Methods and devices for implementing hyperacuity sensing are provided. The imaging device comprises a sensor array including sensor elements, each of which provides intracellular spatial intensity distribution information necessary to render images with subpixel accuracy. The method includes the step of hyperacuity sensing implemented using sensors that output signals that are dependent upon the internal intensity distribution of impinging light. The output signals are processed to determine the internal intensity distribution, which can then be used to produce a hyperacuity image. Such a hyperacuity image has enhanced definition of edges, and reduced artifacts such as jaggies and moire effects.

BACKGROUND OF THE PRESENT INVENTION 
Most image sensing devices operate by projecting an image that is to be 
scanned onto an array of discrete image sensor elements (usually p-i-n 
diodes). The projected image is then determined by interrogating the 
condition of the sensor array elements. For example, FIG. 1 shows a 
4.times.5 element section of an array 10 onto which an image having an 
edge 12 is projected. The term edge is used herein to mean the border 
defined by light illuminated areas and areas under ambient conditions. It 
is assumed that the area of the array 10 above the edge 12 is illuminated, 
while the area below the edge is dark. 
The twenty elements, shown as the twenty squares 14, are organized into 
rows A through D, and columns R through V. To scan the image, the 
illumination state of each of the elements is determined using matrix 
addressing techniques. If a particular element is sufficiently 
illuminated, for example the element at row A, column R, the element is 
sensed as being at a first state (ON). If a particular element is not 
sufficiently illuminated, say the element at row D, column V, that element 
is sensed as being in a second state (OFF). If a particular element is 
partially illuminated, its state depends upon how much of the element is 
illuminated, and the intensity of that illumination. An interrogation of 
all of the illustrated elements of the array 10 results in the rather 
coarse approximation to the image as shown in FIG. 1, with the ON state 
elements in white and the OFF state elements in cross-hatch. This 
cross-hatched representation results from a binary thresholding of the 
pixel (sensor element) values. An alternative prior art implementation 
provides a continuous value for each pixel (gray scale). In both of these 
prior art implementations, the edge position information within a pixel is 
converted to a spatial average. 
When using imaging scanners as described above, an increase in accuracy of 
the image approximation requires smaller and more numerous sensor 
elements. However, the difficulty of fabricating closely spaced, but 
isolated, sensor elements becomes prohibitive when attempting to fabricate 
page width imaging devices that have very high acuity (e.g., an acuity 
approaching that of the human visual system). 
In addition to the discrete sensor elements described above, another type 
of light sensitive element, called a position sensitive detector, exists. 
An example of a position sensitive detector is the detector 200 shown in 
FIG. 2. This detector outputs photogenerated analog currents 202, 204, 
206, and 208, that can be used to determine the position of the centroid 
of the illuminating spot 210. The centroid of the light spot in the 
x-direction (horizontal) can be computed from the quantity (I.sub.206 
-I208)/(I.sub.206 +I.sub.208), while the centroid of the light spot in the 
y-direction (vertical) can be computed from (I.sub.202 
-I.sub.204)/(I.sub.202 +I.sub.204), where I.sub.20x is the current from 
one of the lateral elements. At least partially because position sensitive 
detectors are typically large (say from about 1 cm.times.cm to 5 
cm.times.5 cm), they have not been used in imaging arrays. 
Ideally, an imaging device should be able to match the ability of the human 
visual system to determine edge positions, a capability known as edge 
acuity. Because of the difficulties in achieving high spatial resolution 
by increasing the pixel density, current image scanners cannot match the 
high edge acuity of human perception. Thus, new imaging and scanning 
techniques are necessary. Such new techniques would be particularly 
valuable if they could identify the positions of an edge to a fraction of 
the interpixel spacing. The ability to resolve edge spacings finer than 
the interpixel spacing is referred to as hyperacuity. 
SUMMARY OF THE INVENTION 
The present invention implements hyperacuity sensing. Hyperacuity sensing 
is implemented using an array of sensors whose output signals are 
dependent upon the internal intensity distribution of the impinging light. 
The output signals are processed to determine the intra-sensor intensity 
distribution, which is then used to produce a hyperacuity image. Such 
hyperacuity images enhance the definition of edges, and reduce undesirable 
artifacts such as jaggies and moire effects. 
BRIEF DESCRIPTION OF THE DRAWINGS 
Other aspects of the present invention will become apparent as the 
following description proceeds and upon reference to the drawings, in 
which: 
FIG. 1 shows a schematic depiction of a 4 element by 5 element section of a 
prior art imaging array; 
FIG. 2 shows a simplified depiction of a prior art position sensitive 
detector; 
FIG. 3 provides depictions of a sensor suitable for extracting light 
distribution information within a pixel; 
FIG. 4 helps illustrate a method for using position sensitive detectors to 
extract edge information; 
FIG. 5 shows a schematic depiction of a 2 element by 3 element section of 
an imaging array that is in accordance with the principles of the present 
invention; 
FIG. 6 shows the intensity information obtained using an array of intensity 
distribution sensitive pixels in accordance with the principles of the 
present invention;.

Note that the subsequent text includes various directional signals (such as 
right, left, up, down, top, bottom, lower and upper) which are taken 
relative to the drawings. Those directional signals are meant to aid the 
understanding of the present invention, not to limit it. 
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
As discussed above, the acuity of prior art imaging scanners is limited by 
the separation of the individual elements of the scanner's sensor array 
(for a given image magnification). This limitation is overcome in the 
present invention using hyperacuity techniques which approximate the edges 
of light illuminating the position sensors. The follow description first 
provides a description of a hyperacuity sensor and its use in implementing 
the hyperacuity techniques, then a description of a hyperacuity array is 
provided, and finally, the fabrication of the hyperacuity sensor is 
described. 
HYPERACUITY SENSING 
A sensor 300 that is suitable for use in hyperacuity sensing is illustrated 
in FIG. 3. The sensor 300 is an amorphous silicon position sensitive 
detector that is fabricated small enough to detect spatial distributions 
of light intensity at about 400 spots per inch. The sensor 300 has a 
substrate 301 on which is located a pair of lower electrodes 304 and 306. 
A lower, microcrystalline resistive layer 308 is formed over the lower 
electrodes 304 and 306. Over the resistive layer 308 is a vertical p-i-n 
diode 310 that is overlaid by an upper transparent, resistive layer 312 of 
a material such as indium tin oxide. Over the resistive layer 312 is a 
transparent insulating layer 313. Openings through the insulting layer 313 
to the resistive layer 312 are formed using standard photolithographic 
methods. Then, top electrodes 314 and 316 which electrically connect to 
the resistive layer 312 are located over the formed openings. Except for 
various enhancements in materials and dimensions, the sensor 300 is 
similar to the sensor 200 shown in FIG. 2. 
Currents from the electrodes 304, 306, 314, and 316, the currents I.sub.2, 
I.sub.4, I.sub.1, and I.sub.3, respectively, which are due to the 
intensity distribution of light 320 that is projected onto the sensor 300, 
are applied to external amplifiers (see FIG. 5). The currents are analyzed 
as described below to determine a hyperacuity approximation for the 
distribution of the light which illuminates the sensor. The subpixel 
accuracy relates to the ability to identify spatial intensity 
distributions within the sensor cell as distinguished from intensity 
averaging over the entire cell. This intracellular spatial sensitivity 
allows one subsequently to render images with similar subpixel accuracy. 
To approximate the illuminating light distribution within the sensor, a 
parametric model for the distribution of light within the sensor is first 
determined, and then, using the currents I.sub.1 through I.sub.4, the 
parameters for the model are determined and applied to the model. The 
result is the hyperacuity approximation of the illuminating light 
distribution. A particularly useful model for hyperacuity sensing is the 
edge model. 
In the edge model, the impinging light intensity is assumed to be 
delineated by a straight edge 322 between black regions 324 and some 
uniform gray level regions 326 which subtends the active area of the 
sensor 300. While approximating the edge of the illuminating light, which 
may not be straight, using a straight line approximation may at first 
appear unacceptable, such a linear approximation is quite good because the 
size of the sensor 300 is comparable with or smaller than the smallest 
curvature of interest. The locations at which the edge approximation 
intersects the boundary of the sensor, as well as the gray level, can be 
determined from the four currents from the element 300 (see below). The 
edges of an image can therefore be assigned to a much smaller spatial 
dimension (more accurately determined) than the size of the sensor 300. 
Using the edge model, the determination of the edge position, in terms of 
the measured currents, can be accomplished as follows. First, the current 
ratios x=I.sub.3 /(I.sub.1 +I.sub.3) and y=I.sub.4 /(I.sub.2 +I.sub.4), 
are computed. Then, from the current ratios the intercepts of an 
approximated edge with the sensor boundary can be uniquely determined 
using three separate conditions. First, if the point (x, y) falls within 
the region defined by the black square in the lower, left panel of FIG. 4, 
then the edge approximation intercepts the x axis at w=3Lx and the y axis 
at h=3Ly, where L is the sensor size. This condition, which corresponds to 
the illumination condition shown in the upper, left panel of FIG. 4, is 
referred to as a type 1 condition. 
The second condition occurs when if (x, y) falls within the black region of 
the lower, center panel in FIG. 4. In that case the edge approximation 
intercepts h.sub.1 and h.sub.2 are given by (2-3 x)yL/(1-3x(1-x)) and 
(-1+3 x)yL/(1-3x(1-x)), respectively. This condition, which corresponds to 
the illumination condition in the upper, center panel of FIG. 4, is 
referred to as the type 2 condition. Finally, if (x,y) falls within the 
black region depicted in the lower, right panel of FIG. 4, the edge 
approximation intercepts h and w are found from the relations 
h=(x-1/2)L.sup.2 /(xw/2-w.sup.2 /6 L) and w=(y-1/2)L.sup.2 /(yh/2-h.sup.2 
/6 L). This condition, which corresponds to the illumination condition 
shown in the upper, right panel of FIG. 4, is referred to as the type 3 
condition. Other values of (x, y) yield edge approximation intercepts with 
the sensor boundary by various rotation and mirror operations. The 
determination of a linear approximation to an edge is therefore unique. 
GRAY SCALE HYPERACUITY SENSING 
The above description of hyperacuity sensing used the edge model. Other 
intra-sensor intensity models are possible. For example, a second model 
that is useful in hyperacuity sensing interprets a gray scale 
approximation to the illuminating light. That model involves a 
parameterization in which the intensity within the sensor is taken to be a 
plane in a 3-dimensional (x, y, .PHI.) space where x and y are spatial 
dimensions and .PHI. is the light intensity. The linear approximation is 
determined from the current outputs from the sensor 300. If the light 
intensity, .PHI.(x,y) within the pixel is represented by 
.PHI.(x,y)=C+Ax+By with the origin at the center of the cell, then 
C=I.sub.T /L.sup.2 where I.sub.T =I.sub.1 +I.sub.2 +I.sub.3 +I.sub.4, A=3 
x I.sub.T /L.sup.4 and B=3 y I.sub.T /L.sub.4. Thus, one can readily 
construct a piecewise linear model for the intensity within the sensor. 
HYPERACUITY IMAGING ARRAYS 
The above described hyperacuity sensors are useful in imaging devices. An 
imaging device that makes use of an array of hyperacuity sensors will be 
referred to as a hyperacuity imaging array. FIG. 5 shows a 2.times.3 
section 500 of an array of hyperacuity sensors 502. Each sensor 502 has 
four electrodes 510 which connect to current amplifiers 512 (only four of 
which are shown). The currents from each sensor 502 are resolved as 
described above to form an approximation of the edge of the impinging 
light in each sensor 502. 
For example, FIG. 5 shows an edge 520 defined by the boundary between 
illuminated and dark areas. The position of the edge 520 in each sensor 
502 is approximated using the model described above. The location of the 
overall edge in the section 500 is determined by piecewise fitting 
together the approximations from each sensor 502. The accuracy of the 
approximation of the edge 520 is superior to that produced in prior art 
imaging scanners that use discrete sensor elements that are separated by 
similar distances as the present sensors 502. 
In FIG. 6 we show the improvement in image acuity arising from the use of 
the present invention applied to the same illumination as that used in 
FIG. 1. The array 600 consists of individual pixel sensors 614 which, in 
this example, are position sensitive detectors. The outputs in conjunction 
with the edge model have been used to determine the subpixel edge 
positions as shown by the lines 616. The improved accuracy in the 
approximation to the illuminating boundary 612 is evident. 
While arrays which use hyperacuity sensors have four times as many outputs 
as comparable arrays of integrating pixels, the enhancement of the 
determination of the position of the light edge can be significantly 
greater than two orders of magnitude. 
FABRICATION OF THE HYPERACUITY SENSOR ARRAY 
A hyperacuity array can be fabricated on a glass substrate as follows. 
First, a chrome/molybdenum metal layer is deposited on the substrate by 
sputtering. Then, the chrome/molybdenum metal layer is patterned to form 
the lower electrode pairs (which correspond to the electrodes 304 and 306 
in FIG. 3). Next, a laterally resistive thin film of doped 
microcrystalline silicon (which corresponds to the resistive layer 308) is 
deposited uniformly over the substrate and lower electrodes. An undoped, 
hydrogenated amorphous silicon layer approximately 300 nm thick and a thin 
p-type amorphous silicon contacting layer are laid over the resistive thin 
film by using plasma deposition. A transparent conducting layer of indium 
tin oxide (ITO) is then deposited over the hydrogenated amorphous silicon 
layer. Next, an insulating film of silicon nitride is laid down over the 
indium tin oxide. That insulating film is then patterned to open trenches 
to the indium tin oxide layer. Aluminum is then deposited over the exposed 
top surface. That aluminum is then patterned to form the top electrode 
contacts, the vias, and the leads which apply the current signals to 
contact pads or to thin film pass transistors. Of course, many variations 
of the described fabrication process are possible. Such variations, which 
will be obvious to those trained in the art of semiconductor processing, 
do not change the basic invention. 
From the foregoing, numerous modifications and variations of the principles 
of the present invention will be obvious to those skilled in its art. In 
particular, the shape and compositions of the sensor elements may be 
changed to fit a particular application. Therefore the scope of the 
present invention is defined by the appended claims.