material
which (a) absorbs a percentage of light enerqy passing
through it; (b) displays a high coefficient of thermal
expansion, particularly when compared to the other layers
- of the medium; and (c) displays a high coefficient of
elasticity to the extent that it will expand readily when
heated at the temperatures encountered during a recorda-
tion process without exceeding its upper expansive limit
and contract to its original flat condition upon cooling
unless retained by the retention layer. When at room
temperature, the éxpansion layer material should be near
or above its glass transition temperature, which is pre-
ferably below 30-C. Coefficients of thermal expansion
;~ above about lX10-4/-C are preferred, with those greater
than about 5x10-4/-C particularly preferred and those
greater than about 7.5xlO-~/-C most preferred. The
degree of absorptivity of light energy could be between
20% and 40% in the wavelength range from 850 nm to 650 nm
such that the expansion layer may be heated with a write
beam. To maintain the ability to read data recorded on
the optical media on standard detection mechanisms, such
as those found on conventional compact disk players, a
~ maximum double pass absorption at the compact disk read
; wavelength (780 nm) of about 10% is most preferred.
Accordingly, the expansion layer may be selected from the
group epoxys, polyurethane, polymers, amorphous polymers,
- 30 rubber, natural rubber, butyl rubber, silicone rubber,
styrene-butadiene rubber, cellulose acetate, cellulose
~, acetate-butyrate, polystyrene, polysulfonamide, poly-
carbonate, cellulose nitrate, poly(ethyl-methacrylate),
poly(vinyl butyryl), aromatic polyesters, polyamides,
acrylic polymers, polyvinyl acetate, silicone resins,
alkyd resins, styrene-butadiene copolymers, vinyl
chloride-vinyl acetate copolymers, nitrocellulose,
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PCT1U~9C/ C262 ~
11
ethylcellulose, and mixtures thereof. In a preferred
embodiment the expansion layer is selected from the
materials disclosed in copending application Serial No.
357,377. In one emb~diment, the expansion layer is
between about 1 micron or less thick.
Adjacent the expansion layer, a retention layer
106 is provided. The retention layer is formed of a
material that (a) absorbs a percentage of light energy
passing through it; (b) displa~fs a glass transition tem-
perature which is above room temperature; (c) is rubbery,
when above ito glass transition temperature, with suffi-
cient elasticity to permit it to conform to the contour
of the distortion formed in the second layer by the
expansion of the first layer, when the first active
material layer is heated; and (d) displays sufficient
rigidity and strength below its glass transition tempera-
ture such that it will hold the expansion layer in an
expanded condition, even though the first layer is cooled
to ambient temperature. In preferred embodiments, the
retention layer 106 is formed of material or combinations
of materials which display at least some light absorption
at the wavelength of an era~e beam. The wavelength of
the erase beam light may be chosen from a wide spectrum
oS available light wavelengths. The degree of absorp-
tivity may vary from wavelength to wavelength and from
retention material to retention material but may be for
examplQ about 30~ to 45% at wavelengtho between 650 nm
and 860 n~. Accordingly, the retention layer may be made
Sro~ poxyo, polyurethane, polymer~, amorphous polymers,
rubber, natural rubber, butyl rubber, silicone rubber,
styrene-butadiene rubber, cellulose acetate, cellulose
acetate-butyrate, polystyrene, polysul~onamide, poly-
carbonate, cellulose nitrate, poly(ethyl-methacrylate),
poly(vinyl butyryl), aromatic polyestero, polyamides,
acrylic polymer~, polyvinyl acetate, silicone re~ins,
alkyd resino, styrene-butadiene copolymers, vinyl
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12
chloride-vinyl acetate copolymers, nitrocellulose,
ethylcellulose, and mixtures thereof. In a preferred
embodiment the retention layer is made in accordance
with copending application Serial No. 357,377, entitled
"Recording Media~' and assigned to the assignee of this
invention. The thickness of the retention layer is
approximately 0.5 to 1.5 microns in one embodiment. In a
preferred embodiment, the retention layer is between
about 0.5 and 1.0 microns thick.
Adjacent the retention layer, a reflective
layer 108 is provided. Reflective layer 108 is a reflec-
tive material which serves to reflect light (e.g., more
than 25~ of the light stri~ing it) back through the
expansion layer and retention layer for the purpose of
improved data recordation and data detection. During
the recordation process the reflective properties of the
reflective layer cause an entering light beam to double
pass through the media, thus doubling the e~fective light
beam path inside the media. Energy for the purposes of
heating and thus expanding the various layers is thereby
absorbed for both directions o~ the entering light beams.
In preferred embodiments, the re~lective layer is ~ormed
of materials such as gallium, aluminum, copper, silver,
gold, indium, eutectic alloys of bismuth with tin or cad-
mium, or mixtures thereof.
A protective layer 110 may also be provided.
Tho protective layer serves to absorb data bumps created
in th expansion layer and protect the media from exter-
nal force~. The protective layer may, for example, be
mad- of glas~ or polycarbonate.
In order to record on the active layer~, light
; (indicated by hv) enters the sub6trate 100 and passes
into the expan~ion layer 104, which absorb~ signi~icant
amount~ of energy at the wavelength of the light.
Accordingly, the expansion layer is heated and expand~
into the retention layer, forming bumps (indicated by
112a and 112b). As shown, the data bumps may be either
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W O 90/14663 2 ~ ~ ~ 4 ~ ~ PC~r/US90/02627
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13
at the same location as data bumps in the substrate (asillustrated by bumps 102b/112b) or at different locations
in the substrate (as illustrated by bumps 102a/112a).
A significant portion of the entering light is
passed into the retention layer 106, which is also heated
and softened, thereby accommodating the expansion layer
more easily. Alternatively, the retention layer may be
heated during writing primarily by conduction or the
expansion layer may absorb light at a first wavelength
(e.g., 680 nm) while the retention layer absorbs light at
a second wavelength (e.g., 830 nm). Light may then be
applied including both wavelengths, heating both layers
simultaneously for writing a data bump. Conversely, for
erasing, only the second wavelength is applied to the
media, thereby heating, softening, and releasing the
retention layer. In still another alternative embodi-
ment, the layers absorb light as disclosed in copending
application Serial No. 152,690, which is incorporated
herein by reference.
Differential absorption of light between the
retention and expansion layer is obtained by dye-loading
the layers with different dyes. Dyes or pigments which
may be used singly or in combination are nigrosin blue,
aniline blue, calco oil blue, ultramarine blue, methylene
2S blue chloride, monastral blue, malachite green ozalate,
sudan black 8M, tricon blue, macrolex green G, DDCI-4,
and IR26. In preferred embodiments the expansion layer
is loaded with savinal blue and the retention layer is
loaded with tricon blue and savinal blue.
The permanent data bumps 102a and 102b are read
by a separate, but preferably substantially simultaneous,
mechanism from user-supplied data bumps 112a and 112b.
In particular, in one embodiment the permanent bumps 102a
and 102b are read by differential absorption of an incom-
ing "read" laser beam. At or near the same time, the
user-supplied bumps are read by phase cancellation or
absence of a reflected beam.
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WO90/14663 ~ 3 ~ ~ PCT/US90/02627 -
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- Figure 3 illustrates four possible light paths
into the substrate, i.e., Pl, P2, P3 , and P4 . Light path
Pl passes through an area of the substrate that does not
contain any information, i.e., there is neither a perma-
nent bump or a user-recorded bump in path P1. Light path
P2 passes through a permanent information bump, but not a
user-supplied information bump. Path P3 passes through
both a permanent information bump and a user-supplied
information bump. Path P~ passes through only a user-
supplied information bump. Figure 4 illustrates (quali-
tatively) the intensity of 680 and 830 nm beams reflected
from the media as a function of location.
The expansion layer 104 is provided with a
first dye ("X") w~ich absorbs light at a wavelength of
1~ the "read" or playback beam. In one preferred embodi-
ment, the playback beam has a wavelength of about 830 nm.
The retention layer is provided with a dye "Y" which does
not absorb significant amounts of light at the wavelength
of the read beam but which does absorb significant
amounts of light at the wavelength of an "erase" beam.
While the dye in the retention layer absorbs significant
amounts of the erase beam, the expansion layer does not.
The erase beam may be, for example, about 680 to 780 nm.
In preferred embodiments, the erase beam is
also used for reading from the substrate, but its inten-
; sity is substantially reduced during reading operations.
In a preferred embodiment, the erase beam is operated at
about 10-lS milliwatts when erasing from the media but at
about 1 milliwatt when reading from the media. It will
be apparent to those of skill in the art that erasing and
reading user-supplied data could be performed by provid-
ing two dyes in the expansion layer and using different
wavelengths for reading and erasing, without departing
from the scope of this invention.
In a read mode, the read laser in path P1 (con-
taining no data bumps) has its beam intensity reduced by
an amount proportional to twice the thickness of the
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WO90/14663 ~ ; PCT/US90/02627
expansion layer 104 plus a small loss at the reflective
layer lOa, in the retention layer 106, and in the sub-
strate 100. For purposes of illustration herein, losses
at the reflective layer and in the substrate and reten-
tion layer are assumed to be small and in any eventconstant and are ignored.
In the read mode, the erase beam (operated at
the reduced level) has a beam intensity loss proportional
to the thickness of the retention layer 106. Light
levels at a detector (not shown) for the erase beam and
read beam resulting from path P1 represent latent or zero
signal intensity, i.e., binary zeros for the permanent
and user-supplied data.
Figure ~ qualitatively illustrates the inten-
sity of beams reflected from the media as a function oflocation. The intensities of reflected beams at 680 nm
(the erase beam wavelength) and 830 nm (the read beam
wavelength) are shown as a function of distance along the
substrate in Figure 3. It is assumed that the reflected
beam intensities along path P1 are the baseline or 100%
levels.
When the read beam and erase beam are directed
along path P2, the read beam intensity is decreased signi-
ficantly below the level of path Pl. In particular, the
intensity of the beam is reduced by an amount proportion-
al to twice the thickness of the expansion layer plus
twice the depth of a permanent data bump. There will,
however, be no additional attenuation of the erase beam
along path P2 as compared to path Pl. A reduced signal
intensity of the read beam along path P2 or lower, there-
fore, represents a binary 1, i.e., the presence of a
permanent data bump, and the signal intensity of the
erase beam continues to represent a binary 0 since 100%
reflectance is observed.
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WO90/14663 2 ~ PCT/US90/02627~
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When the read and erase beam are applied along
path P3, the read beam suffers not only an intensity loss
due to the increased depth of the permanent data bump in
the substrate, but also an intensity loss due to phase
cancellation and light scattering due to the temporary,
user-supplied bump. The erase beam will also detect the
presence of a bump due to light scattering and phase
cancellation. Along path P4, both the read beam and erase
beam suffer some loss due to scattering and phase
cancellation due to the temporary bump only.
Therefore, it is seen that the erase beam and
read beam each reflect distinctly different levels of
light depending upon whether a permanent data bump or a
temporary user-supplied data bump are present. A test
for the absolute level of intensity of the reflected
beam(s) can be used to determine whether information is
erasable or non-erasable.
In particular, it is seen that if the reflected
read beam is below some level (e.g., about 50% of the P~
intensity), it is known that a permanent data bump is
present. Similarly, if the intensity of the reflected
erase beam (e.g., about 50% of the Pl intensity), it is
known that a user-supplied data bump is present. In some
embodiments, it may be possible to use a single beam for
detection and segregation of the data bumps, if suffici-
ently sensitive detection electronics are used since
unique levels are reflected for each of the four paths
from the reflected read beam. Further, in some embodi-
ments it may not be necessary to segregate permanent and
user-supplied data. Dual beams are preferred in order to
obtain increased data resolution. Dual beams provide
improved data detection, particularly if, for example,
the data bumps are slightly offset or the like.
Table 1 provides qualitative reflected beam
intensities, along with their corresponding indication of
the presence of user-supplied and permanent data.
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Table l*
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Reflected Reflected
Erase ~eam Read Beam PermanentUser-Supplied
Intensitv Intensitv Data? Data?
80-100% 80-100~ No No
0-80% 80-100% Yes No
80-100% 0-80~ No Yes
0-80% 0-80% Yes Yes
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* All values for reflectance are normalized to the
"no-data" path.
It is to be understood that the above descrip-
tion is intended to be illustrative and not restrictive.
Many embodiments will be apparent to those of skill in
the art upon reviewing the above description. By way of
example the invention has been illustrated primarily with
reference to the use of erasable media for recording
user-supplied information, but it will be readily appar-
ent to those of s~ill in the art that WORM-type media
could be used, although this would enable only a single
use of the media in an interactive manner. Further,
while the invention has been described with regard to
particular wavelengths of light, the roles of these
wavelengths of light could be reversed or different
wavelengths of light could be used. The scope of the
invention should, therefore, be determined not with
reference to the above description, but should instead
be determined with reference to the appended claims,
along with the full scope of equivalents to whïch such
claims are entitled.
