Information recording medium and method for recording and reproducing thereof

Disclosed herein is an information recording medium to be illuminated with an optical radiation beam, comprising a substrate having a surface provided with a preformat comprising a servo track and forming a sectional shape, in transverse to the track, which comprises a concavity provided between two convexities, each having a flat top, and a light reflecting layer being formed by application of a coating liquid over the surface of the substrate, wherein the concavity has the sectional shape of an open trapezoid having a rectangular part, the trapezoidal part has a pair of parallel opposite sides and a pair of sloping sides constituting walls of the trapezoidal part, one of the parallel sides constituting the bottom of the trapezoidal part being shorter than the opposite side, and the rectangular part is adjacent to the side constituting the bottom of the trapezoidal part.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an information recording medium on which 
information is stored and/or reproduced. 
2. Related Background Art 
In recent years, in information recording media having high recording 
density, vigorous research and development of optical recording media, 
such as optical discs, optical cards, etc. have been made. Examples of 
these optical recording media include media of the reproduction-only type 
such as compact discs (CD) and media capable of additionally writing 
information and/or erasing information, such as CD-R and magnetooptical 
discs. 
In the case of a reproduction-only type optical recording media, the 
quality of information is affected by the structure of information formed 
in a substrate. For example, in the case of an optical recording medium in 
which information is recorded in the form of concavities and convexities, 
and the information is reproduced by using their phase differences, 
information signals to be detected are affected by the three-dimensional 
forms of the concavities and convexities. The optical recording media 
capable of additionally recording and/or erasing also has tracking grooves 
for recording, erasing and reproduction of information, and preformed pits 
which correspond to format information, such as addresses for data 
management and synchronizing signals in a substrate (hereinafter the 
information performed in the substrate, including tracking grooves and 
preformed pits, will be referred to as "preformat"). 
As methods for forming a recording layer of such an optical recording 
medium, there have been known a method making use of a vapor phase 
deposition process, such as vacuum deposition or sputtering, and a method 
of applying a coating liquid (hereinafter referred to as "wet coating 
method"). In particular, the method of forming a recording layer by a wet 
coating method is a method attracting attention because it makes it 
possible to form the recording layer at low cost. In this case where a 
recording layer is provided by a wet coating method on a substrate in 
which a preformat such as tracking grooves and preformed pits has been 
formed, however, a coating liquid collects in the concavities, and so the 
resulting coating film becomes uneven in thickness. In the case where a 
recording layer containing a coloring matter has been provided on a 
substrate by a wet coating method, the surface of the substrate and the 
concavities in the substrate differ from each other in the thickness of 
the recording layer, and therefore also differ in reflectance in some 
cases. In the case where a recording layer is provided on a substrate 
having a conventional preformat which has been provided on the premise 
that a recording layer is formed so as to be able to obtain the same 
reflectance at the concavity and at the convex, sufficient contrast cannot 
be obtained. 
In order to solve such a problem, the present applicant has disclosed, in 
Japanese Patent Application Laid-Open Nos. 1-23434 and 2-257444, 
information recording media by which excellent contrast can be provided 
even when a recording layer is formed by the wet coating method. In 
particular, the present applicant has disclosed, in the latter 
application, an information recording medium which achieves high contrast 
by using the characteristics of the recording layer formed by the wet 
coating method, specifically that a recording layer is formed thicker at a 
concavity of the preformat than at a convexity adjacent thereto as 
illustrated in FIG. 5A, and improving the sectional shape of the concave 
preformat. However, this information recording medium tends to cause 
scattering of contrast in reproducing signals of information stored 
between the central portion and peripheral portion of its recording 
region. 
In particular, when the preformat of the peripheral portion of the 
recording region is reproduced, a reproducing signal having a waveform 
with a W-shaped pattern as illustrated in FIGS. 5A and 5B may be obtained. 
Such a pattern makes the contrast worse. Although the reason why such a 
signal pattern is observed is not clearly understood, it is believed to be 
attributed to the fact that a sufficient amount of material for forming a 
light reflecting layer is not filled in the concavity of the preformat 
under the circumstances that the recording layer is formed thinner at the 
peripheral portion of the recording region than at the central portion of 
the recording region, and so the thickness of the recording layer at the 
bottom of the preformat concavity becomes thinner than a thickness for 
which the light reflecting layer will show a minimum reflectance. 
Although such a problem can be solved to some extent, for example, by 
strictly controlling drying conditions, scattering of contrast in 
reproduction signals may be caused by a delicate variation in the 
conditions. In addition, such strict control of drying conditions also 
increases the cost of such an information recording medium. 
SUMMARY OF THE INVENTION 
In view of the above prior art, an object of the present invention is to 
provide an information recording medium which can provide consistent 
signals that are excellent in contrast over its overall recording region, 
and a method for recording and reproducing thereof. 
According to an aspect of the present invention, there is provided an 
information recording medium to be illuminated with an optical radiation 
beam, comprising: a substrate having a surface provided with a preformat 
comprising a servo track and forming a sectional shape, transverse to the 
track, which comprises a concavity provided between two convexities, each 
having a flat top, and a light reflecting layering being formed by 
application of a coating liquid over the surface of the substrate, wherein 
the concavity has a sectional shape having an open trapezoid with a 
rectangular part, the trapezoidal part having a pair of parallel opposite 
sides and a pair of sloping sides constituting walls of the trapezoidal 
part, one of the parallel sides constituting the bottom of the trapezoidal 
part being shorter than the opposite side, the rectangular part being 
adjacent to the side constituting the bottom of the trapezoidal part. 
According to another aspect of the present invention, there is provided an 
information recording-reproducing method, comprising illuminating an 
information recording medium with an optical radiation beam having a 
wavelength .lambda. to effect recording or reproduction of information; 
the information recording medium comprising: a substrate having a surface 
provided with a preformat comprising a track and forming a sectional 
shape, transverse to the track, which comprises a concavity provided 
between two convexities, each having a flat top, and a light reflecting 
layer being formed by application of a coating liquid over the surface of 
the substrate, wherein the concavity has a sectional shape having an open 
trapezoid with a rectangular part, the trapezoidal part having a pair of 
parallel opposite sides and a pair of sloping sides constituting walls of 
the trapezoidal part, one of the parallel sides constituting the bottom of 
the trapezoidal part being shorter than the opposite side, the rectangular 
part being adjacent to the side constituting the bottom of the trapezoidal 
part. 
According to still another aspect of the present invention, thee is 
provided an information recording medium to be illuminated with an optical 
radiation beam, comprising: a substrate having a surface provided with 
tracking grooves, the tracking grooves being separated from each other by 
a recording track which has a flat surface, and a light reflecting layer 
formed by application of a coating liquid over the surface of the 
substrate, wherein each tracking grooves has a sectional shape, transverse 
to the tracking groove direction, having an open trapezoid with a 
rectangular part, the trapezoidal part having a pair of parallel opposite 
sides and a pair of sloping sides constituting walls of a groove, one of 
the parallel sides constituting the bottom of the trapezoidal part being 
shorter than the opposite side, the rectangular part being adjacent to the 
side constituting the bottom of the trapezoidal part. 
The present inventor has carried out an investigation as to the 
above-described problems involved in the prior ar. As a result, it has 
been found that when the sectional shape of a concave preform is that of 
an open trapezoid having a rectangular part, and the trapezoidal part is a 
reversed trapezoid such that the upper side thereof is longer than the 
lower side thereof, and the rectangular part adjoins the lower side of the 
trapezoidal part, a high quality information recording medium, in which 
variation in contrast is small even when drying conditions fluctuate, can 
be provided. Although the reason why this sectional shape provides the 
above-described effect is not clearly understood, it is believed to be 
attributed to the fact that a sufficient width, in the transverse 
direction to the recording track, of a light reflecting layer having a 
thickness showing a minimum reflectance in the concavity of the preformat, 
the formation which has heretofore been dependent upon the coating liquid 
for forming the light reflecting layer, drying conditions and the like, is 
stably and surely provided by the provision of this rectangular part. 
These and other objects, features and advantages of the present invention 
will become more apparent upon a consideration of the following 
description of a number of embodiments of the present invention, which 
will be described by way of example only with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 and 2 are typical cross-sectional views, in the transverse 
direction to a track, of an optical recording medium according to an 
embodiment of the present invention. Referring to FIG. 1, reference 
numerals 1 and 2 indicate an optical recording medium and a transparent 
substrate, respectively. In the surface of the substrate 2, a preformat 
comprising concavities, such as tracing tracks 7, etc. is formed. A light 
reflecting layer 3 is formed by a wet coating method on the surface in 
which the preformat has been formed, and a protective substrate 5 is 
laminated on the light reflecting layer 3, and an adhesive layer 4 is 
provided therebetween. Referring to FIG. 2, the concave tracking track 7 
of the optical recording medium 1 has a sectional shape with an open 
trapezoid having a rectangular or square part (herein after referred to as 
"rectangular part" simply) in a transverse direction to the track. The 
trapezoidal part is the so-called reversed trapezoid in which the length 
(b) of the upper side of a pair of parallel sides for the trapezoidal part 
is longer than the length (a) of the lower side thereof, and the 
rectangular part adjoins the lower side of the trapezoidal part. The 
formation of the concave preformat in such a sectional shape prevents the 
generation of a W-shaped signal waveform. As a result, signals having high 
contrast can be obtained, and unevenness of contrast in reproduction 
signals between the central portion and the peripheral portion of a 
recording region of the optical recording medium can also be restricted. 
In the above embodiment, the shape of the concavity of the preformat is 
defined by an angle (.THETA.) formed between a sloping side of the 
trapezoidal part and a horizontal plane, a depth (d) of the concavity, 
i.e., a depth (d1) of the trapezoidal part and a depth (d2) of the 
rectangular part, and a width (b) of the concavity and a width (c) of the 
rectangular part. The contrast of signals for reproducing the preformat is 
controlled by these factors. More specifically, the light reflecting layer 
formed on the sloping sides diffracts an optical radiation beam for 
reproduction when the optical radiation beam for reproduction crosses the 
concavity of the preformat. Therefore, the angle (.THETA.) defines the 
degree of diffraction. Besides, the depth (d) of the preformat concavity 
is the sum of the depth (d1) of the trapezoidal part and the depth (d2) of 
the rectangular part. The depth (d1) controls the film thickness 
distribution of the light reflecting layer formed at a portion extending 
from the flat top of a convexity adjacent to the concavity of the 
preformat to the concavity of the preformat, and in particular, a part of 
the sloping side, while the depth (d2) controls the film thickness of the 
light reflecting layer at the bottom of the preformat concavity. Further, 
the depth (d) of the concavity controls a phase difference given to 
reflected light of a beam for reproducing the preformat. Furthermore, a 
ratio (c/b) of the width (c) of the rectangular part to the width (b) of 
the concavity defines a proportion of a region in which the light 
reflecting layer shows a minimum reflectance in the concavity of the 
preformat. This ratio also affects the contrast of signals for reproducing 
the preformat. Therefore, it is preferred that the values of the angle 
(.THETA.), depth (d) and widths (a, b, c) of the concavity be controlled 
such a change in the reflectance, which is brought about as a result of 
factors such as the difference in reflectance caused by a difference in 
film thickness between the concave portion and the convex portion of the 
light reflecting layer, the phase difference caused by a difference in 
level between the concavity and the convexity, and the diffraction of 
light at the sloping side of the concavity, alone or in combination, is as 
great as possible, and such that the change in the reflectance can be 
stably caused. 
For example, it is preferable from the viewpoint of providing a 
high-quality optical recording medium for the widths (a, b) of the 
concavity of the preformat and the width (c) of the rectangular part to 
satisfy the following conditions. That is, for example, it is preferred 
that the width (b) of the concavity of the preformat and a spot diameter 
(.PHI.) of an optical recording and reproduction generally satisfies the 
following inequality: 
EQU 0.5&lt;(b/.PHI.)&lt;1.5. 
For example, when the spot diameter (.PHI.) of the optical radiation beam 
on the recording medium is 3.+-.0.3 .mu.m, a preferable width (b) of the 
concavity is in the range of from 1.5 to 4.5 .mu.m, particularly, from 1.8 
to 3.0 .mu.m. In the case of .PHI.=1 to 2 .mu.m, a preferable width (b) of 
the concavity is in the range of from 0.5 to 3.0 .mu.m, and, in 
particular, from 0.5 to 1.0 .mu.m. The width (c) of the rectangular part 
of the preformat concavity defines a proportion of a region in which the 
reflectance is at a minimum in the concavity. This proportion greatly 
affects the contrast of signals for reproducing the preformat. It also 
determines whether the reflectance in this region gives a W-shaped signal 
or not when detecting a signal in the transverse direction to a track A 
ratio (c/b) of the width (c) to the width (b) is preferably greater than 0 
but not greater than 1, more preferably in the range of from 0.14 to 0.96, 
and most preferably in the range of from 0.2 to 0.9, because the contrast 
can be made even and improved by such a rectangular part. 
The angle (.THETA.) formed between a sloping side of the trapezoidal part 
and a horizontal plane of the substrate is preferably greater than 0 but 
not larger than 60 degrees, more preferably in the range of from 10 to 40 
degrees, and most preferably in the range of from 15 to 35 degrees. When 
the angle (.THETA.) is controlled within this range, a film having a 
thickness in which the light reflecting layer shows a minimum reflectance 
when scanning the preformat with the optical radiation beam can be stably 
formed in the rectangular part because the optical recording material is 
reliably filled therein. In addition, the shape of the light reflecting 
layer formed on the sloping sides of the trapezoidal part can also be 
adjusted in such a form that the reflectance of the optical radiation beam 
from the preformat can be steeply lowered. As a result, high contrast can 
be stably ensured, and an optical radiation beam can be introduced to a 
track to be accessed more easily. 
The optimum depth of the concavity varies according to the kind of the 
material used for the light reflecting layer, and the viscosity and 
concentration of the coating liquid used for forming the light reflecting 
layer, and is not defined unqualifiedly. However, when an optical 
radiation beam for recording or reproducing is incident on the light 
reflecting layer through the substrate, it is preferable to make the depth 
greater than at least .lambda./4n, wherein .lambda. denotes a wavelength 
of the optical radiation beam for recording or reproducing, and n denotes 
a refractive index of the substrate. When the concavity is made to have 
such a depth, signals for reproducing the preformat show excellent 
contrast. The reason for this is not clearly understood. However, it is 
believed to be attributed to the fact that the information of a difference 
in reflectance between the convex portion and the concave portion of the 
light reflecting layer and a phase difference caused by this setting of 
the depth act synergistically. 
The depth (d2) of the rectangular part of the preformat concavity varies 
according to the angle (.THETA.) of the sloping side of the trapezoidal 
part. For example, in the case where the angle (.THETA.) is in the range 
of from 15 to 60 degrees, more preferably from 15 to 40 degrees, and most 
preferably from 15 to 35 degrees, the contrast can be improved by 
presetting the depth (d2) of a range of from 5 to 80 nm, from 8 to 60 nm, 
and from 8 to 40 nm, respectively. 
Specific examples according to this embodiment now will be described. 
For example, a polymethine dye represented by the following formula 1! has 
a refractive index (n) of 2.1 and an extinction coefficient (k) of 1.0, 
and the dependence of refractive index on film thickness is as illustrated 
in FIG. 3. 
##STR1## 
When this polymethine dye is used in a material for a light reflecting 
layer, and an optical radiation beam having .lambda. of 830 nm and a 
bisphenol A type polycarbonate substrate is employed, it is preferred that 
the thickness (t2) of a light reflecting layer at convexities of a 
preformat is a thickness showing a maximum reflectance, specifically, 
about 90.+-.10 nm. It is also preferred that the thickness (t1) of a light 
reflecting layer at a concavity is such a thickness that a difference in 
reflectance from the preformat convexity is as great as possible, i.e., a 
thickness showing substantially a minimum reflectance. In the case where 
the thickness (t1) is 180 to 200 nm or greater, excellent contrast is 
expected. If the width (b) of the concavity of the preformat is 3 .mu.m, 
the widths (a) and (c) of the concavity are 1.13 .mu.m, the depth (d) of 
the concavity is greater than .lambda./4n, i.e., 200 to 300 nm, further 
preferably 230 to 270 nm, and most preferably 250 to 270 nm, and 
preferably where d1=250 nm and d2=10 nm, and the angle (.THETA.) is 15 
degrees, the thickness (t1) of the light reflecting layer at the concavity 
can be adjusted to the above range when the viscosity of a coating liquid 
for forming the light reflecting layer is 8 cP. Thus, an even and 
excellent contrast, for example, a contrast of about 0.4 to 0.5 actually 
can be obtained. 
In FIGS. 1 and 2, an embodiment in which the width (a) of the lower side of 
the trapezoidal part is equal to the width (c) of the rectangular part has 
been illustrated. However, the present invention is not limited to such an 
embodiment, and, for example, an embodiment as illustrated in FIG. 4 may 
be used. 
In the above embodiment, the sectional shape of the preformat in a 
direction transverse to the track has been described. However, for 
example, when prepits are provided in a recording track, it is preferred 
that the prepits also have a sectional shape of an open trapezoid having a 
rectangular part in the direction along the track. This trapezoidal part 
is the so-called reversed trapezoid that the length (b) of the upper side 
of a pair of parallel sides making up the trapezoidal part is longer than 
the length (a) of the lower side thereof, and the rectangular part adjoins 
the lower side of the trapezoidal part. 
As a process for producing a substrate for such an information recording 
medium, there may be used a process as described in U.S. Pat. No. 
5,234,633, in which metal films different in etching rate are laminated on 
each other, these metal films are etched to form a mold having a convexity 
corresponding to the concavity having the trapezoidal part and the 
rectangular part, and this mold is used to produce the substrate for an 
information recording medium as described above. 
Examples of a method for molding this substrate including injection 
molding, compression molding, cast molding and a 2P method using a 
photocurable resin. For example, if an optical radiation beam is 
transmitted through the substrate, a material for the substrate is 
preferably transparent to the optical radiation beam. Examples of such a 
material include glass, ceramics, acrylic resins, polystyrene and 
polycarbonate resins. 
A light reflecting layer 3 is then formed by the wet coating method on a 
substrate in which a preformat having the above-described shape has been 
formed. As illustrated in FIG. 1, the light reflecting layer 3 is 
continuously formed from the top surfaces of the convexities 6 to the 
bottom of the concavity though the sloping sides of the concavity. In the 
above-described embodiments, the light reflecting layer can be 
continuously formed on the surfaces of the substrates each having the 
preformat by application of a coating liquid. 
This light reflecting layer is required to have a predetermined reflectance 
in order to reliably read out the information of the preformat formed in 
the substrate. The value is determined in relation to its reproducing 
ability. In order to effect high-precision reproduction without being 
affected by dirt and flaws on the surface of the substrate, however, it 
requires a reflectance of 12% or higher for at least a recording track 
portion thereof. 
A material for such a light reflecting layer varies according to the type 
of the information recording medium according to the represent invention, 
i.e., whether the recording medium is of: 
(1) a ROM type in which an information preformed such as a preformat in a 
substrate is only read out, or 
(2) the additionally recordable type in which new information can be 
additionally recorded in the medium by use of tracking tracks, address 
pits and the like preformed as a preformat in a substrate. In each case, 
the light reflecting layer formed on the concave and convex preformat by 
the application of the coating liquid may preferably change its 
reflectance according to the thickness of the layer. 
In the case of the former ROM type, a dispersion of fine particles of a 
metal in a binder or a heat-resistant dye or pigment is used. In the case 
of the latter type, a material having both properties of absorption giving 
recording sensitivity and reflection determining the contrast of 
reproduction signals against an optical radiation beam for recording and 
reproduction is preferred. Dyes, pigments, and the like conventionally 
known as optical recording materials, for example, cyanine, squalium, 
phthalocyanine, tetradehydrocholine, polymethine, and naphthoquinone dyes 
and pigments and organometallic complexes such as benzenedithiol nickel, 
may preferably be used. 
In the case of the latter type, a light reflecting layer may also be 
provided on only the preformat part by the coating method, and another 
recording medium, for example, a deposited film of a metal, may be 
provided on the residual additional writing part. 
An organic solvent usable on the application of the light reflecting layer 
varies according to whether a coloring matter is dispersed or dissolved in 
the solvent. However, examples thereof include alcohols such as methanol, 
ethanol, isopropanol and diacetone alcohol, ketones such as acetone, 
methyl ethyl ketone and cyclohexanone, and other solvent such as amides, 
ethers, esters, halogenated aliphatic hydrocarbons, aromatics and 
aliphatic hydrocarbons. The alcohol solvents are particularly preferred 
because they do not attack the substrate upon the application of the light 
reflecting layer. 
When the light reflecting layer is provided by a wet coating method on a 
substrate provided with a concave preformat, a solution or dispersion of a 
material for making the light reflecting layer is coated by a method such 
as roll coating, Meyer bar coating, air-knife coating, calendar coating, 
dip coating or spraying. For the coating liquid used for the light 
reflecting layer in this case, its solubility is determined by the 
coloring matter and solvent used. Therefore, the solid concentration and 
viscosity of the coating liquid, which are required to provide a thickness 
giving a maximum reflectance to the light reflecting layer, are determined 
on the basis of this solubility. 
For example, when a coloring matter such as represented by the above 
structural formula 1! or a structural formula 2! which will be described 
subsequently and a diacetone alcohol as a solvent are used, the 
concentration of the coloring matter is preferably 1 to 5% by weight, and 
more preferably 2 to 4% by weight, and the viscosity is preferably 2 to 20 
cP, and more preferably 2 to 8 cP. 
When the shape of the concavity of the preformat is adjusted to such a form 
as has been described in this embodiment, signals that are even and 
excellent in contrast for reproducing the preformat can be provided 
without strictly controlling the drying of the light reflecting layer 
formed by a wet coating method. The conditions for drying may be 
controlled. For example, as conditions for drying the coating film, it is 
preferred that the process in which the coating film be leveled while the 
solvent of the coating liquid is being evaporated is controlled to 
uniformly form the coating film on the concavity of the preformat. More 
specifically, for example, clean air of room temperature is moderately 
blown onto the coated surface to dry the coating film. In this case, the 
flow rate of the clean air is preferably in the range of from 1 to 5 
m/min, and most preferably from 2.5 to 3.5 m/min, and the drying time is 
preferably in the range of from 10 seconds to 2 minutes, and more 
preferably from 20 to 40 seconds. 
The transparent substrate on which the light reflecting layer has been 
formed in the above-described manner is laminated with a protective 
member, for example, through an adhesive. 
In the present invention, as the adhesive layer 4, any conventional 
adhesive may be used, for example, a polymer or copolymer of a vinyl 
monomer such as vinyl acetate, acrylic ester, vinyl chloride, ethylene, 
acrylic acid or acrylamide, a thermoplastic adhesive such as polyamide, 
polyester or polyether, a thermosetting adhesive such as an amino resin 
(urea resin, melamine resin), phenolic resin, epoxy resin, urethane resin 
or thermosetting vinyl resin, or a rubber adhesive such as natural rubber, 
nitrile rubber, chloroprene rubber, or silicone rubber. A hot-melt 
adhesive is particularly preferred from the viewpoint of mass or series 
production because it is a dry process. 
The protective substrate 5 serves to mechanically protect the recording 
layer 3, and a plastic, metal, ceramic or glass sheet or plate, paper, or 
a composite material thereof may be used. As for the protective substrate 
itself, any material may be used irrespective of its transparent or opaque 
properties, so long as it can satisfy the above goals. 
Such a material is rather determined by the system for reading optical 
information. In the case of a transmission type reading system, it must be 
transparent, and the requirement for birefringence is also the same as 
that for the substrate. Therefore, the materials are naturally limited. 
In the case of a reflection type reading system, the protective substrate 
may be opaque, and so its material can be selected from materials of a 
wide range. This protective substrate may be optically closely laminated 
directly on the optical recording layer 3. Besides, the so-called air-gap 
structure in which the protective substrate is provided so as to be an air 
layer between the light reflecting layer and the protective substrate may 
be used as needed. 
According to the embodiments as described above, the following excellent 
effects can be obtained. 
(1) Even when the light reflecting layer is formed by means of a wet 
coating method, the pattern (or waveform) of the quantity of reflected 
light scarcely becomes a W shape upon reproduction of the preformat in the 
resulting optical recording medium, and so the optical recording medium 
can be provided as a recording medium which can provide signals having 
high contrast. 
(2) When the light reflecting layer is formed by a wet coating method, 
scattering of contrast in reproduction signals of the preformat between 
the central portion and peripheral portion of its recording region can be 
reduced. 
(3) Since there is no need for strictly controlling the physical properties 
of the coating liquid for forming the light reflecting layer and the 
conditions for drying the coating film, a high-quality optical recording 
medium can be provided at low cost. 
The embodiments of the present invention will hereinafter be described in 
more detail by the following examples. However, the present invention is 
not limited to or by these examples. 
In each of these examples, the sectional shape of a preformat and the 
thickness of a recording layer were measured and calculated from a 
photograph obtained by cutting the optical recording medium produced in 
the transverse direction to its track and taking a shot of its section by 
using a scanning electron microscope (trade name: S-570 Model; 
manufactured by Hitachi Ltd.; 3,000 magnifications). The measurement of 
reproduction signals of the preformat was conducted by means of an optical 
card recording and reproducing apparatus (manufactured by Canon Inc.). An 
optical system for reproduction in this apparatus uses a semiconductor 
laser of 830 nm in wavelength, in which an optical radiation beam is 
stopped to a spot of 3 .mu.m in diameter at the plane of incidence of the 
medium. 
EXAMPLE 1 
A polymethyl methacrylate substrate (n=1.49) of 10 cm long.times.10 cm 
wide.times.0.4 mm thick was used as a transparent substrate, and a 
preformat was formed in the surface of the substrate by hot pressing, 
thereby producing a substrate as illustrated in FIG. 8. A preformat 
comprising tracking grooves formed in a width of 3 .mu.m at a pitch of 12 
.mu.m and prepits formed in convex portions adjacent to the tracking 
grooves was formed. The sectional shape of each of the tracking grooves in 
the direction transverse to the track was as illustrated in FIG. 2. Its 
respective dimensions were as follows: b=3 .mu.m, a=c=2.84 .mu.m, depth 
(d1)=140 nm, depth (d2)=40 nm and angle (.THETA.)=60.degree.. The 
sectional shapes of each of the prepits in the direction transverse to and 
in a direction along the track were as illustrated in FIGS. 9A and 9B, 
respectively. Respective dimensions were as follows: .THETA.2=55.degree., 
k=2.5 .mu.m, l=2.4 .mu.m, m=2.3 .mu.m, n=140 nm and o=40 nm. In FIG. 9B, 
respective dimensions were as follows: .THETA.3=50.degree., p=3 .mu.m, 
q=2.8 .mu.m, r=2.76 .mu.m, s=140 nm and t=40 nm. 
A mold imparting these tracking grooves was produced in accordance with the 
method described in Japanese Patent Application Laid-Open No. 5-114173. 
A solution (3% by weight) of a polymethine dye represented by the 
structural formula 1! in diacetone alcohol was applied to the surface of 
this substrate by a roll coater, and diacetone alcohol was then vaporized 
to form a light reflecting layer 3. Drying was effected under conditions 
that clean air of 23.degree. C. was blown onto the light reflecting 
layer-formed surface of the substrate for 20 seconds at a flow rate of 3 
m/min. 
A polymethyl methacrylate protective substrate of 10 cm long.times.10 cm 
wide.times.0.35 mm thick was laminated on the light reflecting layer 
through a hot-melt adhesive sheet containing an ethylene-vinyl acetate 
copolymer to fabricate an optical card of a work size. A piece having a 
size of 85.6 mm in length (y) and 54 mm in width (x) was then punched out 
of the work-sized card, thereby obtaining an optical card. 
This optical card was put in the optical card recording and reproducing 
apparatus to measure the contrast of signals in the direction transverse 
to the track, and moreover to observe the waveforms of these signals. The 
measurement of the signals in the direction transverse to the track was 
conducted at 12 positions in the central portion (602) and 36 positions in 
the peripheral portion (603) of a recording region illustrated in FIG. 6 
to calculate a proportion of a standard deviation to the average value of 
the resultant data, thereby evaluating the evenness in the formation 
condition of the light reflecting layer. The respective dimensions in FIG. 
6 were as follows: e=34 mm, f=4.5 mm, g=3 mm, h=15 mm, i=50 mm and j=15 
mm. The results of the measurements are shown in Table 1. 
A new optical card was produced in exactly the same manner as described 
above. This card was used in measuring the thickness of the recording 
layer at the central portion of the recording region. As a result, a 
thickness (t2) at the convex part of the substrate was 100 nm, in which 
the polymethine dye shows a maximum reflectance, while a thickness (t1) at 
the concave part was 180 nm, in which the polymethine dye shows 
substantially a minimum reflectance. 
EXAMPLES 2 TO 9 
Optical cards were produced in the same manner as in Example 1 except that 
the sectional shape of each tracking groove in Example 1 was changed to 
achieve corresponding shapes shown in Table 1, and then evaluated. 
Comparative Examples 1 to 5 
Five optical cards were produced in the same manner as in Examples 2, 4, 5, 
7, and 9, respectively, except that no rectangular part was provided at 
their corresponding tracking grooves, and then evaluated. 
The results of the evaluation in Examples 1 to 9 and Comparative Examples 1 
to 5 are shown in Table 1. The evaluation standards in Table 1 are as 
follows: 
Evaluation standards 
A: A proportion (Y) of the standard deviation to the average value of the 
signals in the direction transverse to the track was not higher than 1.5%; 
B: Y is higher than 1.5% but not higher than 3.5%; 
C: Y is higher than 3.5% but not higher than 5.5%; 
D: Y is higher than 5.5%. 
TABLE 1 
__________________________________________________________________________ 
b c a .THETA. 
d1 d2 
t1 t2 
Example 
.mu.m 
.mu.m 
.mu.m 
deg. 
nm nm 
nm nm CONTRAST 
EVALUATION 
__________________________________________________________________________ 
Example 1 
3.0 
2.84 
2.84 
60 140 
40 
180 
100 
0.22-0.29 
C 
Example 2 
3.0 
2.71 
2.71 
60 250 
20 
190 
100 
0.24-0.31 
B 
Co-Ex. 1 
3.0 
-- 2.71 
60 250 
-- 
180 
100 
0.22-0.28 
D 
Example 3 
3.0 
2.67 
2.67 
60 140 
20 
180 
100 
0.23-0.29 
A 
Example 4 
3.0 
2.40 
2.40 
40 250 
10 
180 
100 
0.25-0.31 
A 
Co-Ex. 2 
3.0 
-- 2.40 
40 250 
-- 
180 
100 
0.22-0.29 
D 
Example 5 
3.0 
2.40 
2.40 
25 140 
20 
180 
100 
0.38-0.42 
A 
Co-Ex. 3 
3.0 
-- 2.40 
25 140 
-- 
100 
80 
0.25-0.31 
D 
Example 6 
3.0 
2.14 
2.14 
25 200 
10 
180 
100 
0.40-0.45 
A 
Example 7 
3.0 
1.90 
1.90 
25 250 
10 
180 
100 
0.43-0.47 
A 
Co-Ex. 4 
3.0 
-- 1.90 
25 250 
-- 
180 
100 
0.38-0.42 
D 
Example 8 
3.0 
1.95 
1.95 
15 140 
20 
180 
100 
0.43-0.46 
A 
Example 9 
3.0 
1.13 
1.13 
15 250 
10 
180 
100 
0.45-0.48 
A 
CO-Ex. 5 
3.0 
-- 1.13 
15 250 
-- 
180 
100 
0.43-0.45 
D 
Example 10 
3.0 
0.45 
1.13 
15 250 
10 
180 
100 
0.44-0.47 
A 
Example 11 
3.0 
0.21 
1.13 
15 250 
10 
180 
100 
0.44-0.46 
A 
Example 12 
3.0 
0.16 
0.16 
10 250 
10 
180 
100 
0.46-0.50 
C 
__________________________________________________________________________ 
EXAMPLE 13 
A coloring matter represented by the following structural formula 2! was 
used as a material for a light reflecting layer to produce an optical 
card. 
##STR2## 
The organic coloring matter represented by the structural formula 2! has a 
refractive index (n) of 3.0 and an extinction coefficient (k) of 0.8, and 
the relationship between thickness and refractive index when formed in a 
thin film is as illustrated in FIG. 7. That is, a maximum value, 26%, of 
reflectance is obtained when the thickness is about 60 to 70 nm, while a 
minimum value, 10%, of reflectance is obtained when the thickness is about 
130 to 140 nm. Further, a substantially constant reflectance of 14% is 
obtained when the thickness is 300 nm or greater. In this example, the 
sectional shape of each of the tracking grooves in the direction 
transverse to the track was as illustrated in FIG. 2. In FIG. 2, 
respective dimensions were as follows: b=3 .mu.m, a=c=2.84 .mu.m, depth 
(d1)--140 nm, depth (d2)=40 nm and angle (.THETA.)=60. 
A coating solution for forming the light reflecting layer used in this 
example was a solution obtained by dissolving 3% by weight of the above 
organic coloring matter in diacetone alcohol. After applying the coating 
liquid to the surface of the substrate, clean air of 23.degree. C. was 
blown onto the coated surface for 30 seconds at a flow rate of 3.5 m/min 
to dry the light reflecting layer. Other procedures were carried out in 
the same manner as in Example 1 to produce an optical card and evaluate 
it. The results are shown in Table 2. 
EXAMPLES 14 TO 20 
Six optical cards were produced in the same manner as in Example 13 except 
that the sectional shape of each tracking groove in Example 13 was changed 
to corresponding shapes shown in Table 2 to evaluate them. The results of 
the evaluations are shown in Table 2. 
Comparative Examples 6 to 9 
Four optical cards were produced in the same manner as in Example 13 except 
that the shape of the tracking groove in Example 13 was changed to 
corresponding shapes provided with no rectangular part, as shown in Table 
2, to evaluate them. The results of the evaluations are shown in Table 2. 
TABLE 2 
__________________________________________________________________________ 
b c a .THETA. 
d1 d2 
t1 t2 
Example 
.mu.m 
.mu.m 
.mu.m 
deg. 
nm nm 
nm nm CONTRAST 
EVALUATION 
__________________________________________________________________________ 
Example 13 
3.0 
2.84 
2.84 
60 140 
40 
130 
60 0.22-0.28 
B 
Example 14 
3.0 
2.67 
2.67 
40 140 
20 
130 
60 0.23-0.30 
A 
Example 15 
3.0 
2.40 
2.40 
25 140 
20 
130 
60 0.39-0.40 
A 
Example 16 
3.0 
2.14 
2.14 
25 200 
10 
130 
60 0.41-0.45 
A 
Co-Ex. 6 
3.0 
-- 2.14 
25 200 
-- 
120 
60 0.36-0.40 
D 
Example 17 
3.0 
1.71 
1.71 
25 300 
10 
140 
60 0.43-0.47 
A 
Co-Ex. 7 
3.0 
-- 1.71 
25 300 
-- 
130 
60 0.41-0.45 
D 
Example 18 
3.0 
1.95 
1.95 
15 140 
20 
130 
60 0.43-0.48 
A 
Example 19 
3.0 
0.76 
0.76 
15 300 
10 
140 
60 0.45-0.49 
A 
Co-Ex. 8 
3.0 
-- 0.76 
15 300 
-- 
130 
60 0.43-0.47 
D 
Co-Ex. 9 
3.0 
-- 2.28 
40 300 
-- 
130 
6D 0.21-0.27 
D 
Example 20 
3.0 
1.41 
1.41 
10 140 
20 
130 
60 0.45-0.50 
B 
Example 21 
3.0 
0.45 
0.76 
15 300 
10 
140 
60 0.44-0.48 
B 
__________________________________________________________________________ 
As is apparent from Tables 1 and 2, according to the disclosed embodiments 
of the present invention, thee is provided an optical card capable of 
obtaining signals in the direction transverse to the track, which are even 
and excellent in contrast, over the entire recording region. 
While the present invention has been described with respect to what is 
presently considered to be the preferred embodiments, it is to be 
understood that the invention is not limited to the disclosed embodiments. 
To the contrary, the invention is intended to cover various modifications 
and equivalent arrangements included within the spirit and scope of the 
appended claims. The scope of the following claims is to be accorded the 
broadest interpretation so as to encompass all such modifications and 
equivalent structures and functions.