Scanner

A scanner comprising a light source, having non-uniformities, for transmitting light through an input sample, and a cassette for holding the input sample, wherein a distance from the light source to the cassette is long enough to cause the effects of the non-uniformities to be substantially reduced.

FIELD OF THE INVENTION 
The present invention relates to optical input scanners generally and to 
scanners incorporating correction apparatus for light source 
non-uniformities, in particular. 
BACKGROUND OF THE INVENTION 
It is well known in the art that an image located on an input sample, such 
as a transparency, may be read by a specific family of optical scanners. A 
scanner of this family generally comprises a light source, a lens, an 
apparatus for holding the input sample, and a detector array. The input 
sample is often held in a cassette comprised of two glass plates. 
It is important that the output of the scanner be uniform in the presence 
of a uniform input. Non-uniform scanner output often occurs because 
individual detectors in the detector array have different sensitivities to 
light and, therefore, do not output identically even while receiving 
identical inputs. This problem is corrected by normalization, i.e., by 
multiplying each detector's output by a suitable factor to produce a 
normalized output, such that each of the normalized outputs is identical. 
Non-uniform scanner output may also result from a variety of 
non-uniformities of the light source. For example, a fluorescent lamp 
light source might have a scratch or a non-uniform phosphor coating on its 
glass tube, or a light source employing fiber optic bundles might have 
non-uniform bundle packaging or a break in one or more fibers. 
Non-uniform scanner output results in non-uniform images of the input 
sample. For example, scratches on the glass of the light source cause 
abrupt changes in light intensity which, in turn, causes thin or sharp 
stripes in the output image. "Slow" changes in light intensity (changes 
which vary slowly as a function of the spatial position on the lamp) cause 
wide or smooth stripes in the output image. 
Thin or sharp stripes in the image are the most undesirable whereas wide or 
smooth stripes are often difficult or impossible for an observer to 
notice. 
As the sharp non-uniformity is undesirable, the light source non-uniformity 
is compensated in a number of ways. The cassette can be moved until a 
portion of the cassette in which there is no transparency is in the 
optical axis. The light distribution on the detector is then measured to 
establish correction factors. 
However, the optical quality of the cassette is rarely consistent 
throughout the cassette. Thus, the light distribution during scanning can 
be different than that during calibration which occurrence will produce 
non-uniform output images. 
Another way to solve the light non-uniformity problem is to remove the 
cassette during calibration and thus, to ensure that the inconsistencies 
of the cassette do not compound the light uniformity error. However, in 
this solution, the optical system for calibration is significantly 
different than that for scanning. The image of the light source on the 
detector with the cassette on an optical axis of the scanner is larger 
than it is without the cassette and therefore, the light non-uniformity 
sensed by the detector is different during scanning than during 
calibration. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to correct errors in 
optical scanner output due to scanner light source non-uniformities such 
that the output is generally the same during scanning and calibration. 
In accordance with the present invention, the object is achieved through 
placing the light source at a distance from the focal plane far enough to 
cause the effects of the non-uniformities to be reduced. The calibration 
of the detector is then performed. 
Alternatively, the large distance is provided by folding the optical axis 
via a mirror or light guide. 
In a further embodiment, the light source is placed close to the focal 
plane and a "low quality" calibration glass is used during calibration. A 
number of measurements are typically taken and averaged together to 
provide calibration of the detector. 
In a still further embodiment, the cassette is placed on the scanner, with 
the light source close to the focal plane. Calibration is performed 
without the cassette and the affect of the different optical properties is 
compensated computationally. 
In a final embodiment, a diffuser is placed between the light source and 
the cassette to blur the non-uniformities of the lamp. 
In accordance with the present invention there is provided a scanner 
including a light source, having non-uniformities, for transmitting light 
through an input sample and a cassette for holding the input sample 
wherein a distance from the light source to the cassette is long enough to 
cause the effects of the non-uniformities to be substantially reduced. 
The scanner hereby provided has an optical axis and also includes a mirror 
or a light guide located between the light source and the cassette, for 
folding the optical axis. 
A 15 mm diameter lens is located 200 mm from the cassette and the distance 
from the light source to the cassette is 100 mm. 
In accordance with an additional embodiment of the present invention there 
is provided a scanner including a light source, having non-uniformities, 
for transmitting light through an input sample, a light detector apparatus 
for detecting light originating from the light source and a calibration 
glass for calibrating the light detector apparatus so as to cause the 
effects of the non-uniformities to be reduced. The scanner thus provided 
also has a focal plane and a cassette for locating the input sample in the 
focal plane and a lens. The calibration glass therein is of good quality 
and is located between the light source and the lens and may be located 
near or away from the focal plane. 
The scanner thus provided additionally includes apparatus for averaging 
multiple readings from the detector apparatus corresponding to a 
multiplicity of regions of the calibration glass, thereby to average 
effects of bubbles in the calibration glass and dust on the calibration 
glass. The calibration glass has a good flatness and parallelism value and 
a low surface scratches and digs quality. 
The scanner may be calibrated without the cassette and without the 
calibration glass and also includes compensation apparatus for 
compensating for the presence of the cassette during operation wherein the 
compensation apparatus perform geometrical correction calculations based 
on the optical qualities of the cassette and the lens. 
In accordance with a further embodiment of the present invention there is 
provided a scanner including a light source, having non-uniformities, for 
transmitting light through an input sample, a cassette for holding the 
input sample and a diffuser located between the light source and the 
cassette for diffusing the light, thereby to cause the effects of the 
non-uniformities to be reduced. 
Additionally, in accordance with a still further embodiment of the present 
invention, the light detector apparatus includes a plurality of detectors 
and the compensation apparatus includes apparatus for determining on which 
detector light from a section of the light source will fall, in the 
presence and absence of the cassette glass. 
Furthermore, in accordance with an embodiment of the present invention, the 
scanner also includes apparatus for determining an expected output signal 
of each of the plurality of detectors as a result of the intensity of 
light received from a corresponding section of the light source. The 
apparatus preferably comprises a) apparatus for measuring the output of 
the detector as a response to a first and second light distributions over 
the light detector apparatus, wherein the first and second light 
distributions are different and b) apparatus for determining, from the 
responses to the first and second light distributions, the output of the 
light detector apparatus in response to a third light distribution over 
the light detector apparatus. 
Still further, in accordance with an embodiment of the present invention, 
the scanner includes apparatus for providing a displacement of at least 
one of the light source, the light detector apparatus and a lens, located 
between the light source and the light detector apparatus, thereby to 
provide the first and second light distributions. 
There is also provided, in accordance with an embodiment of the present 
invention, a method of calibrating a scanner comprising the steps of a) 
measuring an output signal of at least two detectors of a light detector 
array in response to a first light distribution over the light detector 
array, b) displacing a portion of the scanner, in a direction generally 
parallel to a longitudinal axis of the light detector, thereby to provide 
a second light distribution over the light detector, c) measuring an 
output signal from the at least two detectors in response to the second 
light distribution and d) determining, from the responses to the first and 
second light distributions, the output of the detector array in response 
to a third light distribution over the detector array. The step of 
displacing preferably includes the step of non-uniform displacement. 
There is further provided, in accordance with an embodiment of the present 
invention, A scanner including a light source, a lens, a light detector 
which receives light from the light source through the input sample and 
lens, wherein the light source, lens and light detector have 
non-uniformities therein, apparatus for creating an average calibration 
signal to correct for fixed ones of the non-uniformities and apparatus for 
modifying low frequency components of the average calibration signal with 
low frequency components of a pre-scan calibration signal. 
Additionally, in accordance with an embodiment of the present invention, 
the apparatus for creating includes apparatus for dividing the average 
calibration signal into low and high frequency components. 
Moreover, in accordance with an embodiment of the present invention, the 
apparatus for creating includes apparatus for receiving output from the 
light detector corresponding to a first plurality of scans and wherein the 
apparatus for modifying includes apparatus for receiving output from the 
light detector corresponding to a second plurality of scans, wherein the 
second plurality is much smaller than the first plurality. 
There is finally provided, in accordance with an embodiment of the present 
invention, a scanner including a cassette upon which dust falls, a lens, a 
light detector, a light source for transmitting light through the cassette 
and lens to the light source and a diffuser located between the cassette 
and the lens during a calibration pre-scan, thereby to reduce effects of 
the dust on the light detector.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
Reference is now made to FIG. 11 which illustrates features of a prior art 
optical scanner 100. The scanner comprises a long and relatively narrow 
light source 110, such as a fluorescent lamp, typically containing a 
defect 112, for transmitting light in the direction of a lens 122 and an 
input sample (not shown), such as a transparency. Lens 122 is operative to 
focus an image of the input sample onto a detector array 120, such as a 
Charge Coupled Device (CCD) detector array. As known in the art, the 
numerical aperture of the lens 122 establishes the depth of field of the 
scanner 100. 
The scanner further comprises a cassette 116 for holding the input sample, 
located in a focal plane 118, and detector array 120 for electronically 
detecting light transmitted from the light source through the input 
sample. For each point on a scanned line, the scanner 100 has a light path 
from light source 110 to focal plane 118. An example light path, labeled 
114, is shown for a point A. 
Reference is now made to FIGS. 12A-12E which graphically illustrate the 
performance characteristics of prior art scanner 100 (FIG. 11), including 
characteristics of a calibration procedure in which normalization is 
performed in order to correct for the presence of defect 112 (FIG. 11). 
FIGS. 12A-12E plot array detector signal output I.sub.N (x) versus 
detector array position x, where N is a reference index. 
During the calibration procedure, cassette 116 is removed in order to avoid 
artifacts arising from cassette defects or irregularities. Light source 
110 is turned on and the output of detector 120 is observed. This 
demonstrates the signal non-uniformity during calibration when the 
cassette is absent. 
FIG. 12A illustrates a signal output I.sub.A (x) before calibration when 
the cassette 116 is absent. An abrupt signal output deficiency 130 is 
apparent at the region of detector array 120 corresponding to an image 
resulting from defect 112. 
FIG. 12B shows a normalization function I.sub.B (x) containing a 
compensating region 132 typically used to correct for the signal 
deficiency 130. The normalization function is generally calculated 
according to the following equation: 
##EQU1## 
where I.sub.B (x) is the normalization function and I.sub.Amax is the 
maximum signal intensity over the entire signal. 
FIG. 12C illustrates a uniform output I.sub.C (x) resulting from 
multiplying function I.sub.A (x) by normalization function I.sub.B (x) as 
follows: 
EQU I.sub.C (x)=I.sub.A (x)*I.sub.B (x) (2) 
In practice, however, prior art calibration does not typically lead to the 
uniform output I.sub.C (x) during scanner operation, which operation 
includes placing the cassette 116 in the optical path. No uniform output 
is produced because cassette 116, typically made of glass with an optical 
index of refraction greater than 1.0, refracts light that passes through 
it. 
FIG. 12D illustrates a signal output I.sub.D (x) containing a signal 
deficiency 134 resulting from defect 112. The array position of signal 
deficiency 134 is shifted with respect to the array position of signal 
deficiency 130 due to refractive properties of cassette 116. 
FIG. 12E illustrates a signal output I.sub.E (x) with cassette 116 in place 
during scanner operation. When the normalizing function I.sub.B (x) is 
multiplied by the output function I.sub.D (x), the output function I.sub.E 
(x) is obtained which contains two sharp and adjacent peaks 136 and 138. 
The output intensity is obtained by the following equation: 
EQU I.sub.E (x)=I.sub.D (x)*I.sub.B (x) (3) 
Peak 136 arises because compensating region 132 is multiplied by a flat 
portion of output signal I.sub.D (x). Peak 138 arises because signal 
deficiency 134 is multiplied by a flat portion of normalization function 
I.sub.B (x) and remains uncompensated. These peaks manifest themselves in 
an output image as visible artifacts in the form of two thin, sharp 
stripes wherein one stripe, associated with peak 138, is dark and the 
other stripe, associated with peak 136, is bright. 
The present invention provides a multiplicity of solutions, described in 
detail hereinbelow, to the above-identified problems of prior art 
scanners. The solutions of the present invention can be utilized singly or 
in any desired combination. Typically, calibration is performed before 
each scan. 
Providing a light source far from the focal plane: 
Reference is now made to FIG. 1 which illustrates features of an optical 
scanner 10 constructed and operative in accordance with the present 
invention. Similar elements in scanner 10 and scanner 100 (FIG. 11) serve 
similar functions and are referenced by similar reference numerals. 
In accordance with the present invention, scanner 10 further comprises a 
light path 140 which is long relative to light paths in prior art scanner 
100, for modifying transmitted light in order to compensate for effects of 
defect 112. Light path 140 is large, relative to light path 114 in prior 
art scanner 100 (FIG. 11), such that the non-uniformities in the light 
output become out of focus. Therefore, signal intensity peaks associated 
with the non-uniformities are wide and have small gradients and 
amplitudes. Thus, the non-uniformities are rendered negligible in an 
output image. 
The length of light path 140 allows light coming from defect 112 to be 
distributed over a larger number of CCD detectors than in prior art 
scanner 100 and, in this way, reduces the irregularity of the output 
signal. 
For example, for a lens with a diameter of 15 mm which is located 200 mm 
from the focal plane 118, the light source 110 should be located 100 mm 
from the focal plane 118. It will be appreciated that 100 mm is 
approximately twice the prior art distance. 
Reference is now made to FIGS. 2A-2E which graphically illustrate the 
reduced irregularity effected by scanner 10. As in FIGS. 12A-12E, I'.sub.N 
(x) is plotted versus x and is calculated in the same fashion. 
FIG. 2A illustrates a signal output I'.sub.A (x) before calibration. A mild 
signal output deficiency 150 is apparent at the region of the detector 
array corresponding to defect 112 (FIG. 1). This demonstrates the signal 
non-uniformity during calibration when the cassette is absent. 
FIG. 2B shows a normalization function I'.sub.B (x) containing a 
compensating region 152 used to correct for the signal deficiency 150. 
FIG. 2C illustrates a uniform output I'.sub.C (x) resulting from 
multiplying normalizing function I'.sub.B (x) by output I'.sub.A (x). 
FIG. 2D illustrates a signal output I'.sub.D (x) during a scan, which 
signal contains a signal deficiency 154 resulting from defect 112. The 
deficiency is shifted along the X axis due to the presence of the 
cassette. 
FIG. 2E illustrates a signal output I'.sub.E (x) with cassette 116 in 
place. I'.sub.E (x) contains two gradual, adjacent peaks, 156 and 158 
which result from defect 112 in a similar manner to that presented with 
reference to FIG. 12E. These peaks manifest themselves in an output image 
as two barely visible, wide stripes wherein the stripe associated with 
peak 156 is somewhat dark and the stripe associated with peak 158 is 
somewhat bright. 
It will be appreciated that the visibility of the stripes associated with 
peaks 156 and 158 is negligible relative to that of the stripes resulting 
from peaks 136 and 138 (FIG. 12E). In general, as light path 140 becomes 
larger, peaks 136 and 138 become flatter, i.e., smaller and wider. 
Creating a light path colinear with a line connecting array detector 120 
and cassette 116 is sometimes impossible due to mechanical considerations. 
In such cases, a folded optical path is utilized. 
Providing a light source far from the focal plane via folding of the 
optical axis: 
Reference is now made to FIGS. 3A and 3B which respectively schematically 
illustrate features of two alternative optical scanners 160 and 170. 
Similar elements in scanners 10, 160 and 170 serve similar functions and 
are referenced by similar reference numerals. 
In accordance with the present invention, scanner 160 further comprises a 
mirror 162, located close to cassette 116, for directing light coming from 
light source 110 towards lens 122. The resultant light path, formed of 
light path sections 164 and 166 shown with dotted lines, is sufficiently 
large that defects in the signal output of lamp 110 are rendered 
negligible in an output image. 
Light source 110 has a virtual image at a position 168 that is identical to 
the position of light source 110 in scanner 10 (FIG. 1). Additionally, the 
light path formed of sections 164 and 166 has a length similar to that of 
light path 140. As a result of the aforementioned similarities, FIG. 2B 
graphically illustrates the reduced irregularity in output images effected 
by scanner 160. 
Mirror 162 and lens 122 of scanner 160 are chosen with specifications that 
are suitable to particular applications. For example, as is known to any 
person in the art of optics, the mirror width should be at least as large 
as the cross-section of the numerical aperture of the lens 122 in the 
plane of the mirror. The length of the mirror should be at least as large 
as the length of a scanned line corrected by the distance of the mirror to 
the focal pane and to the numerical aperture of the lens 122. 
In accordance with the present invention, the mirror 162 preferably has a 
high optical quality surface since, because mirror 162 is close to 
cassette 116, scratches or other similar defects of the mirror surface 
will result in stripes in the output signal of the type described with 
reference to FIGS. 12A- 12E. 
It will be appreciated that the requirement of a high optical quality 
surface can be realized with conventional manufacturing techniques, as is 
known in the art. The same requirement for the light source 110 is 
typically impractical from a manufacturing standpoint. 
The coating of the mirror 162 can be either front coated or rear coated. 
For a front coated mirror, a good value for the surface scratches and digs 
quality (S&D) is required from only one surface of mirror 162. However, 
the coating is not as well protected as in a rear coated mirror. A rear 
coated mirror, in addition to the requirement of having two surfaces of 
high quality, requires a low value for the bubbles content. 
In the present application, the term "bubbles" refers to internal bubbles 
in optical glass as well as to all other enclosures in the glass that 
disturb the uniformity of the bulk of the glass. 
It will be noted that a very thick, rear surface coated mirror may also 
require that, during geometry design, attention be paid to the refraction 
index of the mirror glass. 
It will be appreciated that the type of coating chosen should fit the 
desired light spectrum of the scanner 160. 
Scanner 170 typically comprises a light guide 172, typically located close 
to cassette 116. Light guide 172 is chosen with dimensions that are 
suitable to the desired applications of scanner 170; dimension 
considerations are discussed hereinabove with respect to mirror 162. 
Light guide 172 is preferably designed for total internal reflection from 
surface 174 in order to provide high reflective efficiency. With total 
internal reflection, there is no need to coat surface 174. 
Principles of design and design considerations for light guide 172 are 
known to those skilled in the art of optics. One example consideration is 
discussed hereinbelow with respect to FIGS. 4A and 4B to which reference 
is now briefly made. 
FIG. 4A illustrates an example light guide 172 in which the material of the 
light guide has an index of refraction of approximately 1.55. With such an 
index of refraction, the minimum incidence angle which provides total 
reflection is 40.2.degree.. 
The length of the example light guide is about 150 mm and is 5 mm thick. 
Light rays, such as light ray 176, are incident at a low angle .alpha.. To 
ensure that all the light rays incident directly on plane 174 are totally 
reflected, angle .beta. is set to 42.4.degree.. 
Furthermore, the light guide 172 is typically located at an angle .THETA. 
to focal plane 118. In the embodiment of FIG. 4A, .THETA. is set to 
8.degree., allowing at least one center light ray 178 to be directed along 
the optical axis of the scanner. 
Because surfaces 174 and 175 are located close to the cassette 116, 
scratches or other similar defects on them will result in artifactual 
stripes in the signal output of the type discussed with reference to FIGS. 
12A-12E. Therefore, a high quality optical surface is preferred for the 
surfaces 174 and 175 in order to provide for optimal performance of the 
present invention. Bulk quality is also required for the glass volume near 
surfaces 174 and 175. 
FIG. 4B illustrates a light guide with light bundles 177 that traverse the 
light guide and undergo multiple reflections off of internal light guide 
surfaces before exiting the light guide. These light bundles 177 emerge 
from the light guide at an angle .mu. relative to central light rays 178 
undergoing only a single reflection off an internal surface. The angle 
.mu. increases with increasing number of internal reflections. 
The light bundles 177 undergoing multiple reflections are generally less 
intense than light ray 178 which undergoes only a single reflection at 
surface 174. Although bundles 177 can be used in the case of mechanical 
constraints of the scanner 170, it is preferable to use those bundles, 
such as rays 178, which are reflected only once from surface 174. 
Calibration of the scanner with a calibration glass: 
Reference is now made to FIG. 5 which schematically illustrates a further 
alternative embodiment of the present invention. Similar elements in FIGS. 
5 and 1 serve similar functions and are referenced by similar reference 
numerals. 
The scanner of the present embodiment, labeled 180, further comprises a 
transparent "calibration glass" 182 to be used during calibration only. 
Calibration glass 182 resembles cassette 116 both in thickness and optical 
index of refraction and thus, during calibration, provides scanner 180 
with optical refractive properties similar to those that are present while 
scanning with cassette 116. 
Calibration glass 182 refracts the transmitted light so that the signal 
deficiency due to light source defect 112 occurs in the same position as 
signal deficiency 134 (FIG. 12D). In this manner, subsequent normalization 
does not lead to stripes in the output image, as discussed with reference 
to FIG. 12E. 
Calibration glass 182 can have three general types of imperfections, 
flatness and parallelism (F&P), surface scratches and digs (S&D) and 
"internal bubbles", which distort scanner calibration as well as the 
resulting output images. In the following description, enclosures in the 
glass will be referred to as internal bubbles. 
Glass with a low F&P quality tends to distort the output image. A low 
quality for parallelism generally causes a shift in the light source image 
position. The extent of the shift is directly proportional to the distance 
of the calibration glass 182 from the light source 110. A low quality for 
flatness generally causes local disposition of the light image. The two 
effects cause a shift of the image of deficiency 112 between calibration 
and scanning and therefore, are source of calibration errors. 
Glass with a low S&D quality causes diffusion of light in a fashion similar 
to light source defect 112 and produces similar stripes. The same is true 
for internal bubbles. 
In accordance with the present invention, the quality of the calibration 
glass 182 typically determines its location, as well as the location of 
light source 110, in the scanner 180. This is illustrated in FIG. 5 by 
arrows 184 and 186 which indicate that the light source 110 and glass 182, 
respectively, are located at different locations depending on the quality 
of calibration glass 182. 
If the glass 182 is of good quality (e.g. high F&P, high S&D and bubble 
qualities such that the image quality is not affected by whatever F&P or 
S&D deficiencies remain in the glass 182), it can be placed anywhere 
between light source 110 and lens 122, and light source 110 is typically 
located near the focal plane 118. 
It will be appreciated that dust which accumulates on the good quality 
calibration glass 182, when the glass 182 is located close to the focal 
plane, will effect the calibrations in a manner similar to the effect of 
defect 112. The effects of the dust can be reduced by either locating the 
glass 182 away from the focal plane 118 or, if that is not feasible, by 
incorporating an averaging unit 188, such as a microprocessor, to average 
a multiplicity of calibration readings from a multiplicity of different 
locations on the glass 182. The effects of the dust will be averaged out. 
The averaging also averages out the effects of bubbles in the glass 182. 
Calibration glass 182 with a good F&P quality but a low S&D quality is 
similar to good quality glass with dust on it. This case is treated as 
described hereinabove with respect to good quality glass with dust on it. 
In accordance with the present invention, calibration glass 182 with a good 
S&D quality but a low F&P quality is located as close as possible to light 
source 110. If the glass accumulates dust, both the lamp and the 
calibration glass 182 are moved away from the focal plane. Alternatively, 
the averaging unit 188 can be utilized, as described hereinabove, to 
average the effects of both dust and bubbles in the glass 182. 
If, in a scanner to be calibrated, the cassette 116 is normally placed very 
close to the light source, then a clear portion of the cassette can be 
utilized instead of the calibration glass 182 with a low F&P quality. 
It is important to note that calibration glass 182 of good quality should 
be placed parallel to focal plane 118. Non-parallel placement of 
calibration glass 182 typically results in image displacement similar to 
that encountered when using a calibration glass with low F&P as discussed 
hereinabove. 
The amount of tilt that may be tolerated depends upon the size of 
individual detector elements in detector array 120. In particular, image 
displacement on the detector array due to tilt of calibration glass 182 
relative to cassette 116, assuming that the optical indices of refraction 
of both are similar, is approximately 0.006 mm per degree of relative tilt 
and per 1.0 mm of thickness of calibration glass 182. The size of the 
detector elements, on the other hand, is normally on the order of 0.01 mm. 
Addition of a light path or insertion of a calibration glass is not always 
convenient or practical for an optical scanner. It is, nevertheless, 
desired to correct for the presence of defect 112 and compensate for the 
refractive properties of cassette 116 so as to avoid artifactual stripes 
as discussed in detail with reference to FIGS. 12A-12E. 
Computational calibration of a scanner: 
Reference is now made to FIG. 6, which illustrates a scanner 190 forming an 
additional embodiment of the present invention. Scanner 190 comprises 
compensating unit 192 for computationally compensating for differences in 
optical refractive properties of scanner 190 in the presence and absence 
of cassette 116. If desired, scanner 190 can have a long light path, as in 
the previous embodiments described hereinabove, however, it is not 
necessary. 
FIG. 6 also illustrates geometrical and trigonometric relationships 
necessary to compensate for the above-mentioned differences in optical 
refractive properties. 
Cassette 116 has an optical index of refraction n and a thickness D. In the 
absence of cassette 116, light rays 194 and 196, emanating respectively 
from points y and x on light source 110, travel in straight lines, in the 
case of a thin lens 122, to points 198 and 200, respectively, on detector 
120. 
When cassette 116 is introduced, light rays 194 and 196 no longer travel in 
straight lines from the light source 110 to the detector array 120. 
Instead, a light ray 206 emanating from point y arrives at point 200. This 
is in accord with the shift in signal deficiency discussed with reference 
to FIG. 12D and leads to stripes in an output image as discussed 
hereinabove with reference to FIG. 12E. 
A distance W between points 198 and 200 is determined by the following 
equation: 
EQU W=m(x-y) (4) 
where m is the optical magnification of the lens 122. The distance (x-y) is 
determined by the following equation: 
EQU (x-y)=D[tan(.sigma.)-tan(.sigma.')] (5) 
where .sigma. is the angle between light ray 194 emanating from point y to 
point 198 and the optical axis 204. .sigma.' is given by the following 
equation: 
EQU .sigma.'=arcsin(sin(.sigma.)/n) (6) 
where n is the optical index of refraction of cassette 116. 
Since the pixel sensitivity is a constant, the change in the output signal 
of a given pixel, for example pixel 198, when the cassette glass 116 is 
included, is a function of the fact that the pixel 198 receives light from 
a different location on the light source 110. 
The change in the output signal can be measured by providing a known change 
in the optical system without introducing the cassette glass 116 and 
measuring the changed output of the detector array. 
An example change in the optical system is shown in FIGS. 7A and 7B 
wherein, in FIG. 7B, the lens 122 is displaced in a direction parallel to 
the longitudinal axis of the detector array 120 by some known distance d 
from its location in FIG. 7A, marked, in FIG. 7B, with a dotted line. Such 
an operation can be performed in any scanner which is capable of 
translating the lens 122. Such a scanner is, for example, the Smart 
Scanner manufactured by Scitex Corporation Ltd. of Herzlia, Israel. 
In the first measurement, shown in FIG. 7A, the light from an area i of the 
light source 110, having a corresponding intensity I.sub.i, impinges on a 
detector i of detector array 120. Detector i produces an outlet voltage 
V.sub.i,i which is a function of the sensitivity S.sub.i of the detector i 
and of the impinging light intensity I.sub.i, as follows: 
EQU V.sub.i,i =S.sub.i I.sub.i (7) 
Equation 7 is operative for each of the plurality of detectors i in the 
detector array 120. 
In the second measurement, the lens 122 is translated the distance d such 
that light from an adjacent area i-1 impinges on a given detector i. Thus, 
for the second measurement, equation 7 becomes: 
EQU V.sub.i,i-1 =S.sub.i I.sub.i-1 (8) 
for each of the plurality of detectors i in the detector array 120. 
The plurality of equations 7 and equations 8 can be rearranged to solve for 
each S.sub.i and I.sub.i, as follows: 
EQU S.sub.i =S.sub.1 (mult(V.sub.k,k-1 /V.sub.k-1,k-1)), k=2 to i(9) 
EQU I.sub.i =I.sub.0 (mult(V.sub.k,k /V.sub.k,k-1)), k=1 to i (10) 
where "mult" indicates a multiplication of the terms for all k indicated to 
the right of the equation, or: 
EQU S.sub.i =S.sub.1 r.sub.i (11) 
EQU I.sub.i =I.sub.0 q.sub.i (12) 
When the cassette glass 116 is placed into the optical path, detector i 
receives light from some area j on the light source 110, as determined by 
equations 4-6 hereinabove. The equation determining the output voltage of 
the detector i is: 
EQU V.sub.i,j =S.sub.i I.sub.j (13) 
which can be rewritten, using equations 11 and 12, as: 
EQU V.sub.i,j =S.sub.1 I.sub.0 r.sub.i q.sub.j (14) 
Since V.sub.1,0 =S.sub.1 I.sub.0 according to equation 13 equation 14 can 
be rewritten as: 
EQU V.sub.i,j =V.sub.1,0 r.sub.i q.sub.j (15) 
where V.sub.1,0 is measured in the second measurement. Thus, it is not 
necessary to know either the sensitivity of each detector nor the 
intensity of light provided by each section of the light source 110. 
If detector i receives light from an area j' which is formed of parts of 
two light areas j and j+1 (i.e. the cassette causes a translation x-+y of 
a non-integral number of pixels), the output of the detector i can be 
calculated via interpolation as follows: 
EQU V.sub.i,j' =V.sub.1,0 r.sub.i ((1-a)q.sub.j +aq.sub.j+1) (16) 
where a is the distance from the center of light area j to the center of 
light area j', in the direction of light area j+1 and one unit of a is the 
distance from light area j to j+1. 
It will be appreciated that the interpolation method of equation 16 is not 
the only method available; other methods of interpolation can be 
alternatively performed. Furthermore, additional measurements, including 
those having sub-pixel displacements, can be used for more accurate 
calculations. 
In accordance with the present invention, compensating unit 192 is 
constructed and operative to modify the output of detector array 120 
according to equations 4-16 in order to recalculate the output each 
detector element when cassette 116 is in place. In this manner, generally 
no artifactual stripes of the type discussed in detail with reference to 
FIGS. 12A-12E will appear. 
Those skilled in the art of optics are familiar with such correction 
calculations to different orders of accuracy using paraxial equations or 
commercially available optical software, such as Code-V available from 
Optical Research Associates, Pasadena, Calif. 
It will be appreciated by those skilled in the art that displacing the 
light source image relative to the detector array, when producing the 
second measurement of FIG. 7B, is not limited to one pixel or to an 
integer number of pixels. 
It will further be appreciated that any translation which produces a 
displacement between the light distribution over the detector array 120 
and the detector array 120 can be performed. For example, any one of the 
light source 110, lens 122 or detector array 120, or a combination 
thereof, can be translated, depending on the capabilities of the scanner. 
Furthermore, a glass plate positioned between the light source 110 and the 
detector array 120 can be tilted to provide the translation. 
Finally, it will be appreciated that the cassette glass 116 provides a 
non-uniform displacement, wherein the displacement is large at the ends 
and small at the center. 
Providing a diffuser between the light source and the cassette glass: 
Reference is now made to FIG. 8 which illustrates a scanner 210 which is a 
further embodiment of the present invention and incorporates a diffuser 
212. Diffuser 212, located between the light source 110 and the location 
of cassette 116, scatters light impinging upon it. Diffuser 212 can be, 
for example, ground glass, and its scattering efficiency is dependent on 
its granulation. 
In prior art scanners, the lens 122 is focussed on cassette 116 and the 
light from a given location, labeled A, on the cassette 116 arrives from a 
given area, labeled 214, of light source 110. The size of area 214 is 
defined by the numerical aperture of the scanner. 
In scanner 210, the light at a given location on cassette 116 arrives from 
an area 216 which is larger than area 214. In both scanners 100 and 210, 
the effect on the light caused by defect 112 is averaged over the areas 
214 and 216, respectively. Since the area 216 is larger than area 214, the 
effect of the defect 112 is reduced. 
The extent of the reduction, called the "averaging power" of the diffuser, 
depends on the ratio of areas 214 and 216. The sizes of areas 214 and 216 
depend on the scattering angle of the diffuser 212 and the geometry of the 
scanner 210. The averaging power increases with increased scattering angle 
and increased distance between light source 110 and diffuser 212. 
Further, the distance between the cassette 116 and the diffuser 212 should 
be increased with increasing granulation and optical resolution of the 
scanner 210. Otherwise, the diffuser 212 can become a further source of 
non-uniformities. 
It should be noted that addition of diffuser 212 can cause a loss of light 
in the scanner 210 for the following sources: reflection of light from the 
diffuser 212 backwards towards the light source 110 and if the area 216 is 
larger than the extent of light source 212. 
The scanner designer should consider the abovementioned effects and select 
a scattering efficiency, for a scanner being designed, which successfully 
trades the desired scattering effects against the loss of light. 
Updating only the low frequency content correction signal: 
Reference is now made to FIGS. 9A-9F which illustrate operations for 
updating only the low frequency content of the correction signal before 
each scan. 
In this embodiment, a general correction signal, shown in FIG. 9A, is 
typically produced during regular maintenance periods, in accordance with 
the method described hereinabove with respect to FIG. 5. This correction 
signal is an averaged correction signal and contains in it both high 
spatial frequency components 300 and low spatial frequency components 302. 
High spatial frequency components 300 are typically caused by 
non-uniformities in single elements, such as a detector, and correspond to 
corrections for fixed non-uniformities in the scanner, such as reduced 
sensitivities in one or more detectors. The high frequency component 
corrections typically do not change much over time and thus, need only to 
be measured, and the corrections calculated, during regular maintenance 
intervals. 
Low spatial frequency components 302 are typically caused by 
non-uniformities measured by a large number of detector elements and 
typically correspond to non-uniformities in the light source and other 
elements of the imaging system. Because the behavior of the light source 
is variable and depends on the power supply, temperature, etc., the light 
source and other low frequency non-uniformities must be corrected before 
each scan. 
Therefore, in accordance with this embodiment of the present invention, a 
calibration scan is performed during a regular maintenance period over a 
large scanning area. The measured outputs of the detector array 120 are 
averaged and a correction signal, shown in FIG. 9A, is calculated and 
stored. It will be appreciated that the averaging procedure can take a 
significant amount of time. 
In one embodiment of this operation, the correction signal of FIG. 9A is 
split, via Low Pass and High Pass Filters, into its low (FIG. 9B) and high 
(FIG. 9C) frequency components, both of which are stored. 
Before every image scan, a calibration scan of a few lines is performed. 
The output (FIG. 9D) will contain low and high frequency components, where 
the high frequency components correspond to dust particles (labeled 312) 
and to reduced detector sensitivities (labeled 310). Since, for the 
calibration update, only the low frequency component is of interest, the 
output is filtered with a low pass filter, to remove the high frequency 
content, and converted to create a low frequency update calibration signal 
(FIG. 9E). 
The low frequency update calibration signal (FIG. 9E) is multiplied with 
the stored high frequency calibration signal (FIG. 9C) to produce the 
calibration signal (FIG. 9F) which will be utilized for the upcoming image 
scan. 
Alternatively, the output calibration signal (FIG. 9F) can be produced as 
follows: 
a) perform the regular maintenance calibration scan, determine a 
maintenance calibration signal, formed of a calibration factor for each 
detector in detector array 120, in accordance with the methods described 
hereinabove, and store the results; 
b) from the output signal, or signals, of the calibration scan or scans, 
determine new calibration factors, in accordance with the methods 
described hereinabove, for each of the detectors of the detector array 
120; 
c) for each detector in the detector array 120, determine the ratio between 
the new calibration factors produced in step b and the maintenance 
calibration factors produced in step a; 
d) average the calibration ratios over a plurality of detectors, to produce 
an average calibration factor. The averaging will reduce high frequency 
calibrations; and 
e) multiply the maintenance calibration signal by the average calibration 
factor found in step d to produce the new calibration signal shown in FIG. 
9F. The new calibration signal has a new low frequency component while 
maintaining the previously determined high frequency component. 
Other methods for modifying the low frequency component of the regular 
maintenance calibration scan can alternatively be performed. 
Both methods described herein shorten pre-scan calibration time by 
requiring less data for the calibration calculations. In addition, they 
are useful for reducing the effects of dust on the cassette glass. An 
alternative method for reducing the effects of dust on the cassette glass 
116 is described hereinbelow with respect to FIG. 10. 
Providing a diffuser between the cassette glass and the lens: 
Reference is now made to FIG. 10 in which a diffuser 320 is placed, during 
the calibration scan only, between the cassette glass 116 and the lens 
122. The diffuser 320 is operative to average the light coming from the 
relatively large area A1 of the cassette glass 116, thereby to spread the 
effects of any dust particles over a plurality of detector elements. 
It will be appreciated by those skilled in the art that the addition of the 
diffuser generally causes a decrease in the intensity of the output signal 
of the detector arrays. The size of the decrease depends on the diffuser 
and is a constant. Therefore, it can be compensated while determining the 
calibration signal, via a scaling factor. 
It will be appreciated by those skilled in the art that more than one of 
the calibration methods described hereinabove may be used together in one 
scanner thereby to provide better results. 
Furthermore, it will be appreciated that partial implementation of one 
correction method in conjunction with partial implementation of a second 
correction method can also provide acceptable results. For example, a 
medium quality mirror (as opposed to a high quality mirror) can be placed 
a distance from the focal plane, where the distance is large, but not as 
large as for a low quality mirror. 
It will be appreciated by persons skilled in the art that the present 
invention is not limited by what has been particularly shown and described 
hereinabove. Rather the scope of the present invention is defined only by 
the claims which follow.