Multi-resolution image scanner

An image scanner is provided that receives light reflected from a same position on a document by first, second, and third one-dimensional image sensors arranged in parallel. The second image sensor is a high-resolution image sensor comprising a plurality of photocells, which is disposed between and in a spaced-apart relationship with the first and third one-dimensional image sensors which are each low-resolution image sensors comprising photocells which are larger in size than the photocells of the second one-dimensional image sensor. Electrical image output signals from the first and third low-resolution image sensors are added, and any high-frequency components in the added signal are removed by a low-pass filter for producing signal data relating to the mean brightness of the region around a pixel of interest. The difference between the mean brightness and the brightness of the pixel of interest, as outputted by the second high-resolution image sensor, is then derived to produce an output signal in which points to where the brightness changes in the image are emphasized. Subsequent processing enables textual and other information of interest in the document to be extracted from background information.

FIELD OF THE INVENTION 
The present invention relates to a document image scanner, and, more 
particularly, to an image scanner than in the course of binarizing image 
data extracts target text and pattern information without hindrance from 
background brightness of the document. 
BACKGROUND OF THE INVENTION 
Image scanners have come into widespread use as a means of inputting data 
to devices. However, if the use of image scanners is to increase further, 
they have to be able to scan text and pattern information with greater 
precision, speed and economic efficiency. To achieve this, with the 
exception of certain areas of application, prior art scanners store, 
transmit and print the scanned information after it has been binarized. 
A problem is that the documents that have to be scanned are varied. In many 
cases the information is written on dark-colored paper, or the documents 
contain extraneous background information such as discolored areas as 
found in, for example, a copy of a document that has been copied many 
times. Thus, it is necessary for an image scanner not only to be able to 
scan a document image, but to be able to accurately extract the target 
textual or other information without being affected by background 
brightness or noise information. In prior art image scanners, the scanned 
information is subjected to the following processing after it has been 
digitized and stored in a system memory. First, the information is read 
out of the system memory and subjected to a prescribed series of iterative 
sum of products operations to obtain the mean brightness of the pixels 
around a pixel of interest. The difference between the obtained mean 
brightness and the brightness of the pixel of interest is then obtained to 
produce an output signal which emphasizes he points where the brightness 
in the image changes. In the prior art image scanners, digital filters are 
generally used to discriminate the textual and other such information of 
interest from the background information. However, such digital filtering 
systems are costly, requiring memories and many other expensive parts and 
components. Additionally, the limited operating speed of such expensive 
parts and components makes it difficult to increase the speed of such 
prior art image scanners. 
SUMMARY OF THE INVENTION 
The present invention is directed to an image scanner that is capable of 
extracting textual and other information of interest without hindrance of 
background information using a simpler and less costly structural 
arrangement than is found in prior art image scanners that use digital 
filters. 
In accordance with the present invention, the image scanner comprises light 
source for illuminating a document being scanned, high-resolution image 
sensor means, low-resolution image sensor means, and differential 
calculation means. The high-resolution image sensor means comprises a 
plurality of photocells that receive light reflected from, or transmitted 
by, the document, and convert the intensity of the received light to 
representative electrical image output signals. The low-resolution image 
sensor means comprises a plurality of photocells which are substantially 
larger in size than the photocells of the high-resolution image sensor 
means. The photocells of the low-resolution image sensor means receive 
light reflected from, or transmitted by, a location on the document that 
is substantially identical to the location from which the photocells of 
the high-resolution image sensor means receive their light, and convert 
the intensity of the received light to electrical image output signals. 
The differential calculation means calculates the difference between the 
concurrent electrical image output signals of the photocells of the 
high-resolution image sensor means and the low-resolution image sensor 
means, and generates a difference output signal. 
Viewed from another aspect, the low-resolution image sensor means comprises 
a first and a second low-resolution image sensor. Each of the first and 
second low-resolution image sensors are disposed on a separate opposing 
side of the high-resolution image sensor means and provide a separate 
electrical image output signal. The electrical image output signals from 
the first and second low-resolution image sensors are added, low-pass 
filtered, and then amplified to a predetermined level before being 
introduced to one input of the differential calculation means. The 
difference output signal from the differential calculation means is then 
processed by an image processing means to extract the textual or other 
information from the document being scanned. 
Viewed from another aspect, the present invention is directed to a method 
of scanning a document to extract textual and other information thereon 
and comprises the following steps. In a first step, the document being 
scanned is illuminated with a light source. In a second step, light 
reflected from, or transmitted by, the document is detected with a 
high-resolution image sensor means comprising a plurality of photocells 
that convert the intensity of the light received from the document to 
representative electrical image output signals. In a third step, 
concurrent with the second step, a low-resolution image sensor means 
detects light reflected from, or transmitted by, a location on the 
document that is substantially identical to the location from which the 
high-resolution image sensor means receives its light. The low-resolution 
image sensor means comprises a plurality of photocells that convert the 
intensity of the received light from the document to electrical image 
output signals. The photocells of the low-resolution image sensor means 
are substantially larger in size than the photocells of the 
high-resolution image sensor means. In a fourth step, the difference 
between the concurrent electrical image output signals from the 
high-resolution image sensor means and the low-resolution image sensor 
means are calculated, and a difference output signal is generated for use 
in subsequent processing to extract textual and other information from the 
document being scanned. 
Thus, with the image scanner according to this invention, light reflected, 
or transmitted from, substantially the same position on a document is 
received by a plurality of image sensors, each having a different 
resolution, which convert the intensity of the received light to 
electrical image output signals. A difference between the electrical image 
output signals is then calculated. This difference is used as a basis for 
producing an output in which portions in the image where there is a change 
in the brightness are emphasized, thereby enabling the textual and other 
information of interest to be correctly extracted from the background 
information. 
The present invention and its various advantages will be better understood 
from the following more detailed description taken with the accompanying 
drawings and claims.

DETAILED DESCRIPTION 
Referring now to FIG. 1, there is shown principal components of an optical 
arrangement of an image scanner 10 in accordance with the present 
invention. The optical arrangement of the image scanner 10 comprises a 
light source 12, an optical system 13, a first one-dimensional image 
sensor 15, a second one-dimensional image sensor 16, and a third 
one-dimensional image sensor 17 which are mounted on a substrate or board 
14. More particularly, in the image scanner 10, the light source 12 is 
used to illuminate a document 11 that is moved at a constant speed in a 
direction indicated by an arrow "S". The pattern of alphanumeric 
characters and other information (not shown) on the document 11 is 
illuminated by the light source 12 and is focussed by the optical system 
13 to form an image thereof on the three one-dimensional image sensors 15, 
16 and 17 which are arranged in parallel on the substrate or board 14. The 
optical system 13 comprises lenses, optical filters and the like (all of 
which are not shown). The one-dimensional image sensors 15, 16, and 17 
generate separate electrical image output signals over separate leads 18, 
19 and 20, respectively. Each of the electrical image output signals on 
leads 18, 19, or 20 are representative of the intensity of the light 
received by the one-dimensional image sensor 15, 16 or 17, respectively. 
Referring now to FIG. 2, there is shown the one-dimensional image sensors 
15, 16, and 17 of FIG. 1, a sensor signal processing circuit 28 (shown 
within a dashed line rectangle) which processes the output image signals 
on leads 18, 19, and 20, and an image processing section 29 connected via 
lead 25 to an output of the sensor signal processing circuit 28. The 
sensor signal processing circuit 28 comprises a serial connection of a 
first amplifier 21, a low-pass filter (LPF) 22, a second amplifier 23, and 
a third amplifier 24. The first amplifier 21 receives the output image 
signals on leads 18 and 20 from the one-dimensional image sensors 15 and 
17, respectively, and the third amplifier 24 receives the output signal 
from the second amplifier 23 on a lead 26 and the output image signal on 
the lead 19 from the one-dimensional image sensor 16. 
Of the one-dimensional image sensors 15, 16, and 17, the one-dimensional 
image sensor 16 is disposed between the image sensors 15 and 17 and is a 
high-resolution image sensor comprising a plurality of small photocells 
which each have a height of "A" and a width of "C". The other two 
one-dimensional image sensors 15 and 17 are arranged on separate sides of 
the one-dimensional image sensor 16, and are low-resolution image sensors 
comprising a plurality of relatively large photocells each having a height 
of "B" and a width of "C". Dimensions "A" and "C" may, for example, be 
approximately 65 micrometers, and dimension "b" may, for example, be 
approximately 1000 micrometers. The photocells making up the three 
one-dimensional image sensors 15, 16 and 17 are arranged to operate in 
mutual synchronization in converting the received light from the document 
11 into a stream of electrical output image signals on leads 18, 19, and 
20, respectively. The electrical image output signals on leads 18 and 20 
outputted from the one-dimensional image sensors 15 and 17, respectively, 
are provided as separate inputs to the first amplifier 21. The first 
amplifier 21 adds the image output signals from leads 18 and 20. The 
resultant output signal from the first amplifier 21 is passed through the 
low-pass filter 22 to remove the high-frequency components, and is then 
amplified to a prescribed level by the second amplifier 23 which has a 
variable gain. An amplified output signal on lead 26 from the second 
variable-gain amplifier 23 is provided to one input (a negative input) of 
the third amplifier 24 (e.g., a differential amplifier). The image signal 
on lead 19 outputted from the second one-dimensional image sensor 16 is 
provided directly to a second input (a positive input) of the third 
amplifier 24. The third amplifier 24 derives the difference between the 
image signal on lead 19 from the one-dimensional image sensor 16 and the 
amplified signal on lead 26 from the second amplifier 23, and outputs this 
difference as differential signal on lead 25. The differential output 
signal on lead 25 from the third amplifier 24 is provided as an input to 
an image processing section 29. The image processing section 29 processes 
the difference output signal from the third amplifier 24 to extract 
textual and other information that is the object of interest on the 
document 11 being scanned. 
In operation, the pattern of alphanumeric characters and other information 
on the document 11 is illuminated by the light source 12 and focussed by 
the optical system 13 to form an image thereof on the one-dimensional 
image sensors 15, 16, and 17 arranged on the board 14. The respective 
photocells forming the three one-dimensional image sensors 15, 16, and 17 
synchronously convert the light received from the document 11 to separate 
electrical signals which are outputted as image signals on leads 18, 19, 
and 20, respectively. When a complete set of output image signals for each 
scanned line of the document 11 are provided on leads 18, 19, and 20 from 
the one-dimensional image sensors 15, 16, and 17, respectively, the 
document 11 is moved in the direction of the arrow S by an amount 
corresponding to one line. It is to be understood that the one line 
referred to hereinabove by which the document 11 is moved corresponds to 
the scanning resolution of the central one-dimensional image sensor 16. 
The above-described procedure for processing one line of the document 11 is 
repeated for the ensuing lines, with the respective photocells forming the 
one-dimensional image sensors 15, 16, and 17 working in unison to output 
the image signals on leads 18, 19, and 20, respectively, that correspond 
to the intensity of the received light, until the whole of the document 11 
has been scanned. 
Referring now to FIG. 3, there is shown the positional relationship of 
regions 31, 32, 33, 34, and 35 scanned by the photocells used with each of 
the one-dimensional image sensors 15, 16, and 17. The arrow designated "M" 
indicates the direction within a line in which photoelectric conversion by 
the photocells takes place, i.e. the primary scanning direction. The arrow 
designated "S" indicates the direction in which the document 11 is moved, 
i.e., the secondary scanning direction. 
In operation, a small picture element (pixel) 31, the size of which 
corresponds to the scan resolution, is scanned by the one-dimensional 
image sensor 16 and is provided as the electrical image output signal on 
lead 19 to the second input of the third amplifier 24 of FIG. 2. 
Concurrent therewith, after being scanned by the respective 
one-dimensional image sensors 15 and 17, the mean brightness levels of the 
larger regions 32 and 33 that bracket pixel 31, extending in the secondary 
scanning direction, are added by the first amplifier 21 of FIG. 2. Because 
the low-pass filter 22 of FIG. 2 blocks high-frequency components received 
in its input signal, the input signal to filter 22 is converted to an 
output signal that indicates the mean brightness of a surrounding region. 
The surrounding region includes regions 34 and 35 that are extended in the 
opposite direction to that of the primary scanning direction shown by 
arrow "M". The output signal from the low-pass filter 22 is then passed 
through the second variable-gain amplifier 23 to produce the amplified 
signal on lead 26 which is provided to the negative input of the third 
amplifier 24. 
The third amplifier 24 produces a differential output signal on lead 25 
that corresponds to the difference between the image output signal on lead 
19 derived from the pixel 31, and the amplified signal on lead 26 that 
includes the mean brightness of the surrounding regions 32, 33, 34 and 35. 
This enables the pixel 31 to be clearly discriminated from the background 
brightness. Thus there is provided a basis for the document patterns of 
alphanumeric characters and the like to be emphasized and extracted by the 
image processing section 29. 
It is preferred that the one-dimensional image sensors 15, 16, and 17 are 
arranged on a single substrate or board, which facilitates fabrication by 
eliminating the need to align the one-dimensional image sensors 15, 16, 
and 17 in parallel. The signal processing circuit 28 can comprise 
inexpensive amplifiers, resistors and other such components, which permit 
the operating speed of the present image scanner to be higher than that of 
prior art image scanners using digital filter systems. 
It is to be appreciated and understood that the specific embodiments of the 
invention described hereinbefore are merely illustrative of the general 
principles of the invention. Various modifications may be made by those 
skilled in the art which are consistent with the principles set forth.