Optical image defocus correction

In an optical/electrical imaging system, optical depth of focus corrections are accomplished electronically. A plurality of defocus states of the optical system are measured and/or determined and image restoration coefficients corresponding to each of these states are stored. The state of defocus of the imaging system is then determined by deriving an image from a target having known characteristics, which are then correlated to determine a state of defocus. A microprocessor is utilized to provide appropriate defocus correction information to an electronic filter, thereby providing an adaptive filtering arrangement.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to optical imaging systems in electronic 
reprographic equipment. More specifically, the invention relates to a 
system for correction of optical defocus and related optical system errors 
by correction of electrical signals representative of the optically 
produced image data. 
2. Description of Related Developments 
Electronic reprographic equipment and facsimile transmission equipment 
employ scanners for optically scanning an image, such as a document, and 
converting the optical information to an electrical image signal. Several 
types of optical systems are commonly employed to achieve the raster 
scanning of input documents and produce the representative electrical 
signal. FIG. 1 illustrates one of several configurations frequently used 
with reduction optical systems to scan a document and produce a reduced 
size image on a linear array of photosensors. This configuration is 
relatively tolerant of errors in the object conjugate of the lens; i.e., 
errors in the optical path length from the lens, through the mirrors, to 
the original document. Variations in this path length of .+-.1 
millimeters, while causing noticeable change in magnification, will 
typically have little degrading effect on the focus, or sharpness, of the 
optical image formed on the photosensor array. Since only one compound 
lens is used, however, there may exist a variation in image quality along 
the linear portion of the image sensed by the photosensor array. 
Variations in image quality from the end of such a scanned image line to 
the center of this line frequently occur with a reasonable degree of 
symmetry about the center of the scanned image. These variations in 
quality may, for example, be caused by the curvature of the surface of 
best focus of the lens, or by other related lens aberrations which are 
well known to vary with the distance from the center of the optical field. 
The mathematical description of these aberrations can be approximately 
determined from lens design data, but further aberrations occur due to 
small errors which result from the lens fabrication process. Other single 
lens optical systems, such as those frequently referred to as 
"full-rate/half-rate" scanners, have similar optical characteristics. As a 
result, scanners using single reduction lens optics may illustrate a 
decrease in image quality at the edges of the scan line, even when the 
center of scan is in good focus. 
A second class of optical systems frequently used for document scanning in 
electronic reprographic equipment is the "full-width" scanner type, shown 
in FIG. 2. Here, an array of lenses extends (into the page in the figure) 
the full extent of the line on the input document which is to be imaged at 
unity magnification onto the full-width photosensor array. Full-width 
scanners have been developed which utilize amorphous or crystalline 
silicon photosensor arrays that offer the advantages of high responsivity 
(which yields high scanning speeds), low illumination requirements (which 
reduces power consumption) and compactness. These scanners require 
compact, full width lens arrays to achieve these performance advantages. 
The most commonly used lenses for this purpose are gradient index fiber 
lens arrays, as illustrated in FIG. 3. While commercially available 
gradient index lens arrays provide good optical efficiency and excellent 
control of the unity magnification requirement, they have poor 
depth-of-field capabilities when compared with, for example, reduction 
optics designs; i.e., they are considerably more sensitive to errors in 
the object conjugate length than reduction optics designs. Typically, the 
depth-of-field for high efficiency gradient index lens arrays is 
approximately .+-.0.25 to .+-.0.50 millimeters. 
FIG. 4 illustrates the depth-of-field characteristics of a typical gradient 
index lens array. The graph of FIG. 4 plots the modulation transfer 
function (MTF) achievable by the lens array as a function of defocus 
distance at a predetermined spatial frequency; for example, FIG. 4 shows 
these characteristics for a commercially available lens at 6 cycles per 
mm. The MTF value correlates directly to the fidelity level of the image 
from the lens array. By selecting a desired level of MTF (and thus image 
fidelity) the curves show the defocus distances (or depth-of-field) of the 
lens array necessary to maintain the desired MTF. In FIG. 4, the zero on 
the abscissa represents the best focus position and the small divisions 
along the abscissa are tenths of a millimeter. Curve M represents the 
characteristics of the lens in the main scanning direction along a line 
coincident with the line of photosensors, and curve S represents the 
characteristics of the lens in the subscanning direction; i.e., 
perpendicular to the main scanning direction. From FIG. 4, it is evident 
that at high levels of image fidelity (MTF), small variations in defocus 
distance can cause unacceptable blurring and that the amount of blur 
varies with the scan direction (i.e., the lens becomes increasingly 
anamorphic in this loss of quality as the focus error increases). As a 
result, optics/sensor architectures employing such full-width lens arrays 
frequently do not provide sufficient image quality or resolution to meet 
image fidelity requirements, thereby limiting the use of such scanner 
designs. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide a system for improving image 
fidelity in optical/electrical imaging systems. 
It is an object of this invention to provide image defocus correction in 
systems using optics having a limited depth-of-field. 
It is a further object of this invention to provide correction of other 
predictable optical errors which may occur in varying amounts throughout 
the field of the imaging system. 
These objects are achieved in a system wherein the characteristics of the 
optical image data representative of a plurality of states of defocus or 
optical field position of the system are predetermined, and filter 
coefficients for the correction of these states are stored in the scanner 
system. The defocused or degraded state of a specific scanner system is 
then determined by use of one or more standard reference targets. 
Electrical image data from the photosensor array is then filtered with a 
fixed or time-varying image restoration filter which utilizes correction 
coefficients preselected to be appropriate for the state of system defocus 
or degradation previously determined.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 1 of the drawings, there is shown an image input 
terminal, designated generally by the numeral 10, having a platen 12. The 
exemplary image input terminal 10 is a multi-mode, in this case, dual 
mode, input terminal which in a first mode scans a document original 11 
resting face down on platen 12 line by line at a first scan station 13, 
and in a second mode scans a moving document at a second scan station 14. 
As will appear more fully herein, image input terminal 10 converts the 
document image being scanned to video image signals or pixels which are 
output to a suitable user (not shown) such as a memory, communication 
channel, raster output scanner, etc. 
Image input terminal 10 has a suitable frame or housing 18 with base member 
20, side members (not shown), end members 24, 25 and top member 26 which 
cooperate with platen 12 to provide an interior 27 within which a scan 
carriage 32 is movably disposed. Platen 12, which is made of a suitably 
transparent material, normally glass, is typically rectangular in shape 
with a length and width sized to accommodate the largest sized document to 
be scanned by input terminal 10 in the first mode plus the platen area 
necessary for the second mode. 
For the first mode, scan carriage 32 is supported for back and forth or 
reciprocating scanning movement (in the direction shown by the solid line 
arrow of FIG. 1) within the interior 27 of image input terminal 10 by a 
pair of parallel carriage support rods 34. Support rods 34 are suitably 
mounted on frame 18 in predetermined spaced relation below platen 12 with 
carriage 32 supported for slidable movement on rods 34 by suitable 
bearings (not shown). 
To impart controlled scanning movement to carriage 32, a drive screw 37 is 
threadedly engaged with carriage 32. A reversible drive motor 39 rotates 
screw 37 in either a clockwise or counter-clockwise direction to move the 
carriage 32 back and forth along carriage support rods 34. 
A linear scanning or image reading photosensor array 40, which may, for 
example, comprise a Toshiba Model TCD141C CCD chip, is mounted on carriage 
32. Array 40 has a series (i.e. 5,000) of individual photosensitive 
elements adapted to generate signals having a potential proportional to 
the reflectance of the object line viewed by the array 40. The signals 
output by array 40 are thereafter input to suitable signal processing 
circuitry (described below) to provide video image signals or pixels 
representative of the image scanned. 
An optical system consisting of imaging lens 55 and folding mirrors 56, 57, 
58 cooperate to form an optical imaging path 54 through which array 40 
views platen 12 and a line-like portion of the document being scanned, the 
light rays reflected from the document line passing downwardly through 
platen 12 to mirror 56 and from mirror 56 through mirrors 57, 58 to lens 
55 and array 40. To illuminate the document line being scanned, an 
illumination assembly 64 consisting of an elongated exposure lamp 65 and 
cooperating reflector 70 is provided on carriage 32 adjacent the underside 
of platen 12. Lamp 65 and reflector 70 extend in a direction generally 
perpendicular to the direction of scanning movement of scan carriage 32. 
As will be understood, reflector 70 serves to enhance and concentrate 
light emitted by lamp 65 onto platen 12 at the document line being scanned 
by array 40. 
In the first scan mode, scan carriage 32 is moved by motor 34 from a Start 
of Scan (SOS) position 72 at one end of platen 12 to an End of Scan (EOS) 
position 73 and back to SOS position 72. Array 40, imaging lens 55, 
folding mirrors 56, 57, 58 and illumination sensor 64 are fixedly attached 
to scan carriage 32 and move in unison with the carriage 32. EOS position 
73, which cooperates with SOS position 72 to delineate first scan station 
13, is slightly upstream of the platen end to leave room for a second scan 
station 14. As will be understood, the distance between SOS and EOS 
positions 72, 73, respectively is chosen to accommodate the largest size 
document image to be scanned at first scan station 13. 
In the second scan mode, scan carriage 32 is moved beyond EOS station 73 to 
a predetermined fixed scan position 74. During scanning in this mode, scan 
carriage 32 is stationary while the document being scanned is moved past 
the fixed scan position 74. 
To move the document 11 to be scanned past the fixed scan position 74, a 
Constant Velocity Transport (CVT) 80 is provided. CVT 80 has a plurality 
of spaced document transport rolls 82 disposed opposite scan position 74, 
rolls 82 cooperating with the surface 15 of platen 12 opposite thereto to 
form a document feeding nip 83 therebetween. 
Referring to FIG. 2 of the drawings, there is shown an image input 
terminal, designated generally by the number 90, that employs a full width 
scanning arrangement. Elements of image input terminal 90 having a 
structure and function similar to like elements of the image input 
terminal shown in FIG. 1 are similarly numbered. The image input terminal 
90 is also a multi-mode input terminal, which in a first mode scans a 
document original 11 resting face down on platen 12 line by line at a 
first scan station 13, and in a second mode scans a moving document at a 
second scan station 14. In the same fashion as imaging input terminal 10, 
imaging input terminal 90 converts the document image being scanned to 
video image signals or pixels which are output to a suitable user (not 
shown) such as a memory, communications channel, raster output scanner, 
etc. 
Image input terminal 90 has a suitable frame or housing 18 with base member 
20, end members 24, 25 and top member 26, which cooperate with the platen 
12 to provide an interior 27 within which a scan carriage 33 is movably 
disposed. A document 11 placed on platen 12 is scanned in the same manner 
as described with respect to the FIG. 1 terminal. In the first or 
reciprocating scan mode, the controlled scanning movement is imparted to 
carriage 33 by drive screw 37 which is threadedly engaged with carriage 
33. A reversible drive motor 39 rotates screw 37 in forward or reverse 
directions, thereby moving carriage 33 back and forth along carriage 
support rods 34. 
In this scanner, the carriage 33 has mounted therein a photosensor array 82 
that extends across the carriage 33 in a direction normal to the plane of 
the drawing. The width of the photosensor array 82 corresponds to the 
maximum width of the document to be imaged on platen 12. The illumination 
assembly 64 comprising the exposure lamp 65 and reflector 70 is mounted on 
carriage 33 by a suitable mounting arrangement (not shown). A gradient 
index lens array 85 is also mounted on carriage 33. The optical axis of 
the lens array 85 coincides with the optical axis between platen 12 and 
photosensor array 82, which in the FIG. 2 embodiment is substantially 
vertical. The gradient index lens array 85 extends transversely across the 
carriage 33 in a direction normal to the plane of the drawing. The width 
of the lens array 85 corresponds to the maximum width of the document to 
be scanned on platen 12 and thus generally corresponds to the width of 
photosensor array 82. 
Alternatively, an image input terminal can include a two dimensional area 
array of individual photosensors. In such an array, the photosensitive 
elements can extend along one direction of the imaged portion of the 
platen 12, for example the width as in the FIG. 1 and FIG. 2 scanners, and 
also along all or a portion of the length of the imaged portion of the 
platen 12. In this manner, all or a significant portion of the area of a 
document can be electronically scanned. 
Referring to FIG. 3, a typical scanning arrangement using the gradient 
index lens array 85 is schematically illustrated. Successive lines L.sub.0 
on the document 11 are scanned as the lens array 85 is moved in the 
direction of arrow S1, which comprises the subscanning direction. Line 
L.sub.0 is scanned in a transverse direction of arrow S2 (main scanning 
direction) electronically by successive scanning of the photosensor array 
82, which may, for example, comprise multiple CCD chips, each of which 
comprises a series of individual photosensitive elements adapted to 
generate signals having a potential proportional to the reflectance of the 
object line viewed by the array 82. As described previously, the signals 
output by array 82 are thereafter input to suitable signal processing 
circuitry (described below) to provide video image signals or pixels 
representative of the image scanned. 
The gradient index lens array 85 can comprise, for example, two aligned 
series of gradient index glass fiber lenses 88. An example of such a lens 
array 85 is the type SLA09 lens sold under the tradename SELFOC by Nippon 
Sheet Glass. Successive lines L.sub.0 are imaged through the lens array 85 
to form image line L.sub.i on photosensor array 82. As shown in FIG. 3, 
optical image information from a given point (pixel location) on line Lo 
may pass through one or more of the individual fiber lenses 88, to be 
imaged on the photosensor array 82. 
Referring to FIG. 5, the basic functions of a typical electronic 
reprographic scanner capable of embodying the invention are illustrated. 
Raster scanning of document 11 on platen 12 occurs as a result of the 
electronic scanning of the linear photosensor array 82, combined with the 
mechanical motion of the lens 85 and sensor 82 in the subscanning 
direction. At each position in the subscanning direction, the output of 
the linear photosensor array 82 is an electronic image signal 
representative of the reflectance information along the single line on the 
document which lens 85 has imaged onto the sensor array 82. This signal 
thus represents the information in a row of picture elements, or pixels, 
which is sampled in the electronic or main scanning direction. The 
sampling pitch in this direction on the document 11 is frequently 300 to 
600 samples per inch, as determined by the lens 85 magnification and the 
spacing of the individual sensors in the linear photosensor array 82. 
Mechanical motion of the sensor 82 and lens 85 in the subscanning 
direction permits a sequence of rows of pixels to be sensed, with the 
subscanning pitch or spacing determined by the distance the optical 
assembly has advanced from the beginning of one electronic line scan to 
the beginning of the next electronic line scan. This subscanning pitch, or 
separation between rows of pixels, is also typically in the range of 300 
to 600 rows per inch, although it is not necessarily identical to the 
pixel pitch within one row. 
Sensor control clocks 91 provide the necessary timing information to the 
photosensor array 82 to determine the rate at which pixels in a single row 
are clocked out into the analog signal buffer 92 and the time at which 
each line scan in the sequence of electronic line scans is to be 
initiated. The analog signal buffer 92 receives this sequence of pixels, 
one line at a time, and provides the necessary analog signal conditioning 
(such as signal gain and offset) to properly drive the analog-to-digital 
(A/D) converter stage 93. In the A/D converter 93, each analog pixel 
signal is converted into a digital value so that it may be subsequently 
stored and further processed by digital electronic methods. For example, 
analog pixel signals may be conditioned to vary from 0 volts (representing 
a point where the document reflectance was 0%) to a value of 1 volt 
(representing a point where a document reflectance of 95% was sensed). The 
A/D converter 93 will then convert this continuous analog voltage range 
into a discrete set of digital numbers. For example, if an 8-bit A/D 
converter is used, the signal is converted to one of the 2.sup.8 =256 
levels which can be represented by an 8-bit binary number. The signal, so 
represented, may be manipulated by conventional digital electronics. 
It is also common practice to include pixel correction capability in the 
signal processing, as shown in FIG. 5. A linear photosensor array 82 
frequently contains between 2000 and 9000 individual photosensor sites 
which sample the line of information on the document 11 in the electronic 
scan direction, as previously described. Because of small variations in 
size and other related parameters, each of these photosensor sites will 
vary slightly in its photoresponse and dark offset voltage. In high 
quality reprographic systems it is necessary to compensate for these 
variations to prevent image defects, such as streaks in the captured 
image. One technique for obtaining such a calibration utilizes a 
calibration strip 101 which is located on the platen 12 and outside the 
area to be occupied by the document 11 placed on the platen 12. A common 
position for calibration strip 101 is between the top of the platen 
surface 15 and the bottom surface of the document registration guide 100. 
The calibration strip 101 may consist of a uniform high-reflectance area 
approximately 6 to 10 mm wide and extending the full distance sensed by 
one line of the photosensor array 82, and an adjacent low-reflectance area 
of the same size, both are as facing into the scanner towards the lens 85. 
Under control of the system CPU 99, the lens 85 and photosensor array 82 
are positioned so that the linear photosensor array 82 views only the 
uniform, high-reflectance area of this strip 101. The multiplicative 
factors required to adjust each pixel to a constant output voltage are 
computed and stored in the pixel calibration RAM 96, one factor for each 
sensor site in the photosensor array 82. The lens 85 and photosensor array 
82 are next moved to a position which permits the photosensor array 82 to 
view the adjacent low-reflectance or black area of the calibration strip 
101. Here an additive factor is computed for each pixel which will cause 
all pixels to produce the same output signal in the presence of little or 
no light. The additive correction factor for each of the individual sensor 
sites in the photosensor array 82 is also stored in the pixel correction 
RAM 96. During subsequent document scanning, as the signal from each pixel 
passes through the pixel correction circuit 94, the multiplicative and 
additive correction factors for the individual sensor element which sensed 
that pixel are read from the pixel calibration RAM 96 and applied to the 
current pixel signal. In this manner each pixel is corrected for the small 
deviations in gain and offset which result from the variation in 
characteristics of the individual sensor elements in the photosensor array 
85. This calibration technique also compensates for variations in 
illumination along the photosensor array 82 caused by the lens 85 and lamp 
65 characteristics. 
Several scan lines of the corrected pixels are then stored in the pixel 
line buffer 95. This is required because subsequent image processing may 
utilize the signals from pixels which are adjacent to the currently 
processed pixel in both the main scanning and subscanning direction. 
Referring to FIG. 6, the pixel context 102 for image processing may be 
expressed in terms of the number of adjacent rows and the number of 
adjacent pixels in each row required to process the current pixel. For 
example, a 3.times.5 pixel context 102 implies that three adjacent scan 
lines (or pixel rows) with five pixels from each row are to be used for 
image processing. The position of the pixels may be labeled as shown in 
FIG. 6. The pixels are numbered with two indices. The first index 
represents the scan line position relative to the scan line of the current 
pixel. The second index represents the pixel position along the same scan 
line relative to the current pixel. If the pixel currently being processed 
is designated as p.sub.0,0, then the pixel in the scan line directly above 
p.sub.0,0 is p.sub.-1,0 ; directly to the right is p.sub.0,1, etc. When a 
new pixel is to be processed, the new pixel becomes p.sub.0,0 and the 
3.times.5 pixel context 102 shifts to the same relative position about the 
new pixel. In this example, it is clearly necessary to have pixel signal 
information available for 3 adjacent scan lines, and this information is 
stored in the pixel line buffer 95. 
These multiple lines of pixel signal information are next made available to 
the digital image processing circuit 97. In this circuit, each pixel in 
the selected pixel context 102 is multiplied by a preselected coefficient 
stored in the filter coefficient read only memory (ROM) 98 and the 
resulting products are summed to produce a new value for the current 
pixel. This process is repeated for every pixel, with the selected set of 
coefficients applied to the new pixel and the surrounding pixels in the 
new pixel's context. For example, in the 3.times.5 context 102 previously 
cited, a set of 15 coefficients would be stored in the filter coefficient 
ROM 98. If these are labeled as 
______________________________________ 
a.sub.-1,-2 
a.sub.-1,-1 
a.sub.-1,0 
a.sub.-1,1 
a.sub.-1,2 
a.sub.0,-2 a.sub.0,-1 
a.sub.0,0 
a.sub.0,1 
a.sub.0,2 
a.sub.1,-2 a.sub.1,-1 
a.sub.1,0 
a.sub.1,1 
a.sub.1,2 
______________________________________ 
to correspond with the associated pixel in the 3.times.5 context 102, the 
digital image processing circuit will convert the current pixel, 
p.sub.0,0, to a new value, p.sub.new, according to 
##EQU1## 
When the next pixel is to be processed, its new context may be similarly 
processed by this set of coefficients to generate a new value for this 
next pixel. Multiple coefficient sets may be maintained in the filter 
coefficient ROM 98, with each set preselected to process a specific type 
of image. One set may be preferable for continuous tone images, and 
another set for text images. The appropriate set of coefficients may be 
manually selected by the operator of the scanner, or may be automatically 
selected by a circuit (not shown) which recognizes a particular class of, 
images. The coefficient set which is applied to the current pixel may be 
changed very rapidly; i.e., it may be changed on a pixel-to-pixel basis, 
if there is a need to do so. It is understood that the System CPU 99 
generally coordinates the circuits and activities described above, 
operating from computer code stored in its associated read only memory 
(not shown). 
In order to improve the quality of the image data supplied by the 
photosensor array 82 in the presence of defocus errors introduced by lens 
85, a three-step image restoration process is applied. The three steps 
are: 
(1) Determine by measurement and/or computation the defocus imaging 
characteristics of the lens 85 at several levels of defocus throughout the 
expected range of focal errors, and compute appropriate image restoration 
filters for each level; 
(2) Enable the image input terminal to dynamically measure its present 
state of defocus by scanning appropriately located focus measurement 
targets, thus establishing the level of defocus correction required to 
compensate for component and assembly errors; and 
(3) Using digital image processing circuits, select the set of image 
restoration filter coefficients indicated by the present state of defocus 
and apply these coefficients to the image signal. 
This process will be described with reference to a full-width scanning 
system as shown in FIG. 2 which is assumed to have the functional 
capabilities of a scanner detailed in FIG. 5. The further applicability of 
this process to scanners of the single lens type as shown in FIG. 1 will 
be made apparent by subsequent descriptions. 
In a first step, the defocus characteristics of lens 85 are measured and/or 
computed at several steps or levels of defocus throughout an expected 
range of focal errors for the purpose of determining appropriate image 
restoration filters for each step. In order to do so, a point spread or 
line spread function of the selected lens design is determined as a 
function of defocus error, typically at several steps over a range of 0-4 
millimeters of defocus. The number of steps or levels within the expected 
range of defocus at which the point spread function is determined is 
dependent upon the type of lens 85, the anticipated assembly error for the 
scanner optical system, and the possible document position errors. 
Typically, the number of levels at which the point or line spread function 
would be determined is greater than 2 and less than 10. 
Because of the anamorphic nature of the lens array characteristics as 
previously described with reference to FIG. 4, it is preferable to 
characterize these lenses using the two-dimensional point spread function, 
thus capturing the image degrading characteristics for all directions on 
the document. Referring to FIG. 7, a typical point spread function is 
illustrated which was measured in green light for a Nippon Sheet Glass 
SLA09 lens array when a defocus error of 0.5 mm was introduced. This 
figure illustrates that a point of light on the document 11 is not imaged 
on the photosensor array 82 as a corresponding point, but has a defocused 
intensity distribution which blurs over an x-position range of 
approximately 0.15 mm and over a y-position of approximately 0.10 mm at 
the photosensor array 82. The multiple peaks in this point spread function 
are caused by the failure of the contributions from adjacent lenses in the 
array to converge to the same point in the defocused image plane. Point 
spread functions of the type shown in FIG. 7 are thus computed or measured 
and recorded at several defocus positions throughout the range of 
anticipated defocus errors. For example, they may be recorded for steps of 
0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5 and 4.0 mm of defocus, producing a set 
of eight descriptions of the defocus blur. If the scanner operates in a 
color separation mode, it may also be necessary to capture this set of 
eight point spread functions in each of the illuminant colors, if the lens 
behavior changes significantly with the spectral properties of the 
illuminant. In this manner, the chromatic aberrations of lens 85 may be 
taken into account in determining the defocus characteristics of the 
optical system. 
It is common practice, and useful in the design of digital filters, to 
describe each of the defocus point spread functions in a two-dimensional 
spatial frequency domain. This is achieved by computing the 
two-dimensional Fourier transform of the point spread function and 
normalizing the result at the zero-frequency origin to unity. This is a 
procedure well known in the fields of optical system analysis and digital 
filter design, and the resulting function is known as the two-dimensional 
modulation transfer function, or 2-D MTF. For example, FIG. 8 is a plot of 
the 2-D MTF so computed from the 0.5 mm defocus point spread function of 
FIG. 7. In this manner, each of the recorded point spread functions over 
the focal range is transformed to produce a corresponding set of 2-D 
MTF's. 
In order to correct the undesirable blurring of the signal it is necessary 
to design a digital image restoration filter for each of the recorded blur 
functions, using the information computed in the set of 2-D MTF's above. 
One approach to the design of a compensating filter is to use the inverse 
filter. If the 2-D MTF surface for a particular blurred image condition, 
such as the shown in FIG. 8, is represented by the symbol, H(f.sub.x, 
f.sub.y), then an inverse restoration filter may be computed from 
##EQU2## 
where H.sub.I (f.sub.x, f.sub.y) describes the frequency response of the 
filter in the two-dimensional spatial frequency domain, and f.sub.x and 
f.sub.y represent the frequency axis variables in the main and subscanning 
directions, respectively. Because the original blurred 2-D MTF, H(f.sub.x, 
f.sub.y) may take on very small values, approaching zero, the computed 
inverse restoration filter frequency response, H.sub.I (f.sub.x, f.sub.y) 
may take on very high values (relative to unity) at the higher spatial 
frequencies. This leads to the significant enhancement of any noise in the 
originally captured image during the remainder of the restoration process 
which will be subsequently described. An alternate formulation for 
computing a desired restoration filter for the 2-D MTF function, 
H(f.sub.x, f.sub.y) is the Wiener type filter expressed by 
##EQU3## 
where H.sub.W (f.sub.x, f.sub.y) describes the frequency response of the 
filter in the two-dimensional spatial frequency domain, S.sub.n (f.sub.x, 
f.sub.y) is the noise power spectrum and S.sub.f (f.sub.x, f.sub.y) is the 
signal power spectrum. A common variant on the Wiener formulation is to 
replace the ratio of power spectra in the denominator of the above 
equation with a single constant approximating the system's noise 
power-to-signal power ratio. The inverse and Wiener filter formulations 
are two of a variety of restoration filter formulations employed in 
digital filter designs. An aspect of this invention is that one of these 
generally known techniques is employed to compute the appropriate 
restoration filter frequency response for each of the recorded defocus 
blur responses; i.e., there is a specific restoration filter design for 
each step at which the blur response has been recorded. While the symbol 
H(f.sub.x, f.sub.y) was used above to describe the 2-D MTF due to the 
blurring properties of lens 85 at each level of defocus, it should be 
clear that other predictable blurring errors, such as the blurring due to 
the finite size of the sampling aperture on each photosite of the 
photosensor array 82 may, optionally, be included in H(f.sub.x, f.sub.y). 
As a final part of the filter design process, the pixel coefficients, 
described previously as a.sub.i,j, which are to be applied to the current 
pixel context are obtained from the computed restoration filter frequency 
response, typically by performing an inverse Fourier transform on the 
frequency response H.sub.W (f.sub.x, f.sub.y) or H.sub.I (f.sub.x, 
f.sub.y) of the restoration filter. While this theoretically leads to an 
infinite set of pixel coefficients, all but a few of the coefficients 
surrounding a.sub.0,0 may be discarded while still maintaining a good 
approximation to the restoration properties of the computed restoration 
filter. As an example of this final operation, FIG. 9 illustrates a 
restoration filter frequency response, H.sub.I (f.sub.x,f.sub.y), which 
was computed using the inverse filter formulation, above, for the 
restoration of an image blurred according to the 0.5 mm defocus error 
illustrated by the H(f.sub.x,f.sub.y) of FIG. 8. The inverse Fourier 
transform of the frequency response shown in FIG. 9, after dropping the 
less significant values, yields a set of coefficients, a.sub.ij, which may 
be applied to the current pixel, as above described, to restore the 
blurred image to a quality approaching the original image. The resulting 
a.sub.ij for this example use a 5.times.5 pixel context, and are given by 
__________________________________________________________________________ 
a.sub.-2,-2 
a.sub.-2,-1 
a.sub.-2,0 
a.sub.-2,1 
a.sub.-2,2 
-0.0014 
-0.0007 
0.0501 
-0.0007 
-0.0014 
a.sub.-1,-2 
a.sub.-1,-1 
a.sub.-1,0 
a.sub.-1,1 
a.sub.-1,2 
0.0742 
-0.2281 
0.0141 
-0.2331 
0.0770 
a.sub.0,-2 
a.sub.0,-1 
a.sub.0,0 
a.sub.0,1 
a.sub.0,2 
= 0.1222 
-1.7161 
4.6879 
-1.7161 
0.1222 
a.sub.1,-2 
a.sub.1,-1 
a.sub.1,0 
a.sub.1,1 
a.sub.1,2 
0.0770 
-0.2331 
0.0141 
-0.2281 
0.0742 
a.sub.2,-2 
a.sub.2,-1 
a.sub.2,0 
a.sub.2,1 
a.sub.2,2 
-0.0014 
-0.0007 
0.0501 
-0.0007 
-0.0014 
__________________________________________________________________________ 
In this manner, each of the recorded defocus levels for lens 85 has 
derived, according to known techniques, a corresponding set of restoration 
coefficients. The computation of these restoration coefficients completes 
the first step of the invention. These restoration coefficients are 
utilized to significantly reduce the detrimental effects of defocus blur 
and associated imaging errors on the digitally processed image signal. 
In a second step, the actual state of focus of the optical system as 
mounted in a particular piece of equipment is determined. In order to 
dynamically measure the state of defocus in a specific scanner structure, 
a target is placed on the platen glass 12 or on some other convenient 
structure for establishing the level of defocus correction required to 
compensate for assembly and component errors existing in a specific 
assembly. The target may comprise an isolated line or a small multiple bar 
target of known modulation, having a spatial frequency sufficiently high 
so that it is sensitive to defocus errors. Ideally, the target should be 
located in a plane which is representative of the position (i.e., object 
conjugate), of the document or image to be scanned. For example, the 
target may be located in a region of the platen frequently used for 
calibration targets 101 or under a side registration strip, where it can 
be monitored periodically during the scanning process. The optical system, 
including the lens 85, the photosensor array 82 and the illuminator 64 are 
caused to be positioned under the known location of the focus measurement 
target under control of the system CPU 99. The image of the target is then 
projected onto the photosensor array 82, this image containing an amount 
of optical blur similar to that which will be experienced when document 11 
is imaged. The digital signal from the photosensor array 82 may then be 
used, under control of system CPU 99, to compute the reduced modulation of 
the target, for example, by storing the maximum and minimum signal values 
obtained while scanning across the multiple bar target and computing 
modulation as the ratio of the difference of these maximum and minimum 
values to the sum of these maximum and minimum values. Since the blur 
characteristics of lens 85 have been previously determined and the focus 
target has a known spatial frequency and modulation, the reduction in 
modulation of this target can be correlated directly with one of the 
previously measured states of defocus. For this purpose a look-up table 
which identifies one of the previously measured states of defocus with the 
currently determined reduction in modulation of the focus measurement 
target is placed in a ROM (not shown) of the system CPU 99. In this 
manner, the specific state of defocus of the optical system comprising the 
platen glass 12, the lens 85 and the photosensor array 82 is determined, 
and this state may be directly identified with one of the previously 
evaluated levels of defocus and the associated digital filter coefficient 
set required to correct this defocus level. 
The frequency at which a defocus determination is made is variable. For 
example, the target may be sensed once after final assembly and alignment, 
using a removable target on the platen, and the required level of defocus 
correction may be stored for the life of the machine. Alternatively, a 
target may be built into the machine and may be sensed during each machine 
warmup so that any changes in the optical components (for example, 
resulting from field replacements) can be compensated. Alternatively, the 
target may be sensed several times during the scan of a single image to 
correct for object conjugate variations during the scanning process. For 
color document scanners, the target may be sensed in each color to detect 
and correct for defocus resulting from chromatic differences in focus. In 
this manner, the restoration filter coefficient set used in the digital 
image processing circuit 97 can be modified for each color, thereby 
compensating for the chromatic aberrations of the lens 85. 
In the final step, the restoration of the blurred image is undertaken in 
the digital image processing circuit 97 using filter coefficients stored 
in the filter coefficient ROM 98. Each of the image restoration 
coefficient sets determined for the multiple levels of possible optical 
defocus are stored in the filter coefficient ROM 98. This is preferably 
accomplished prior to the assembly of the image input terminal, but could 
be achieved subsequently by replacing the ROM chips with random access 
memory into which coefficients may be dynamically downloaded at any time. 
After the current state of defocus of the image input terminal is 
determined according to the previously described process, the system CPU 
99 provides information to the digital image processing circuit 97 
indicating which of the predetermined levels of defocus most accurately 
represents the current machine defocus state. The digital image processing 
circuit is thus enabled to read and apply to the pixel signal the set of 
image restoration filter coefficients most appropriate to the machine's 
current state of defocus. 
While the automatic detection of focus state and subsequent selection of 
optimum restoration filter coefficients is the preferred operating mode, 
it may be desirable under certain circumstances to manually override this 
selection. For example, if a relatively noisy document is to be imaged, 
the automatically selected restoration filter may provide excessive 
enhancement of high frequency noise. In such cases it is desirable to 
permit the user to enter information on a user interface (not shown) which 
signals the system CPU 99 to substitute a user designated filter selection 
in place of the automatically sensed selection. 
In the foregoing description it has been assumed that a single defocus 
state can be detected and the appropriately selected single set of image 
restoration filter coefficients will be applied to all pixels in the 
scanned document's image signal. It is possible, however, that the state 
of focus may vary from one part of the document to another part. This may 
occur if the gradient index lens array 85 is not accurately parallel to 
the platen glass. It may also occur due to the natural increase in blur 
that can occur as the ends of a scan line are sensed with a single lens 55 
as previously described in reference to the reduction optical 
architectures of FIG. 1. In these instances there may be several distinct 
states of defocus along the main scan line. Such multiple states of 
defocus along one scan line may be detected and corrected using a 
multiplicity of focus detection targets in a location which may be sensed 
by the scan line. Referring to FIG. 10, there is shown a strip of material 
104 which may be, for example, mounted on the platen 12 with focus 
measurement targets 105 facing the full-width lens 85 or single lens 55. 
Mechanically placing the optical system such that a line in the main 
scanning direction senses targets 105 as indicated along line L.sub.0, 
will permit the state of focus to be detected at multiple positions along 
a single scan line. Such multiple states are computed and communicated to 
the digital image processing circuit 97 prior to the start of the scanning 
of document 11. Thus during the scanning of document 11, the restoration 
filter coefficients appropriate to the current pixel's position along the 
scan line are correctly identified and applied. This approach to 
correction of multiple states of defocus within a document may similarly 
be extended to focus variations in the subscanning direction by mounting 
and sensing a multiple target strip of the type shown in FIG. 10 along the 
edge of a document which is perpendicular to the main scanning direction. 
In this latter arrangement it may be desirable to rotate the individual 
focus measurement targets 105 by 90.degree., or provide targets with 
0.degree. and 90.degree. rotation to permit defocus measurement in both 
scanning directions. 
From the foregoing, it can be seen that the depth of field of a scanner 
system can be electronically enhanced in an adaptive manner to maximize 
image reproduction fidelity. The system compensates for focus errors 
induced by optical component and alignment tolerances. The system permits 
relaxation of these tolerances while maintaining image quality. 
While the invention has been described with reference to the structure 
disclosed, it is not confined to the details set forth, but is intended to 
cover such modifications or changes as may come within the scope of the 
following claims.