Note: Descriptions are shown in the official language in which they were submitted.
RCA 71,62~
l This invention ~elates to an improved information
record for use in optical recording appara-tus and optical
playback appara-tus to recover information from such informa-
tion record.
S
French Published Application of Spong,
Serial No. 2,344,920, announced in the Official Gazette,
No. 41, October 14, 1977, describes an
ablative recording medium such as a flat glass disc having
a layer of a highly reflecting material, such as aluminum,
overcoated with a thin layer of a material which is high:Ly
absorptive of light at the recording light frequency. During
recording, modulated laser light, both incident and reflected,
passes through the thin absorptive layer, increasing its
temperature so that it ablates or melts to form a pattern of
holes in the absorption layer representative of recorded
information. The thickness of the light absorption layer is
chosen to establlsh an antireflection condition for the
coated information record at the recording light frequency.
Very efficient heating to melt or ablation temperatures is
realizable and a sequence of regularly shaped holes of micron
or submicron dimensions is formed along a track. The length
and spacing of these holes carry the information, as for
2S example, by frequency modulation of video signals, to provide
excellent records having signal to noise ratios of up to 50
decibels (hereinafter dB) at a very high density of packing
of stored information. For example, one television frame
occupies an area of only about l square millimeter. The
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RCA 71,62~
above described recordLng meclium is thus capable of high
quality playhack of recorded television signals of broadcast
quality.
However, the above recording medium has the dis-
advantage that it is ~oxpensive. The substrate on which the
reflective and ahsorptive layers are deposited must have a
high degree of surface perfection, since defects of micron
size or even less cause signal dropouts or disturbances to
the retrieved video slgnal.
Several dropout compensator schemes are known for
which certain signal dropouts can be compensated, but only
when they are of such magnitude that the signal perturba-
tion goes beyond the normal range of vicleo signal levels
which correspond to picture information. Deviations from
flatness oE the recording substrate can also be compensated
for by means of focus servos to keep the objective lens in
focus at a fixed distance from the recording surface.
However, microscopic imperfections in the record-
ing medium substrate heretofore were eliminated by careful
mechanical polishing of the glass disc surface. While effec-
tive, this method is costly. Further, glass is a very brittle
material subject to breakage, particularly when spinning at
high speeds, typically at 1800 rpm, during recording and
readout. Not only is the recording destroyed, but some
danger to the operator is apparent. Less brittle materials,
such as photographic quality polyester sheet, acrylic sheet
and pressed vinyl, do not have the degree of surface perfec-
tion required and are totally unsuitable for the present
application. Thus an inexpensive, safe disc surface that
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~CA 71,624
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1 has a surface free o~ macroscopic and microscopic imperfec~
tions and is suitable for broadcast quality television
reeordings has been sought.
We have found that plastie substrates ean be made
free of maeroseopic and microscopic imperfections by coating
with a thin layer of a material that is able to flow so as
to fill in any surface defects in the substrate and form a
very smooth layer and that can cure or dry rapidly at low
temperatures to form a glossy, smooth, inert, hard and
durable finish onto which the reflecting layer can be
deposited.
In the drawings:
FIG. l is a eross seetional view of an embodiment
of the present reeording medium.
FIG. 2 ls a cross seetional view oE another
embodiment of the reeordinq medium.
FIG. 3 is a eross seetional view of still another
embodin,ent of the medium.
FIG. 4 is a sehematie view of the system of reeording
~nd playbaek in whieh the present reeording medium ean be
employed.
Various eoatings and finishes available eommereially
ean be employed in the present applieation. ~urprisingly,
we have found that eommereial laequers, varnishes, epoxy
paints, polyurethane floor and furniture finishes and
aerylie latex floor finishes, as well as hardenable gelatins
and photoeurable polymers, ean be applied to plastie
substrate materials having highly imperfeet surfaees and
dried or eured to produee a substrate having a mieroseopieally
smooth surfaee suitable for reeording and playbaek of
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RCA 71,624
L3r5~
1 broadcast quality video information.
The recording medium of the invention can be
described with reference to FIGS. 1, 2 and 3,and is
designated as lO. A substrate 12 can be any inexpensive
plastic such as polyvinylchloride (PVC), acrylic,
polyethyleneterephthalate polymers and the like, and
can be in the form of a tape or disc, but will be hereinafter
described in disc form.
Useful substrate ma-terials can be polyester sheet,
polymethylme-thacrylate or acrylic sheet, and PVC record
blanks similar to those commonly employed in the audio
record industry. Even carefully prepared, high quallty
materials of the above types do not have surfaces of adequate
perfection for the preparation of optical discs for the
present application.
Materials found to be suitab:Le to coat the plastic
substrate include acrylic latex floor finishes, epoxy
paints without pigment, diluted with known reducers;
dichromated gelatin, e.g., 2 - 5 percent by wèight of
gelatin solution to which about 2 percent by weight of
sodium dichromate is added; polyurethane varnishes; alkyd
varnishes such as a 35 percent by weigh-t ester gum modified
soya-linseed alkyd resin in 65 percent of an aliphatic
hydrocarbon solvent; and monomers which cross link or cure
when irradiated with ultra violet light and the li~e.
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RCA 71,62
The coating layer I~ can be applied to the
substrate disc 12 in FIGS. 1, 2 and 3 in any convenient manner,
as by spinning and the like. It is important that the coat-
ing be applied so as to prevent the formation of ripples or
other gross surface defects. A few drops o a leveling agent
can be added to paint and varnish formulations advantageously.
The coating material can be diluted to obtain a coating of
the desired thickness. The coating layer must be thick
enough so as to fill in any impèrfections in the disc surface.
The coating is then dried or cured until hardened.
For example, varnishes can be applied by spinning and drying
in air or at slightly elevated temperatures, e.g., about
45C. Water based materials can be dried in a circulating
hot air oven to remove residual water. Care should be taken
to maintain a dust-free atmosphere during application and
dryin~ of the coating layer.
A metallic light reflecting layer 16 is applied,
generally by vacuum evaporation deposition to the dried and/
or cured coating layer. For example, a layer of aluminum
about 500 angstroms thick, a layer of rhodium about 1000
angstroms thick or a layer of gold about 800 angstroms thick
can be employed.
A light absorbing layer 18 is then applied to the
light reflecting layer. A suitable light absorbing, anti-
reflective layer can be either a single organic dye layer,
as is disclosed in the French Published Application of Spong,
referred to above and shown in FIGS. 1 and 2, or a two
layer system consisting of an absorbing layer 2~ and a
non-absorbing spacer layer 26 as shown in FIG. 3.
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RCA 71,6
l Suitable dye layers ir~clude fluorescein or
4-phenylazo-l-naphthylamine abou-t 400 - 500 an~stroms -thick.
Suitable two layer systems include a bismuth or titanium
layer about 75 angstroms thick, or a rhodium layer about
300 angstroms thic~ over a silicon oxi~e spacer layer.
In either case, recording is accomplished by
melting or ablating holes, or otherwise altering the absorb-
ing layer. ~he heat associated with this process tends to
diffuse through the light reflecting layer into the sub-
strate. Since the present coated substrate materials may
be sensitive to heat, it is desirable to apply a thermalbarrier layer (not shown) between the coating layer of the
present invention and the light reflecting layer. A layer
of silica about 3000 angstroms thick is suitable, although
the thickness of such a layer is not critical.
Alternatively, the optical spacer layer of the
above described two layer light absorbing system can serve
both as an optical spacer layer and a thermal barrier layer.
In this case, however, the thic~ness of the spacer layer
is important and it is dependent upon the optical constants
o~ the light reflecting layer, the spacer layer and ~-
the light absorbing layex because o~ the design re~uirements
to achieve the antireflection condition.
~ An overcoat layer 20 to protect the
recording medium ~rom the effects of surface dust and other
contaminants can be applied over the light absorbing layer
as shown in FIGS. 2 and 3. Suitably, a layer o~ a transparen-t
room temperature vulcanizable silicone resin can be employ~d.
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I The use of the present recording medium can be
explained in qreater detail by referring to FIG. 4. For
recording, the light emitted by an argon laser 110 is fed
to a modulator 112 which intensity modulates the light in
response to an input electrical signal from source 114.
The mddulated ligh-t is enlarqed by recording optics 116 to
increase the diameter of the intensity modulated laser beam
so that it wlll fill the desired aperture of an objective
lens 118. The enlarged modulated laser beam is totally
reflected by a polarizinq beam splitter 120 and passes
thro~gh a beam rotating one~quarter wave pla-te 122 to the
objective lens 118. The modulated recording beam then
impinges upon a recording medium 10 as shown in FIGS. 1,
2, and 3 and alters the light absorbi.ng layer of the
recording medium, as by ablating a series o:f pits, such
as pit 22 in FIG. 2 and pit 32 in FIG. 3,in the abso~bing
layer. The recording medium 10 is scanned in a spiral
track by the turn-table drive 124 rotating at about laO0 rpm. .
A focus servo 126 maintains a constant distance between the
objective lens 118 and the surface of the recording medium 10.
For readout, an unmodulated and less intense
laser beam, that is, one that will not cause changes in the
recording medium, follows -the same path as the recording
beam to the recording medium 10. The recorded reflection-
antireflection pattern modulates the reflected light back
through the objective lens 118 and the one-quarter wave
plate 122. The light, now rotated by 90 in polarization
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RCA 71,624
by the two passages through the one-quarter wave plate
122, passes through the polarizing beam splitter 120 and is
directed by playback optics 128 to a photodetector 130.
Photodetector 130 converts the reflected light beam to an
electrical output signal at terminal 132 which corresponds
to the input signal. A tracking servo 134 monitors the
l.ight through the playback optics 128 to insure that the
track in the recording medium 10 during playback is the
same as that used for recording.
The recording medium described hereinabove pro-
vides a record blank on which video information may be
recorded as above described. The intensity of the record-
ing light is controlled in accordance with the information
to be recorded, as by carrier waves modulated in frequency
by picture representative video signals, with the light
beam intensity varying between a level high enough to cause
changes in the absorptive layer and a lower level insufficient
to cause such changes, the frequency of the level alterations
varying as the video signal amplitude changes.
An informa~ion track comprising a succession of
holes or pits such as pit 22 in FIG. 2 and pit 32 in FIG. 3
is formed in the absorptive layer of the disc through the
thermal alteration of the absorptive material in response to
the modulated light beam exposure. The present recording
medium provides video recordings with broadcast quality
signal-to-noise ratios.
The invention will be further illustrated by the
following Examples, but it is to be understood that the
invention is not meant to be limited to the details described
therein.
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RCA 71,624
EXAMPLE l
~ 12 inch disc (30.5 cen~imeters) pressed from
polyvinylchloride was spin coated with a layer of
acrylic floor finish in aqueous solu-tion and allowed to dry
at room temperature. A layer of silicon dloxide about 3000
angstroms thick was evaporated onto the acrylic layer to
act as a thermal barrier.
In sequence, a layer of aluminum about 500 ang-
stroms thick, overcoated with a layer of silicon dioxide
10 about 800 angstroms thick and a layer of titanium about 75
angstroms thick was app]ied to the coated disc in ]cnownmanner. Finally, an overcoat layer of a room temperature
vulcanizable silicone resin about 75 microns thick was
applied~
A recording was made using the apparatus as
described in FIG. 4 varying the recording laser power.
The recording characteristics are summarized in Table 1
below where Control 1 was made without the barrier or over-
coat layers and Control 2 was made without the overcoat20
layer.
TABLE 1
Si~nal-to-Noise Ratio, dB
Laser Power, Example Control Control
mW 1 1 2
. .
400 46 ~2 45
25350 48 43 44
300 50 42 40
250 49 <35 <35
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RCA 71,624
RXAMPI.E ?
Using the coated disc of Example 1, a record blank
was made by applying sequentially a reflecting layer of gold
about 800 angstroms thick, a light absorbing layer of 4-phenyl-
azo-l-naphthylamine about 400 angstroms thick, a barrier
layer of silicon dioxide about 1670 angstroms thick and a
room temperature vulcanizable silicone protective layer
about 75 microns thick.
A record made as in Example 1 at a laser power of
10 about 300 mW had a signal to noise ratio of 47 dB.
EXAMPLE 3
.
A filtered solution of 2 percent dichromated
gelatin in water was spun onto a polyvinylchloride grooveless
record blank and allowed to dry in clean air at 45C for
24 hours. A record medium was made as in Example 1. Only
1 - 5 dropouts per television frame were noted.
A similar recording medium, except that the poly-
vinylchloride blank was not coated, was recorded. This
picture had hundreds of dropouts per frame.
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