Note: Descriptions are shown in the official language in which they were submitted.
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Background of the invention.
Field of the invention.
This invention relates generally to the fabrication of master
records and direct read after write ("draw") records using an information
modulated radiation write beam, and it relates more particularly to video
master records and video direct read after write records. The invention
; also relates to records particularly adapted to the method of fabrication,
but also having advantages unrelated to the method of fabrication.
Description of the prior art.
10Several methods have been developed in the past for writing in-
formation onto an information storage medium using an information modulated
radiation write beam. The information storage medium, which may be a
video master record or a video "draw" record, has a radiation sensitive
layer on at least one of its faces and is exposed to the radiation write
beam during writing. In the article "A Review of the MCA Disco-Vision
System" (Journal of the SMPTE, July '74, Volume ~3) a method, typical to
the prior art, for fabricating a master video record is described. The
master record comprises a glass disc base on which a thin metal film is
evaporated.
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This film is sensitive to a radiation write beam provided by
an argon-ion laser. The write beam is modulated by a Pockels
cell and directed by optics onto the metal film of the rotating
disc, to produce an information track consisting of small holes
in the metal film where the metal film is evaporated by the
beam. The last few optical elements in the write beam are
mounted on a carriage that is moved along the record's radius
by a motor-driven lead screw. The ob;ective lens is supported
on an air bearing, which is loaded against the surface of the
record. A relatively small air flow at moderately high pres-
sure maintains the head and objective lens at a constant dis-
tance of approximately 0.0005 inch (0.013 mm) from the metal
film surface.
A master record thus fabricated may also be used as a
direct read after write record. Since reproduction from a
direct read after write record provides a higher quality
television picture than may be provided by video magnetic
tapes, it is believed that there may be a potential in the
future for the use of direct read after write records for re-
cording purposes as a high quality alternative to magnetic
tape recording. -~
The problems presently encountered in master and "draw"
record fabrication are mainly due to dust particles settling
on the surface of the information layer impairing the formation
and read-out of the information track. Also, with methods
wherein the write beams evaporates material out of a metal
film, evaporated material may settle on the objective lens,
thus interfering with the optical pro-
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pertLes Or the lens.
The last mentioned problem could be overcome by using
a dlfferent kind of information layer, such as a photographic
or photochemlcal layer. At the present state of the art
however, it is not possible to monitor the recording with
these layers as compared to a photomechanical layer such as
the already described metal film, due to the fact that the
virtual image immediately available on recording must be
developed before it can be formed into distinct radiation
read beam modulating "pits", measuring roughly 1 ~m x 2 ~m
at a mean distance in the track of 1.66 ~m.
The first mentioned problem may be overcome by carrying
out the method in an ultra clean room, having special and
elaborate equipment to filter most of the dust out of the air.
It is an ob~ect of the invention to provide a method
that will alleviate the above mentioned problems and that may
be used outside of an ultra clean room, for instance for the
direct recording prior to broadcasting of television programs
in or outside a television studio.
It is a further object of the invention to provide a
record which is particularly adapted for use with the new
method. Another object is to pravide an information storage
record that has information tracks provided in a surface layer
and which may be reproduced without loss of quality of the
reproduced information due to dust particles on the information
tracks.
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Summary of the invention.
In accordance with one aspect of this invention there is provided
a method for writing information with an information modulated radiation write
beam onto a radiation sensitive information layer provided on at least one
face oE an information storage medium, the information being stored in the
information layer by local changes in the physical properties of the informa-
tion layer, the method comprising the steps of:
a. providing a write-beam-transparent flexible foil attached to
at least part of said information storage medium and in cli~nging relationship
to said information layer prior to recording,
b. immediately prior to writing said information providing a gas
layer between said foil and said information layer, thereby separating said
transparent foil from said information layer,
c. modulating the read beam with the information to be stored in
the storage medium,
d. focussing the modulated write beam through the protective foil
and through said gas layer onto said information layer,
e. moving the write beam in relation to the storage medium and
the protective foil in order to write information tracks in said information
layer,
f. maintaining said gas layer between said transparent foil and
said information layer throughout said writing process, and - -
g. exhausting said gas from between said foil and said informa-
tion layer subsequent to said writing process to reestablish said clinging
, relationship between said foil and said information layer, thereby protecting
Il said information layer from dust contamination both prior and subsequent to
writing, while providing a gas-filled space abutting said information layer
`1 durlng the writing process to permit by-products of the writing process to
pass freely from said information layer.
In the method according to the invention the objective for focus-
sing the write beam is separated from the surface of the information layer by
a distance greater than the thickness of the transparent protective means,
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which may be a foil. Dust particles present in the ambient air, may settle
on the surface of the protective means. However, this surface being out of
focus of the objective, the detrimental effect on the quality of the recording
will be much less than in the prior art method, where the dust particles may
settle on the very surface on which the objective is focussed. Also, if a
write method similar to the earlier described prior art method is used, the
protective means will protect the objective against the metal evaporated by
the write beam.
The optics of the objective used in the method according to the
invention will have to be dified, since the optical path of the radiation
beam is increased by the thickness of the protective means. The required
modification however calls for techniques which are entirely state of the art
and identical to the known techniques used in optical microscopes to adapt the
microscope objective to the thickness of the cover-glasses used in microscopic
slides.
In accordance with the invention there is also provided an informa-
tion storage medium having a protective mean~s, such as a protective foil,
attached to it. This has the advantage that the first step of the method
according to the invention may be carried out in the manufacture of the
storage medium. The first step may thus be carried out more economically and
blank storage media may be kept in stock ready for use without the need of
keeping them in a dust tight enclosure or of cleaning the surface of the in-
formation layer prior to the information-wri-tlng s*eps.
Thus in accordance with another aspect of this invention, there is
provided an information storage medium, comprising a base, a radiation-sensi-
tive information layer on a surface of the base, a radiation-transparent pro-
tective foil attached to at least part of the storage medium and in separable
clinging relationship to said radlation-sensitive information layer, and said
storage medium being provided with a gas-admitting aperture communicating
with the interface of said protective foil and said information layer and com-
prising a means for providing a layer of gas between said protective foil and
said information layer during a writing process and for permitting said gas
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layer to be exhausted subsequent to said writing process.
Brief description of the drawings.
The features of the invention which are believed to be novel are s
set forth with particularity in the appended Claims. The inventionJ together
with further objects and
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advantages thereof, may best be understood by reference to the
following description taken in conjunction with the accompany-
ing drawings in which like reference numerals identify like
elements, and in which:
Fig. 1 is a perspective view of a video master recording
machine with a video master record in place on a turntable,
part of the protective foil of the record broken away,
Fig. 2 is a sectional view of part of the machine and
the record of Fig. 1,
Fig. 3 is a plan view of the master record used with
the machine of Fig. 1,
Fig. 4 is a sectional view of part of the master record
of Fig. 3 when the record is not rotating, and
Fig. 5 illustrates an alternative method of fabricating
a master record, wherein a rigid protective means is employed
which is connected to stationary part of the recording machine.
In the figures like parts are designated with like
reference nurnerals.
Description of the Preferred Embodiment
The method according to the invention for writing inform-
ation with an information modulated write beam onto an inform-
ation storage medium can be most easily understood by reference
to Fig. 1 and Fig. 2. A video master record 1 is rotated at
a predetermined speed by a motor
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assembly 2 and is positioned on a turntable 3 with the aid of
a locking member 4. The term "master record" is herein con-
fined to that of describing the record which is initially
exposed to the write beam and which may later by treated to
produce a record which can be used to make reproductions.
The master record 1 comprises a base 5 which is a
plastic or glass disc and has a radiation sensitive layer 6 ~
consisting of a metal film evaporated onto a face of the ~ -
disc. A write beam transparent protective means, a plastic
foil 7, is provided over the surface of the information
layer 6.
An objective 8 is connected to an arm 9 belonging to
a frame which is not shown in the drawing. Optical elements,
such as a mirror 10 are present for directing a radiation
write beam to the optical elements in the objective 8. The
beam 11 comes from a suitable laser source, such as an argon
laser, and video information is modulated on it in a suitable
way. The beam is focussed through the protective foil 7 on
the information layer 6 by means of any known optical focus
sensing device (not shown) and a voice coil (not shown) con-
nected thereto. The arm 9 is moved radially while the master
record 1 is rotated so that a spiralling information track
consisting of small pits in the metal of the information
layer is written in the manner known from the prior art.
The protective foil 7 is connected to the central por-
tion of the disc 5 by adhesive means, and prior to
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recording, clings to the recording surface by electrostatic
force. During rotation a gas filled space 12 exists between
the foil 7 and the information layer 6 in which gas, preferably
highly purified, may flow from gas admission openings 13
radially outwards and expelled through an annular gap 14
between the outer edges of the foil 7 and the disc 5. The
turntable 3 is provided with an annular groove 15, correspond-
ing to the gas admission openings 13, and gas is fed into
the groove via a passage 16 in the turntable 3. These
measures provide a number of advantages, especially in the
method here described wherein the pits in the metal inform-
ation layer are formed by evaporation of metal. The vapor
will be transported by the gas stream and will not be able
to reach the last optical element of the objective 8.
The disc 5 is provided with a central part 17 which is
raised a little, for instance 0.25 mm, above the information
layer 6, the width of the space 12 is determined by this
dimension. The foil 7 may be of any thin, optically homo-
geneous foil which can have relatively large flatness and
thickness tolerances, such as a suitable plastic foil of
0.25 mm, i.e. polyvinyl chloride foil.
In the middle of the raised part 17 a center hole 18
is provided, for cooperation with the locking member 4.
The protective foil also has a center opening 19. The foil
and the disc are connected adhesively to each other exclusively
near these center openings over an area roughly identical to
the raised part 17.
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The foil and the air filled space form a good protection
of the information layer against dust, finger prints and
scratches. The number of degrees of freedom in manufacture
of the disc and the information layer are significantly in-
creased. As illustrated in Fig. 4, electrostatic charges
will tend to hold the foil in contact with the information
layer when the record is not rotating.
In order to seal the information layer effectively
against dust particles, the gas supply to the gas admission
openings 13 may be stopped while the turntable 3 still
rotates. The vacuum which will be created in the space 12
will pull the foil against the information layer.
A video record such as shown in Figures 3 and 4, not
being a master record or a "draw" record but being a mass
produced reproduction for the consumer video record market,
could have some noticeable advantages over prior art video
records which are known from published Netherlands patent
application 7211999 and which comprise a light transparent
base, a metallic light reflecting information layer on one
of the faces of the base and a protective layer applied over
the information layer, which protective layer may be light
absorbing. Read-out of the information is effected by
focussing a laser read beam through the transparent base ;
onto the information layer and subsequent processing of the
reflected, modulated beam, which travels through the base
and back into the video record player.
Only a limited number of available materials
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are suited for the base of this known video record. The
base has to be light transparent and optically homogeneous,
should not include any entrapped air bubbles or dust part-
lcles near the beam focussing plane, that is the information
layer, and also lend itself well to mass production methods.
Furthermore, the record should not be too expensive. Some
of the plastics which have been used up till now, such as
polyvinyl chloride, have relatively little dimensional
stability, especially at temperatures above normal room
temperature and may not stand up too well against frequent
manual handling.
A video record according to the invention may have
a base, the material of which may be chosen from a much
greater number of available materials while, if a new mat-
erial would have to be developed for it the number of degrees
of freedom in development and manufacture is increased.
While the invention has been described in conjunction
with a specific embodiment thereof, it is evident that many
alterations, modifications and variations will be apparent
to those skilled in the art in the light of the above dis-
closure. Accordingly, it is intended to embrace such alter-
ations, modifications and variations which fall within the
spirit and scope of this invention as defined by the appended
claims. Fig. 5 shows one such modification, in which the
protective means is a thin glass strip 20, connected to a
stationary frame 21. The objective 8 is thus protected
against metal vapor during writing of information on record 22.
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