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Claims  |
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What is claimed is:
1. An apparatus for reading a selected hologram of a plurality of holograms
formed on a sheet, each hologram having an information portion and an
identification portion, the apparatus comprising:
reconstruction beam means for producing a reconstruction beam to form a
reconstructed light image of one of the holograms;
sensor means for reading the light image and thereby the hologram;
first scan means for producing relative scanning movement between the
reconstruction beam means, sheet and sensor means so that the holograms
are sequentially moved to a predetermined position in the reconstruction
beam and read by the sensor means;
second scan means for producing relative scanning movement between the
reconstruction beam means, sheet and sensor means for reading the
information portion of a hologram in the predetermined position; and
logical control means for controlling the sensor means, first scan means
and second scan means in such a manner that the sensor means reads the
identification portion of the hologram in the predetermined position and
the logical control means determines whether the hologram in the
predetermined position is the selected hologram, controls the first scan
means to continue scanning if the hologram in the predetermined position
is not the selected hologram and controls the second scan means to read
the information portion of the hologram in the predetermined position if
the hologram in the predetermined position is the selected hologram;
the identification portion of each hologram comprising a density reference
section, the sensor means being operative to read a light image of the
density reference section of the hologram in the predetermined position
and adjust a magnitude of a bias reference signal in accordance therewith;
each density reference section comprising a pattern of lines, the sensor
means comprising a sensor for reading the density reference section,
spacings between adjacent lines being such that spacings between light
images of said adjacent lines are smaller than a minimum dimension of a
light receiving area of the sensor;
the sensor means being constructed to combine electrical signals produced
by reading the information area of the selected hologram with the bias
reference signal and produce output signals in accordance therewith;
the sensor means comprising comparator means receiving as inputs the
electrical signals produced by reading the information area of the
hologram and the bias reference signal, a quantization threshold of the
comparator means corresponding to the bias reference signal.
2. An apparatus as in claim 1, in which the density reference section
comprises two patterns of intersecting parallel lines.
3. An apparatus as in claim 1, in which the sensor means comprises first
and second sensors for reading the information portion and density
reference section respectively, the first and second sensors having
substantially identical electrical characteristics.
4. An apparatus as in claim 1, in which the density reference section
comprises a random pattern of lines having a predetermined average
spacing.
5. An apparatus for reading a selected hologram of a plurality of holograms
formed on a sheet, each hologram having an information portion and an
identification portion, the apparatus comprising:
reconstruction beam means for producing a reconstruction beam to form a
reconstructed light image of one of the holograms;
sensor means for reading the light image and thereby the hologram;
first scan means for producing relative scanning movement between the
reconstruction beam means, sheet and sensor means so that the holograms
are sequentially moved to a predetermined position in the reconstruction
beam and read by the sensor means;
second scan means for producing relative scanning movement between the
reconstruction beam means, sheet and sensor means for reading the
information portion of a hologram in the predetermined position; and
logical control means for controlling the sensor means, first scan means
and second scan means in such a manner that the sensor means reads the
identification portion of the hologram in the predetermined position and
the logical control means determines whether the hologram in the
predetermined position is the selected hologram, controls the first scan
means to continue scanning if the hologram in the predetermined position
is not the selected hologram and controls the second scan means to read
the information portion of the hologram in the predetermined position if
the hologram in the predetermined position is the selected hologram;
the sensor means comprising an array of sensing elements arranged in a row,
the second scan means producing relative scanning movement between the
reconstruction beam means, sheet and array perpendicular to the row of
sensing elements, the sensor means further comprising means for
sequentially enabling the sensing elements;
the identification portion of each hologram comprising a plurality of
marks, thicknesses of reconstructed light images of the marks being
greater than a spacing between adjacent sensing elements;
the sensing elements being constructed to, when enabled, produce logically
high and low electrical signals in accordance with incident light
intensity, the logical control means comprising bistable means responsive
to the electrical signals and constructed to change state from logical low
to high in response to a logically high signal preceded by a logically low
signal and to change state from high to low in response to a logically low
signal preceded by a logically high signal.
6. An apparatus for reading a selected hologram of a plurality of holograms
formed on a sheet, each hologram having an information portion and an
identification portion, the apparatus comprising;
reconstruction beam means for producing a reconstruction beam to form a
reconstructed light image of one of the holograms;
sensor means for reading the light image and thereby the hologram;
first scan means for producing relative scanning movement between the
reconstruction beam means, sheet and sensor means so that the holograms
are sequentially moved to a predetermined position in the reconstruction
beam and read by the sensor means;
second scan means for producing relative scanning movement between the
reconstruction beam means, sheet and sensor means for reading the
information portion of a hologram in the predetermined position; and
logical means for controlling the sensor means, first scan means and second
scan means in such a manner that the sensor means reads the identification
portion of the hologram in the predetermined position and the logical
control means determines whether the hologram in the predetermined
position is the selected hologram, controls the first scan means to
continue scanning if the hologram in the predetermined position is not the
selected hologram and controls the second scan means to read the
information portion of the hologram in the predetermined position if the
hologram in the predetermined position is the selected hologram;
the identification portion of each hologram being formed in two sections on
opposite sides of the respective information portion, the sensor means
being operative to read the two sections in opposite directions
respectively.
7. An apparatus for reading a selected hologram of a plurality of holograms
formed on a sheet, each hologram having an information portion and an
identification portion, the apparatus comprising:
reconstruction beam means for producing a reconstruction beam to form a
reconstructed light image of one of the holograms;
sensor means for reading the light image and thereby the hologram;
first scan means for producing relative scanning movement between the
reconstruction beam means, sheet and sensor means so that the holograms
are sequentially moved to a predetermined position in the reconstruction
beam means and read by the sensor means;
second scan means for producing relative scanning movement between the
reconstruction beam means, sheet and sensor means for reading the
information portion of a hologram in the predetermined position; and
logical control means for controlling the sensor means, first scan means
and second scan means in such a manner that the sensor means reads the
identification portion of the hologram in the predetermined position and
the logical control means determines whether the hologram in the
predetermined position is the selected hologram, controls the first scan
means to continue scanning if the hologram in the predetermined position
is not the selected hologram and controls the second scan means to read
the information portion of the hologram in the predetermined position if
the hologram in the predetermined position is the selected hologram;
the identification portion of each hologram comprising a density reference
section, the sensor means being operative to read a light image of the
density reference section of the hologram in the predetermined position
and adjust a magnitude of a bias reference signal in accordance therewith;
each density reference section comprising a pattern of lines, the sensor
means comprising a sensor for reading the density reference section,
spacings between adjacent lines being such that spacings between light
images of said adjacent lines are smaller than a minimum dimension of a
light receiving area of the sensor;
each density reference section having a length substantially equal to a
length of the information portion of the respective hologram. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to an optoelectronic reading apparatus
especially suited to Fourier or Fraunhaufer transform holography.
Fourier transform holography provides an advantageous means for recording a
large amount of information in a small space. Generally, a large number of
closely spaced holograms are formed on a sheet or plate and selectively
reconstructed through illumination with a coherent reconstruction beam
from a laser. The holograms may represent digital or analog information in
two dimensional form. The analog information may be, for example,
pictures, printed pages or the like.
A scan means is provided to move the sheet relative to the reconstruction
beam and a sensor so that the selected or desired hologram is read.
However, a major problem has limited the packing density of such holograms
heretofore. The problem is that due to the close spacing of the holograms
the scan means must operate with such precision as to render the reading
apparatus economically unfeasible. More specifically, the sheet must be
positioned with microinch precision to ensure that the selected hologram
is in the reading position.
Another major problem has existed heretofore which has limited the
resolution of Fourier transform hologram readers. Due to variations in the
output of the laser which produces the reconstruction beam, the
reconstruction efficiency of various holograms, temperature dependence of
dark current in optoelectronic sensors and other effects, it has been
excessively difficult to obtain high and stable resolving power of fine
patterns. In digital applications, the quantization threshold varies in
accordance with the parameters described above. If the quantization
threshold is too high fine patterns will appear as unresolved dark areas.
If the quantization threshold is too low the fine patterns will appear as
unresolved light areas. This effect is compounded by the fact that the
average image intensity of fine patterns is lower than that of coarse
patterns due to the optical transfer function. In analog applications
these variations cause incorrect bias and resulting loss of resolution.
In digital applications, it has been proposed to overcome this problem in
high signal-to-noise ratio processing to integrate the output of a sensor
which scans a predetermined area of the hologram having a pattern
consisting of a predetermined number of bits of logically high or low
information. This integrated value is compared with a predetermined value
and the quantization threshold adjusted until equality is obtained.
Although this method has practical application in digital systems, it
cannot be used in analog applications.
SUMMARY OF THE INVENTION
The present invention overcomes the above problems of the prior art by
providing each hologram with an identification portion in addition to the
digital or analog information portion comprising an address keyword
identifying the respective hologram. A sensor reads the address keyword
and controls the scan means to search the sheet in a predetermined scan
pattern until the selected hologram is located as evidenced by the address
keyword.
The identification portion further comprises a density reference section
which is read by a sensor to produce a bias reference signal. This signal
is used to determine the quantization threshold in digital applications
and the bias level in analog applications. The electrical characteristics
of the sensor which reads the density reference section are the same as
those of the sensor which reads the information portion.
It is an object of the present invention to provide an optoelectronic
reading apparatus especially suited to Fourier or Fraunhaufer transform
holography which overcomes the deficiencies of the prior art.
It is another object of the present invention to increase the packing
density of holograms and provide means for positively locating a selected
hologram.
It is another object of the present invention to increase the resolution of
hologram reading.
It is another object of the present invention to stabilize the resolving
power of hologram reading.
It is another object of the present invention to provide a generally
improved holographic reading apparatus.
Other objects, together with the foregoing, are attained in the embodiments
described in the following description and illustrated in the accompanying
drawing.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a partial block diagram of a holographic reading apparatus
embodying the present invention;
FIG. 2 is a diagram of a sheet or plate on which a plurality of holograms
are formed;
FIG. 3 is a diagram showing the information and identification portions of
each hologram;
FIG. 4 is a diagram showing the process of forming Fourier transform
holograms;
FIGS. 5a to 5d are graphs illustrating electronic information conversion;
FIG. 6 is a block diagram of important portions of the apparatus;
FIG. 7 is a diagram illustrating an alternative arrangement of information
and identification portions of each hologram;
FIG. 8 is a diagram illustrating the reading of information and density
reference areas of a hologram;
FIGS. 9a to 9c are graphs illustrating electronic conversion of digital
holographic information or data;
FIG. 10 is a diagram illustrating the arrangement of address keyword and
density reference sections of each identification portion;
FIGS. 11 and 12 are diagrams illustrating alternative patterns of the
density reference sections;
FIG. 13 is a diagram illustrating an alternative arrangement of the density
reference section; and
FIG. 14 is an electrical schematic diagram of a portion of the apparatus
utilized to generate and process a bias reference signal.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
While the holographic reading apparatus of the invention is susceptible of
numerous physical embodiments, depending upon the environment and
requirements of use, substantial numbers of the herein shown and described
embodiments have been made, tested and used, and all have performed in an
eminently satisfactory manner.
Referring now to FIG. 1 of the drawing, a holographic reading apparatus
embodying the present invention is partially shown and designated as 11.
The apparatus 11 comprises an inverse Fourier transform or reconstruction
lens 12 which is disposed between a hologram plate or sheet 13 and an
optoelectronic sensor 14. A reconstruction beam 17 from a laser 16
illuminates the sheet 13.
The method of forming the sheet 13 is illustrated in FIG. 4. A transparency
19 is placed between a random phase or diffuser plate 18 and a Fourier
transform lens 21. A coherent beam 22 from a laser (not shown) illuminates
the transparency 19 through the random phase plate 18. The lens 21
focusses a light image of the transparency 19 onto the sheet 13 which is
here designated as 13'. A coherent reference beam 23 from a laser (not
shown) is radiated onto the sheet 13' at an angle.
The transparency 19 may contain digital data in the form of alternating
transparent and opaque areas. The transparency 19 may also represent
analog data such as pictures or printed pages. The reference beam 23
interferes with the light image from the lens 21 to form a hologram on the
sheet 13' in the form of an optical interference pattern of fringes. The
hologram features high redundancy and is not effected by damage such as
abrasion of localized areas. The sheet 13' may be photographic, in which
case the sheet 13' is developed after exposure. The sheet 13' may also be
thermoplastic and the hologram formed by means of heating and cooling. The
transparency 19 and sheet 13' are provided in the object and image focal
planes respectively of the lens 21.
Although FIG. 4 illustrates the formation of only one hologram on the sheet
13' actually it is advantageous to form a number of holograms thereon in a
closely spaced pattern. This is illustrated in FIG. 2 in which the sheet
13 is designated unprimed since it is completely formed. The holograms are
designated as 24 (only one hologram 24 is designated for simplicity of
illustration) and are arranged in a rectangular pattern. A large number of
holograms 24 are formed on the sheet 13 by forming one hologram 24 and
then replacing the transparency 19 with a different transparency and
shifting the sheet 13' in its own plane to bring a new hologram position
to the exposure position in a rectangular pattern. The sheet 13' is then
exposed with a light image of the new transparency to form another
hologram. The process is repeated until all of the holograms 24 are
formed.
FIG. 3 shows the arrangement of each hologram 24 as comprising an
information portion 24a and an identification portion 24b. The information
portion 24a constitutes the digital or analog data of the hologram 24. The
identification portion 24b comprises an address keyword which is shown as
being constituted by a number of marks arranged in a code which
distinguishes the holograms 24 from each other. The transparency 19
preferably comprises both the information and identification portions so
that each hologram 24 is exposed in an integral manner.
Referring back to FIG. 1 it will be seen that the hologram reconstruction
process is the reverse of the exposure process. The sheet 13 and sensor 14
are arranged in the object and image planes respectively of the lens 12.
The sensor 14 is advantageously provided in the form of an array of
optoelectronic sensing elements arranged in a row such as photodiodes,
although not illustrated in detail. The width of the sensor 14 is
typically equal to the width of the optical image of the hologram 24
formed by the lens 12, this image being symbolically indicated by an arrow
26. The sensing elements are enabled or strobed in sequence so as to
effect horizontal scan of the hologram 24. More specifically, when enabled
the sensing elements produce electrical signals corresponding to the
intensity of the portion of the light image incident thereon. These
signals are fed to a signal processing unit 27 which processes the signals
in a manner which will be described in detail below. A power source 30 is
shown for the signal processing unit 27. The output of the signal
processing unit 27 is fed to a video processing unit 28 which applies the
signals to a display unit 29 such as a cathode ray tube to display the
holographic information.
The output of the signal processing unit 27 is also applied in modified
form to an address processing unit 31 which has an output connected to a
scan control unit 32. The output of the scan control unit 32 is applied to
a scan drive unit 33. The purpose of the scan drive unit 33 is to move the
sensor 14 in a direction perpendicular to the row of sensing elements in a
synchronized manner to effect vertical scan of the hologram 24.
The output of the scan control unit 32 is also applied to another scan
drive unit 34 which is used to position the sheet 13 relative to the
reconstruction beam 17, lens 12 and sensor 14. Whereas the scan drive unit
33 is used to scan an individual hologram 24, the scan drive unit 34
positions the sheet 13 in a plate perpendicular to the optical axis of the
lens 12 for selecting the particular hologram 24 which is desired to be
reconstructed. The scan drive unit 34 is constructed to scan the sheet 13
in the same rectangular pattern which was used to form the holograms 24.
In operation, the address keyword of the selected hologram 24 is entered
into the address processing unit 31 through a keyboard or the like (not
shown). The scan control unit 32 controls the scan drive unit 33 to move
the sensor 14 to a position to read the identification portion 24b of the
hologram 24. The address keyword is typically one dimensional, as is the
sensor 14, although the information portion 24a of the hologram 24 is two
dimensional. The scan control unit 32 also controls the scan drive unit 34
to move sheet 13 to a position at which the selected hologram 24 is in a
predetermined reading position so as to be illuminated by the
reconstruction beam 17 and be read by the sensor 14.
The signal processing unit 27 then controls the sensor 14 to read the
address keyword of the hologram 24 in the reading position, and the
address processing unit 31 compares the address keyword with the address
keyword of the selected hologram to see if they are the same. If the
address keywords match, indicating that the hologram 24 in the reading
position is indeed the selected hologram, the address processing unit 31
feeds a signal to the scan control unit 32 indicating the same. The scan
control unit 32 then controls the scan drive unit 34 to hold the sheet 13
in position and controls the scan drive unit 33 to move the sensor 14
vertically to scan the image 26 and thereby read the selected hologram 24.
A signal is applied to an enable input of the video processing unit 28
causing the output signals from the sensor 14 to be gated to the display
unit 29.
As mentioned hereinabove, the holograms 24 are formed in a very closely
packed arrangement on the sheet 13 and are provided in a very large
number. Thus, due to the limited accuracy of a reasonably priced
mechanical scan drive unit 34 it is quite probable that the scan drive
unit 34 will move the wrong hologram 24 into the reading position. More
specifically, the hologram 24 in the reading position will be spaced from
the selected hologram by a maximum distance which can be calculated once
the accuracy of the scan drive unit 34 is determined. In such a case, the
address processing unit 31 will determine through comparison of the
address keywords that the wrong hologram 24 is being read. In this case,
the scan control unit 32 will control the scan drive unit 34 to execute a
scan or search pattern to locate the selected hologram 24. The search
pattern will be rectangular and will center about the initially
incorrectly selected hologram 24. The search pattern will cover the
maximum distance the selected hologram may be spaced from the incorrectly
selected hologram 24 in two dimensions with, preferably, a safety factor
added to the maximum distance. The search pattern is executed in
increments equal to the spacing between adjacent holograms 24. As each new
hologram 24 is moved into the reading position, the address keyword is
read to determine if the selected hologram is being read. It the selected
hologram 24 is found, the search will be terminated and the hologram 24
read. The search will be continued under control of the scan control unit
32 until the selected hologram 24 has been located, at which time the
search or scan will be terminated and the selected hologram 24 read.
It will be understood that the present invention allows perfect selection
of holograms utilizing a relatively inexpensive mechanism regardless of
the packing density of the holograms. This allows increased packing
density and greater information storage efficiency.
Although the sensor 14 is illustrated as being moved relative to the lens
12 for scanning the selected hologram 24, the sensor 14 may be held
stationary and the lens 12 moved by a scan drive unit 33' which is
indicated in broken line. It will be further understood that rather than
moving the sheet 13, the scan drive unit 34 may be adapted to shift the
reconstruction beam 17 relative to the sheet 13 in a parallel or
non-parallel manner. As yet another alternative, a translating or rotating
mirror or prism may be provided between the reconstruction beam 17 and
sheet 13 to adjustably deflect the reconstruction beam 17.
To ensure reliable reading of the address keyword, the individual marks are
preferably made large enough that the images thereof are larger than the
distance between adjacent sensing elements of the sensor 14. As a typical
example, the marks may be as wide as the distance between five sensing
elements. This is illustrated in FIGS. 5a to 5d. FIG. 5a illustrates the
intensity I of the address keyword as a function of displacement d along
the hologram 24. FIG. 5b illustrates the voltage V produced by the outputs
of the corresponding sensing elements of the sensor 14 as a function of
time t. It will be seen that the bright marks of the address keyword
produce electrical signals in the form of five positive spikes each. It is
extremely inconvenient to attempt to read the address keyword in this
form.
To overcome this problem, the present invention provides the address
processing unit 31 with a clock generator 41 which produces clock pulses
which are illustrated in FIG. 5c. A frequency divider 42 divides these
clock pulses by a factor of two and produces output pulses .phi.1 and
.phi.2 which have half the frequency of the clock pulses and are
180.degree. out of phase relative to each other. These output signals are
applied to shift inputs of a shift register 43 which is part of the sensor
14. A start pulse is also applied to the shift register 43 which is
ultimately shifted out of the shift register 43 as an end pulse. The
parallel outputs of the shift register 43 are applied to the individual
sensing elements (not shown) of the sensor array of the sensor 14 which is
here designated as 44. The outputs of the sensing elements are connected
in a bus arrangement which is not shown in detail so that only the sensing
element which is enabled produces an output. The paralleled outputs of the
sensor array 44 are fed through an amplifier 46 to the D input of a D-type
flip-flop 47. The clock pulses from the clock pulse generator 41 are
applied to the clock input CK of the flip-flop 47. The Q output of the
flip-flop 47 produces signals illustrated in FIG. 5d corresponding to the
marks shown in FIG. 5a as will be described in detail below.
Scanning is initiated by feeding the start pulse into the shift register
43. With the logically high start pulse in the first stage of the shift
register 43, the high first stage output of the shift register 43 is
applied to the corresponding first sensing element of the sensor array 44
thereby enabling the first sensing element which produces an output signal
which is applied to the D input of the flip-flop 47. More specifically,
the .phi.1 pulse is used to cause the shift register 43 to shift and the
.phi.2 pulse is used to enable the parallel outputs. This provides
positive action of the circuitry. The next .phi.1 pulse causes the shift
register 43 to shift and the start signal to be shifted to the next stage.
The next .phi.2 pulse enables the parallel outputs of the shift register
43 thereby applying the high second stage signal to the second sensing
element of the sensor array 44 thereby enabling the same. The second
sensing element thus produces an output signal. In this manner, the
sensing elements of the sensor array 44 are enabled in sequence.
In response to a high output signal from the array 44, if the flip-flop 47,
which is a bistable device, was in the low state, the flip-flop 47 will be
changed over to the high state. The flip-flop 47 produces logically high
and low output signals at the Q output when the states thereof are high
and low respectively. More specifically, in response to a clock pulse at
the CK input, the flip-flop 47 assumes the logical state indicated by the
signal at the D input. Thus, when the D input is high and the state of the
flip-flop 47 is low, the next clock pulse will cause the flip-flop 47 to
change over to the high state. However, if the flip-flop 47 was already in
the high state, no change will occur. In response to a low output signal
from the sensor array 44 applied to the D input, the opposite effect
occurs. If the flip-flop 47 were already in the low state no change will
occur. However, if the flip-flop 47 was in the high state it will change
to low.
Comparing FIGS. 5b and 5b, it will be seen that the flip-flop 47 is changed
to high by the first high output signal from the sensor array 44 and
remains high until the sensor array 44 produces a low signal. Then, the
flip-flop 47 is changed over to low and remains low until receipt of
another high signal. In this manner, the output of the flip-flop 47
corresponds to the intensity of the marks of the address keyword and
greatly simplifies reading of the address keyword. The output of the
flip-flop 47 is applied to other portions of the address processing unit
31 which are not the subject matter of the present invention and are not
shown in detail. A gate 48 feeds the output of the amplifier 46 to the
video processing unit 28 in response to the enable signal applied to a
gate terminal thereof. An indicator 40 indicates the status of the address
processing unit 31.
Although the sensor array 14 has been described as being in the form of a
linear array, it may be in the form of a two-dimensional array which
greatly increases the scan speed. For low cost applications, however, the
sensor 14 may be in the form of a signal sensing element such as a
photodiode. In this case, the sensing element is mechanically moved in two
dimensions for scanning. As illustrated in FIG. 7, the speed of scanning
the identification portion may be doubled where horizontal mechanical scan
is utilized. A hologram 51 comprises an information portion 51a as
described above. However, the identification portion comprises two address
keywords 51b and 51c provided on opposite sides of the information area
51a. Although the address keywords 51b and 51c are identical, they are
written backwards relative to each other. Thus, a one dimensional sensor
will produce the same output signals reading the address keyword 51b from
left to right or reading the address keyword 51c from right to left. This
eliminates the time required for the sensor to return to an initial
position after a scan.
FIGS. 9a to 9c illustrate another problem which has existed heretofore in
the prior art but is overcome by the present invention. FIG. 9a shows the
intensity of various sinusoidal interference fringe patterns as a function
of displacement along the hologram. It will be seen that the intensity of
a coarse pattern is greater than the intensity of a fine pattern. This is
due to the optical transfer function and is common to other optical
devices besides holographic apparatus. FIG. 9b illustrates the output
signals of a sensor scanning the pattern of FIG. 9a. The output signals
are added to a D. C. component which corresponds to the dark current of a
typical photodiode sensor. If a quantization threshold is taken as the
average value of the A. C. component, digital quantization of the signals
of FIG. 9b will produce the signals illustrated in FIG. 9c.
Due to the effects described hereinabove, the intensity of the signals as
well as the dark current will vary with time. In the case of the dark
current, the major variation is caused by temperature. Variation of either
or both of the A. C. and D. C. components will shift the peak value of the
waveform relative to the threshold. Whereas the effect may be tolerated
with coarse patterns (large amplitude), fine patterns may be completely
lost. If the entire waveform is below the quantization threshold, the
pattern will appear as an unresolved dark area. If the entire waveform is
above the threshold, the pattern will appear as an unresolved light area.
Although coarse patterns will not be completely lost due to their large
amplitude (they will be distorted), fine patterns may be completely lost.
This results in instability of the resolving power of the holographic
apparatus and limits the reliable resolving power thereof to an
unnecessary extent.
FIG. 8 shows how this problem is overcome in accordance with the present
invention, in which like elements are designated by the same reference
numerals. Here, the hologram sheet is designated as 61 and another sensor
62 is provided to read a density reference sention 63c of a hologram 63
formed on the sheet 61 (see FIG. 10). The identification portion of the
hologram 63 comprises, in addition to the density reference section 63c,
an address keyword 63b. The information portion 63a of the hologram 63 is
the same as described above.
During the time in which the sensor 14 is reading the address keyword 63b,
the sensor 62 is reading the density reference section 63c which typically
consists of a pattern of intersecting parallel lines. The spacing between
adjacent lines is such that the spacing between the images of the lines is
smaller than the smallest dimension of the light receiving area of the
sensor 62. Preferably, it is much smaller. The spacing between the lines,
or the corresponding spatial frequency of the pattern, is selected to
correspond to the mean spatial frequency of the type of pattern formed on
the hologram 63. The pattern of the density reference section 63b is shown
in enlarged form in FIG. 11. As shown in FIG. 12, the pattern may comprise
only one set of parallel lines as designated at 63c'.
The output signals of the sensors 14 and 62 are applied to inputs of
amplifiers 64 and 66 as illustrated in FIG. 14. The outputs of the
amplifiers 64 and 66 are applied to inverting and non-inverting inputs of
an operational amplifier connected as a comparator 67. The output of the
comparator 67 is grounded through a resistor 68 and a zener diode 69.
The sensors 14 and 62 are selected to have identical electrical
characteristics. Therefore, the output of the sensor 62 corresponds to the
intensity of the density reference section 63c added to the dark current
of the sensor 62. The dark current of the sensor 14 is identical to that
of the sensor 62 and the sensor 14 is influenced by intensity variations
in the same manner as the sensor 62. Thus, the output of the sensor 62
will be the average value of the A. C. component of the output of the
sensor 14 under all conditions. The output of the sensor 62 is therefore
used to set the quantization threshold of the comparator 67 since it is
applied to the non-inverting input thereof.
Where the amplitude of the output signal of the sensor 14 is higher than
the amplitude of the output signal of the sensor 62, the comparator 67
will produce a positive output. The resistor 68 and zener diode 69 act as
a series regulator to limit the output of the comparator 67 to a suitable
constant value. When the output signal of the sensor 14 is below the
amplitude of the output signal of the sensor 62, the comparator 67 will
produce a low output. This action produces the signals shown in FIG. 9c
from the signals shown in FIG. 9b in a precise manner.
It will be understood that the automatic biasing action of the sensor 62
prevents loss of fine patterns since it maintains the quantization
threshold at precisely the average A. C. value of the output signals from
the sensor 14. Although the pattern of the density reference portion 63c
has been described as being sinusoidal, it may be made rectangular for
ease of manufacture without significant loss of effectiveless. It will
thus be seen that the output signal of the sensor 62 constitutes a bias
reference signal. While the comparator 67 is utilized in digital
applications, it may be replaced by a differential amplifier in analog
applications, in which case the bias reference signal establishes the bias
point.
The pattern of the density reference section 63c may be modified in a
number of ways. For example, it may be a random pattern having a
predetermined average spatial frequency. It may furthermore have an
average spatial frequency corresponding to maximum or minimum spatial
frequencies of the information portion 63a. It may furthermore be a blank
area, with a filter provided (not shown) to reduce the image intensity.
It is further possible to utilize the sensor 14 for reading the density
reference section 63c if a memory means (not shown) is provided. In this
case, the sensor 14 would be used to read the density reference signal
which would be stored in an analog memory and used as the bias reference
signal. The sensor 14 would then be used to read the address keyword. This
system produces extremely small errors since the variations which effect
the reading occur over relatively long periods of time.
FIG. 13 shows a modification of the density reference section as being
elongated to the length of the information portion, the corresponding
elements being designated by the same reference numerals double primed.
This modification allows the sensor 62 to produce the bias reference
signal continuously to compensate for variations in intensity in the
vertical dimension during scanning of the information portion 63c".
In summary, it will be seen that the present invention provides greatly
increased information storage density, resolving power and reliability
although comprising low cost mechanism. Various modifications will become
possible for those skilled in the art after receiving the teachings of the
present disclosure.
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