Image reader capable of setting density and illumination

An image reader sets the level of illumination provided by a light source and the gamma correction provided by a density correction circuit in dependence upon a user selected density setting button on a control panel.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an image reader for image processing and, more 
particularly, to an image reader for reading characters or images and 
generating a corresponding digital image signal (e.g. a digital scanner or 
a digital copier). Such image readers comprise a light source for exposing 
an image to light, and an image sensor (e.g. a CCD image sensor) for 
generating a corresponding image signal, and means for digitizing the 
image signal and digitally processing the digitized image signal. 
2. Description of the Prior Art 
A digital image reader is disclosed in our European Patent 0267793B granted 
to the present Asignee. In a reader of this type, an image of characters 
or pictures written or drawn on pages of a book, or some other document, 
or alternatively recorded on a microfilm or other transparent medium, is 
read using a CCD image sensor to provide an image signal which is then 
either recorded, processed or transmitted. 
One example of an image reader is arranged to read an image from microfilm, 
and to print a corresponding output copy, and comprises a CCD image sensor 
or the like, means for electrically processing the image sensor output, 
and means for providing a digital signal to a printer unit such as a laser 
beam printer to obtain a copy. FIG. 14 is a block diagram of an image 
reader provided within such a reader/printer apparatus. In FIG. 14, 
reflected light from an original placed on, for example, an original table 
or light projected through a microfilm, is scanned by an image pick-up 
section 21 comprising an image pick-up device such as a CCD, and is 
converted into an electrical signal which is quantized into a multi-valued 
signal by an A/D conversion section 22. Multi-level quantized data thereby 
obtained is corrected by a gamma correction section 29 to take account of 
the difference between the photo-electric conversion characteristic of the 
image pick-up device in the image pick-up section 21 and the human 
gradation perception characteristic, illustrated in FIGS. 15, 16 and 17. 
FIG. 15 shows an example of the photo-electric conversion characteristic of 
an image pick-up device (e.g. a CCD device). The abcissa represents a 
density scale which is linear with respect to the human gradation 
perception characteristic. It will be seen that the photo-electric 
conversion characteristics of image pick-up devices in general differ from 
the human gradation perception characteristic. For this reason, a gamma 
correction table embodying a correction characteristic such as that shown 
in FIG. 16 is used to provide density correction or density conversion, so 
that the output D' is linear with respect to the human gradation 
characteristic as shown in FIG. 17. The gamma correction characteristic is 
therefore essentially the reverse of the conversion characteristic of the 
image pick-up device with respect to the human gradation perception 
characteristic. 
The signal processed by the gamma correction section 29 is converted, by a 
gradation processing section 25, into binary (bi-level) image data from 
which a graded image can be reproduced by, for example, a dither or error 
diffusion method, and the processed data is output from the reader for 
subsequent printing or other processing or transmission. 
Microfilm images are provided either as negative images or positive images 
on corresponding negative or positive films, which are selectively used 
for different purposes. A known image reader enables either negative or 
positive films to be copied; in this reader apparatus an image from the 
film is directly projected onto a photo-sensitive member from which it 
maybe printed (e.g. electrostatically). This direct, or analog, type 
printer therefore has two separate image forming processes, one for 
negative film and one for positive film, and requires toners having 
different polarities and two different development devices, a changeover 
mechanism for interchanging between the two, a circuit or mechanism for 
changing the polarity of a high voltage output from a transfer charger or 
other charger, and two separate high voltage power sources for different 
load characteristics, and requires also a changeover for providing blanks 
for blank exposure. Analog type reader/printers of this type are proposed 
by the present asignee in U.S. Pat. No. 4,341,463 and U.S. Pat. No. 
4,627,703. This type of apparatus can be used to provide images having 
good gradation characteristics (grey scale reproduction) by selecting the 
reproduction voltages for each of the positive and negative film processes 
so as to provide optimal gamma characteristics (exposure/density 
characteristics). However, since two separate developing processes and 
associated changeover mechanisms are required, the apparatus is 
necessarily undesirably complex. 
Digital reader/printers (for example the Canon DMP100) are, on the other 
hand, less mechanically and electrically complex. In principle, such a 
reader/printer can be used to obtain a positive print copy from a film of 
either type merely by selectively inverting the digital signal output. 
However, if the digital signal is merely inverted in this way, it is not 
possible to obtain an image having good gradation characteristics. One of 
the reasons for this is that the gamma characteristic of a film is 
non-linear and, referring to FIG. 9, curved portions of the characteristic 
in the vicinity of A and B are not symmetrical. 
Analog type reader/printers of this type are proposed by the present 
asignee in U.S. Pat. No. 4,341,463 and 4,627,703. 
Thus, when a positive original is photographed onto a film, the portion A 
corresponds to a line drawing portion and the portion B corresponds to a 
background image portion whereas when a negative original is photographed 
onto the film, the portion B corresponds to the line drawing portion and 
the portion A corresponds to the background portion. Thus, if a 
reader/printer with a gamma correction characteristic set for one type of 
original is used to read and print the opposite type of original, by 
merely inverting the digital image data, the result is that line image 
portions receive the gamma correction appropriate for background portions 
and vice versa. 
Japanese laid open applications nos. 3-150977 and 3-160877, and 
corresponding U.S. applications Ser. No. 604,955 filed on 25th Oct. 1990, 
all assigned to the present assignee and published after the present 
priority date, describe a reader/printer in which this problem is solved 
by providing means for selecting the gamma correction in dependence upon 
whether the film is a negative or positive original film. 
In such a digital reader/printer, ordinarily, the signal from the image 
pick-up device is amplified and digitized, and a look-up data table is 
used for converting the digitized data to gamma corrected data, the 
look-up table operating in accordance with a linear or non-linear scale, 
which in either case does not have a one-to-one output characteristic. 
In general, in the case of processing image data from a microfilm (or like 
transparent medium) the print density level required is found to depend 
not only on the non-linearity of the gamma characteristic of the film but 
also on the original type (positive or negative), the density 
characteristic of the original image (imaged by a camera), the resolution 
of the camera lens which imaged the original, the extent of defocus 
thereof, the kind of film (silver salt, diazo, vescicular and so on) the 
film developer liquid, the development conditions, and other factors, as 
well as the purpose and the user's preference. 
SUMMARY OF THE INVENTION 
In view of the foregoing, one object of the present invention is to provide 
means for reading an original image at a desired density. 
Another object of the present invention is to suitably read an image 
recorded on a microfilm. 
Another object of the present invention is to suitably read either positive 
or negative images as desired from a film or other type of original. 
In one aspect, the present invention makes use of the inventor's discovery 
that particular gamma correction and light level combinations are 
preferable for each different film/image density setting. Accordingly, an 
image reader in this aspect of the invention varies the illumination level 
and the density correction jointly, in dependence upon an image control 
signal (typically a user selected density level). 
In another aspect, the present invention provides an image reader which 
allows a continuously variable user adjustment of image density, in which 
the image density selected varies the density conversion applied, in such 
a way as to avoid sudden changes in the image appearance (such as the 
appearance fogging) with small changes in the selected image density. This 
is preferably achieved by providing that the density correction is 
progressive with varying user selected image density over a central or 
normal operating density range, and that the variation of density 
correction is reduced (for example held constant) outside this normal 
range. 
In a preferred embodiment according to this aspect, the light level of 
illumination is also controlled over the density range; for example, the 
light level may vary more outside the normal range than inside so as to 
compensate the reduction in variation of density conversion 
characteristic. 
Preferably, in either aspect, edge enhancement filtering is provided and 
very preferably this filtering can by varied (for example in dependence 
upon density, or upon film type). 
The foregoing and other objects of the present invention, and the 
advantages thereof, will become further apparent from the following 
description in conjunction with the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a block diagram of a reader/printer comprising one embodiment of 
the present invention, The reader/printer comprises an image sensor 1 
(e.g. a CCD device, or any other suitable image pick-up sensor) for 
reading an enlarged projected image of a microfilm, an amplifier 2 
connected to the output of the image sensor 1 for generating an amplified 
analog image signal, an A/D converter 3 connected to the amplifier 2 for 
converting the analog signal into a digital signal, a density correction 
circuit 4 connected to receive the digital signal as input data and to 
provide corrected output signal data in accordance with a predetermined 
gamma correction curve, an edge enhancement circuit 5 comprising a digital 
filter for converting the digital image signal into an edge enhanced 
digital signal, a pseudo half-tone processing circuit 6 for processing a 
digital signal so as to provide a pseudo half-tone output (e.g. by using 
an error diffusion method well known in the prior art), a binarizing 
circuit 8 which thresholds multi-level image data in accordance with 
threshold reference data, and a selector circuit 7 for selecting either 
the binary coding circuit 8 or the half-tone processing circuit 6, for 
routing to, for example, a binary printer. 
The reader/printer also comprises a light source 9 such as a halogen lamp, 
for illuminating a microfilm for reading by the image sensor 1, and a 
light source drive circuit 10 for controlling the quantity of light 
emitted by the light source 9 by controlling the electric power for 
energizing thereof, according to digital light quantity data EN or EP. The 
light source 9 may be as described in EP0267793, for instance. 
A control unit 11 (for example, a microcomputer unit comprising a 
microprocessor and associated ROM and RAM storage) is connected to a 
control panel 12 to receive control signals provided by a user operating 
the control panel 12, and is connected to provide control data to the 
density correction circuit 4, the edge enhancement circuit 5 and the 
selector 7. 
In operation, the reader/printer illuminates a microfilm via the light 
source 9, and an enlarged projected image of the microfilm is scanned line 
by line by the image sensor 1 to provide a line by line analog serial 
image signal which is amplified by the amplifier 2 to a suitable range to 
provide a signal input to the A/D converter 3. The amplifier 2 may also 
provide any offset voltage adjustments required. 
The analog signal input to the A/D converter 3 is converted into a 
multi-bit (for example 8 bit) digital signal between 0-255 (hexFF) coupled 
to the input of the density correction circuit 4. 
Density Correction Circuit 4 
The circuit 4 comprises, for example, a look-up table ROM as shown in FIG. 
2; in this case the input signal is coupled to the address lines of the 
ROM density correction circuit 4, and an output signal is extracted from 
the data bus terminals D0-D7 thereof. 
Also provided from the density correction circuit 4 (as an output thereof) 
is the light quantity data EN-EP to drive the light source drive circuit 
10. 
Also coupled to the input (address) lines of the density correction circuit 
4 is a multi-bit characteristic selection signal SEL (for example a 5 bit 
signal), a single bit signal (N/P) indicating. Whether the microfilm 
represents a positive or negative original, and a single bit signal 
indicating whether the microfilm image is a character image or a 
photograph image. 
The density correction controlling signals SEL, N/P and 
character/photograph are supplied, as discussed in greater detail below, 
in dependence upon controls on the control panel 12. 
FIG. 8 shows the relationship between input data values and output data 
values from the density correction circuit 4. The density correction 
circuit 4 provides an output data value which is related to the input data 
value by a gamma correction curve. FIG. 8 shows two types of gamma 
correction curves. A first type (labelled "A") is used with negative 
original films. In this case, the gamma correction curve runs between an 
upper point with coordinates hex (FF,FF) and a lower point with 
coordinates (.delta..sub.0,0). Thus, when the film contains an original 
negative image, the range of input data values between .delta..sub.0 and 
FF is expanded to an output data range of 0-FF, and low input data values 
are suppressed. 
The curve B is defined by an upper point (.delta..sub.FF, FF) and a lower 
point (0,0), and corresponds to a positive original film. In this case, 
input data values between 0 and .delta..sub.FF are expanded to fill the 
output data range between 0 and FF, and high input data values are clipped 
to FF. As shown, the gamma correction curves may be linear, or could be 
non-linear functions defined by the corresponding point .delta..sub.FF or 
.delta..sub.0 ("rise points"). 
The density correction circuit 4 embodies a plurality of different density 
conversion characteristics of the type illustrated in FIG. 8; a first 
subset being for use with N type films and a second Subset for use with P 
type films. The characteristic curve which is selected for processing the 
image signal depends upon the control signals input to the density 
correction circuit 4; the N/P signal selects whether a characteristic of 
the type "A" corresponding to a negative film or of the type "B" 
corresponding to a positive film is selected, and the SEL signal (and 
optionally the character/photograph signal) selects the value of the rise 
point defining the slope (and hence the input data value range) of the 
gamma correction applied by the density correction circuit 4. 
As mentioned above, the amount of light emitted by the light source 9 is 
also controlled in accordance with same control signals N/P, SEL and, 
optionally, character/photograph; conveniently the light quantity EN/EP is 
supplied from the output of the density correction circuit 4. 
FIG. 3 shows, for a negative film with a character image, the variation of 
.delta..sub.0 (defining the slope of the gamma correction applied by the 
density correction circuit 4) and the light level E to be applied by the 
light source 9, in dependence upon the density level selected by the 
operator from the control panel 12. FIG. 4 correspondingly shows a curve 
of rise point and light level E for a positive film. Data corresponding to 
FIGS. 3 and 4 is stored in the density correction circuit 4 and is 
accessed by the SEL signal to vary the light level and gamma correction 
selected in dependence upon the density levels selected by the user. 
Control Panel 12 
FIG. 5 is a diagram of the construction of the control panel 12. The 
control panel comprises, from the right hand side, a print button 51 for 
initiating printing; print density selection buttons 52 and an associated 
LED indicator array 53, so that the operator can increase or reduce the 
print density ("thick" and "thin" respectively), print run number setting 
buttons 54 for increasing, reducing or cancelling the number of prints 
selected; a film type selection button 55 and associated indicator lamps, 
for selecting between negative originals, positive originals or automatic 
original selection; a character/photograph selection button 56 for 
selecting either character or photograph mode, and associated indicators; 
and a sharpness selection button 57 and associated indicators for 
selecting the sharpness of reproduction of the image. The state of the 
buttons is selected by the operator according to preference. The outputs 
of the buttons are connected to the control unit 11, which monitors the 
button states and sets corresponding print density level data (SEL), 
sharpness data (SHARP), character/photograph indication data and 
negative/positive original data (N/P). 
Edge Enhancement Section 5 
Referring back to FIG. 1, the digital signal output from the density 
correction circuit 4 is input to the edge enhancement circuit 5 and is 
processed by digital filtering using a Laplacian convolution (3.times.3) 
mask shown in FIG. 6, for example. The mask can be seen to execute two 
dimensional high pass spatial filtering. The degree of edge enhancement 
depends upon the mask coefficients shown in FIG. 6, and the degree of edge 
enhancement is varied by varying sharpness data .alpha. in accordance with 
the sharpness selected by the button 57 on the keyboard 12; the edge 
enhancement circuit 5 therefore permits the varying of the Laplacian 
convolution mask coefficients. The mask need not, of course, be a 
3.times.3 mask; it may be a 5.times.5 pixel mask, for example. Likewise, 
diagonal filtering coefficients may also be present as shown in FIG. 7. 
The edge enhancement circuit 5 therefore comprises line buffer or picture 
buffer means for retaining the two dimensional coefficients of neighboring 
lines and arithmetic means for performing the convolution. 
The edge enhancement circuit 5 could provide a fixed degree of edge 
enhancement (although this is not a preferred embodiment). The edge 
enhancement circuit 5 could comprise a plurality of edge enhancement 
stages, to increase the degree of edge enhancement. 
In a preferred embodiment, the degree of edge enhancement is changed 
according to whether the film is a positive or negative original film, as 
disclosed in the above referenced Japanese laid open applications and U.S. 
unpublished application. The reason for this is that dust, extraneous 
material, scratches or the like have an image density close to that of 
line image portions in the case of a positive film, and are hence more 
noticeable if the degree of edge enhancement is increased, whereas in a 
negative film the density of such unwanted or extraneous image portions is 
closer to that of the background and consequently is less noticeable even 
at higher degrees of edge enhancement; accordingly a higher degree of edge 
enhancement is preferred if the film has a negative original, and the 
sharpness data .alpha. is changed accordingly in dependence upon the 
negative/positive data generated by the control unit 11. 
The sharpness of the film also varies according to the nature of the 
original prior to recording on the film; the resolution of the camera lens 
which imaged the original onto the film; the extent of defocus; the kind 
of film; and development materials and conditions, and accordingly the 
sharpness control button 57 on the control panel 12 permits the user to 
continuously vary the degree of sharpness (i.e. the magnitude of the 
coefficients of the Laplacian convolution mask) to suit the user's 
preference. 
The degree of edge enhancement is preferably also selected depending upon 
whether the image is a character image or a photograph image, as selected 
by the button 56 on the control panel 12. In the photograph mode, the 
Laplacian convolution mask coefficients are set to be about half of their 
value in the character mode, so as to increase the gradation effect, by 
changing the sharpness data .alpha.. 
Half-Tone Processing Circuit 6 
The signal, edge enhanced by the edge enhancement circuit 5, is input to 
the half-tone processing circuit 6 and, in parallel, the binary coding 
circuit 8 for processing by pseudo half-tone processing or sample binary 
coding. Preferably, the half-tone processing circuit 6 comprises an error 
diffusion circuit. In such a circuit, the density of each pixel is 
compared with a certain predetermined threshold value, and the error 
between the two is diffused to neighboring pixels, to affect the density 
of the neighboring pixels. For example, the target pixel may be quantized 
to the predetermined threshold value, and predetermined portions of the 
quantizing error added to the neighbouring pixels. This method as is 
typical of pseudo half-tone processing methods, which also include, for 
example, dither quantization methods. 
Binary Coding Circuit 8 
The binary coding circuit 8 compares each multi-level (e.g. 8 bit) pixel 
Signal with predetermined reference data (representing a threshold level) 
and generates a corresponding binary pixel output (e.g. depending upon 
whether the multi-level pixel data is greater or less than the 
predetermined threshold). 
Mode Selector Circuit 7 
The mode selector circuit 7 selectively passes either the output of the 
half-tone processing circuit 6 or the binary coding circuit 8 to an output 
port for connection to a binary printer or other binary output device, 
under control of the ASEL signal from the control unit 11. A half-tone 
processed image has an improved gradation characteristic to the human 
viewer, and this effect is particularly high in a case of a photographic 
image. It is also possible to provide a density difference between thick 
and thin characters in the case of a line image. However, the effect of 
pseudo half-tone processing in a binary printer (in which on-off dots are 
determined according to a binary input signal, and all dots have a uniform 
density) reduces the spatial resolution of the image slightly, and it is 
therefore preferable to inhibit the half-tone processing when the film 
image is comprised only of lines (e.g, as a character image). The control 
unit 11 is therefore arranged to select either half-tone processing, or 
binary coding, via the selector unit 7, in response to operation of the 
character/photograph selection button 56 on the keyboard by the user. 
Deriving the Gamma Correction Data 
The process by which the gamma correction data used in the density 
correction circuit 4 was derived by the inventors will now be described. 
The rise points .delta..sub.0 and .delta..sub.FF which are illustrated on 
FIG. 8 and are plotted in FIGS. 3 and 4 will first be described. The rise 
points are related to density levels of the film image which can be used 
to separate the image from the noise level of the background. 
.delta..sub.0 for a negative film is a low value near the background 
level, so that background noise density variations are cut or suppressed 
(portion B of the curve of FIG. 9) whilst image portions are reproduced 
with suitable fidelity (portion A of FIG. 9). Likewise, for a positive 
original, .delta..sub.FF is a high density level close to the background 
density level, so that background noise above this level is cut whilst 
reproducing image data below this level. 
In FIG. 8, the gamma correction curve is shown as a straight line 
connecting 00 and the rise point (for a positive film) or FF and the rise 
point (for a negative film). The slope of the density correction curve 
varies if the rise point value varies, and thus a suitable image can be 
obtained by a user by changing the rise point, according to the kind of 
film and user selected density. The straight line correction curves shown 
in FIG. 8 are provided as an aid to Understanding. If the A/D converter 3 
provides a logarithmic conversion, as in some types of known image 
processing apparatus, this linear curve can suitably provide gamma 
correction of the logarithmic A/D output. If the input data is not 
non-linearly converted in this way, however, the gamma correction curves 
shown in FIG. 8 are preferably non-linear (e.g. generally exponential) in 
shape but, as described above, are defined by the rise points and either 
00 or FF. 
FIG. 10(A) is a diagram showing the frequency (n) of appearance of data of 
each density level on a negative film on which a character image is 
provided, and FIG. 10(B) is a corresponding diagram showing the frequency 
(n) of occurrence of image density levels on a positive film; in other 
words, these diagrams are density histograms. In each case, the film has a 
predetermined base density D and is illuminated with light of a 
predetermined level. 
In FIG. 10(A), for a negative film, the solid line indicates the density 
histogram for a film of transmission density D=1.4 with respect to the 
background portion, and the broken line indicates a film in which the 
transmission density D is 0.6, and the solid and broken curves in FIG. 10b 
correspond. 
It will be seen that in each case, a higher peak corresponds to background 
data, and lower peaks correspond to the image (character) portion of the 
film. In the case of the negative film illustrated in FIG. 10(A), the rise 
point .delta..sub.0 is set to a point approximating to the inflection 
point between the background portion and the character portions of the 
histogram, and likewise the rise point .delta..sub.FF is set at this 
inflection point in the positive film of FIG. 10(B). 
FIGS. 11(A) and 11(B) show the variation of the rise point values with 
amount of light with which the film is illuminated, in the case of films 
of different transmission densities D. It will be seen that in each case 
the rise point varies monotonically with increasing light level. In each 
of FIGS. 11(A) and 11(B), the abelsea represents the quantity of light E 
(LUX) and the ordinate represents the value of the rise point; 
.delta..sub.0 in FIG. 11(A) for the negative image and .delta..sub.FF for 
the positive image in FIG. 11(B). The curves shown in FIG. 11(A) and 11(B) 
were derived by using a plurality of different evaluation test figures 
photographed on microfilm, including typed characters, characters written 
with a pencil, gradation step frames having densities from solid black to 
half turn, and a mixture of different types of characters such as thin and 
thick line characters. 
In FIGS. 11(A) and 11(B), firstly, rise points are determined from density 
histograms of the types shown in FIG. 10(A) and 10(B) for each of a 
plurality of different light levels, for each of several different film 
densities (D=0.6-D=1.4). 
In each case, however, one particular light level is found, by human 
inspection, to give the best reproduction, and from inspection of FIGS. 
11(A) and 11(B), this optimum light level and rise point combination is 
indicated as the intersection of the dashed line with each of the rise 
point curves corresponding to different film base densities. Data values 
of optimum rase point .delta..sub.0 =70 hex, 48 hex and 20 hex are 
obtained for films of base-densities D=0.6, 1.0 and 1.4, respectively, in 
the case of the negative film of FIG. 11a, and values of .delta..sub.FF 
=60 hex, 70 hex and 90 hex for films of D=0.6, 1.0 and 1.4 respectively 
are obtained in the case of the positive film of FIG. 11(B). 
Referring now to FIGS. 3 and 4, on these figures the density level setting 
F set by the density control buttons 52 on the control panel 12 is plotted 
as the abcissa and the optimum light quantity E and optimum rise point 
.delta. are plotted as the ordinate, being shown as a chain--dash line and 
a solid line respectively. FIG. 3 is a plot for a negative original image, 
and FIG. 4 is a corresponding plot for a positive original image. 
The black triangle corresponds to that on the control panel 12 and 
indicates the direction in which density increases whereas the white 
triangle indicates the direction in which density decreases. 
To derive FIGS. 3 and 4, the film base density values D=0.6, 1.0 and 1.4 
are converted to a print density scale F, and for each film base density 
the corresponding rise point .delta.0 or .delta.FF, and light level E are 
plotted. The light level E in FIG. 3 for a negative film is denoted EN, 
and the light level E in FIG. 4 for a positive film is denoted EP. 
It will be seen from FIG. 3 that for a negative film, the rise point 
.delta..sub.0 decreases with increasing density level whereas the light 
required E rises with increasing density level; likewise it will be seen 
from FIG. 4 that the rise point .delta..sub.FF increases with increasing 
density level whereas the light level decreases. 
After the curves of FIGS. 3 and 4 have been derived, data corresponding to 
the rise point curve and the light level curve in each figure is tabulated 
as shown in Table 1 for storage in the density correction circuit 4. 
Table 1 illustrates the combined data shown in FIGS. 3 and 4, in an 
exemplary form in which it may be stored in the density correction 4. The 
value F represents the image density set by the density selection button 
52 on the control panel. The density increases towards F1 and reduces 
towards F33. EN and EP, as discussed above, represent respectively the 
optimum light level for negative and positive originals, in hexadecimal 
notation. .delta..sub.0 and .delta..sub.FF represent the rise points 
respectively for a negative and a positive film in hexadecimal notation. 
It will be seen that EN and EP are reverse--symmetrical; EN varies 
logarithmically with F from a higher value at F1 to a low value at F33, 
whereas EP varies logarithmically in the reverse sense. Thus, for a given 
user selected density level F and original type N-P, a unique pair of rise 
point (defining the gamma correction curve) and light level settings are 
derived from the look-up table corresponding to Table 1 and employed to 
operate the image reader. 
______________________________________ 
F EN .gamma.O EP .gamma.FF 
______________________________________ 
1 B5 14 0F A0 
2 AA 15 12 9E 
3 A0 17 14 9C 
4 97 18 16 99 
5 8E 1A 18 97 
6 86 1C 1B 95 
7 7E 1E 1D 93 
8 76 20 20 90 
9 6F 22 23 8E 
10 68 24 26 8B 
11 62 27 2A 89 
12 5C 2A 2D 86 
13 56 2D 31 83 
14 5D 3D 34 81 
15 4B 33 38 7E 
16 46 37 3D 7B 
17 41 3A 41 78 
18 3D 3F 46 75 
19 38 43 4B 72 
20 34 48 5D 6F 
21 31 4D 56 6C 
22 2D 52 5C 68 
23 2A 58 62 65 
24 26 5E 68 62 
25 23 65 6F 5E 
26 20 6C 76 5B 
27 1D 73 7E 57 
28 1B 7B 86 53 
29 18 84 8E 50 
30 16 8D 97 4C 
31 14 97 A0 48 
32 12 A2 AA 44 
33 0F AD B5 3F 
______________________________________ 
Second Embodiment 
A suitable density correction process improving in handling for the user, 
based on the above-described experimental results in accordance with a 
second embodiment of the present invention, will be described below. 
FIG. 12 shows .delta. characteristic curves of the second embodiment, 
improved from the .delta. characteristic curves for, in particular, 
negative films in accordance with the first embodiment. In the first 
embodiment, a wide-range arrangement with non-linear curves of FIGS. 3 and 
4 is adopted to cover all films (densities, kinds, and so on) with respect 
to the density level variable range. However, the second embodiment is 
arranged by considering printing with fidelity in a central film density 
range of 0.8 to 1.2 in particular and ease of density control. 
At present, a system including camera photographing techniques and film 
development techniques for forming films with a finished base portion 
density ranging from 0.8 to 1.2 has generally been established to provide 
laboratory services improved in terms of commercial value. There is 
therefore a problem of density control operability and image 
reproducibility with respect to the image output operation of outputting 
an enlarged film image to a printer. 
Thus, the density dial operation is performed according to a user's 
preference. However, it is important for the image output to be free from 
fog, on the paper base portion, and to have the image density level stably 
maintained while preventing any abrupt density change (an abrupt increase 
in fog, an abrupt reduction in density, or the like) when the density dial 
52 is changed by one step (density increasing or reducing direction) so as 
to bring the rise point into the fog level. 
In FIG. 12, inflection points (2) and (3) of the dot-dash line A and the 
solid line B correspond to the film density range D=0.8 to 1.2, the 
inflection point (2) corresponding to the limit 1.2 and the inflection 
point (3) corresponding to the limit 0.8. The region (1)-(2) is set to 
cover film densities higher than 1.2, and the region (3)-(4) is set to 
cover film densities lower than 0.8. 
That is, the density dial operation is performed according to the user's 
preference, and an image output can be obtained without fog on the paper 
base portion while the image density level is stably maintained. Also an 
abrupt density change (an abrupt increase in fog, an abrupt reduction in 
density, or the like) can be prevented when the density dial 52 is changed 
by one step (density increasing or reducing direction). Over the density 
region 0.8 to 1.2, the rise point is varied depending on the user density 
setting from the control 52. Outside this range, the rise point is held 
constant and instead the light level E is varied to change the density. If 
a fine control is required according to user's preference, the density 
dial can thus be moved by one step to suitably change the density without 
the sudden appearance of fog. 
FIG. 13 corresponds to FIG. 12 and shows .delta. characteristic curves of a 
third embodiment arranged to suitably process data from films having a 
finished film density distribution which is considerably one-sided on the 
high-contrast side or low-contrast side. 
At present, a system including camera photographing techniques and film 
development techniques for forming films with a finished base portion 
density ranging from 0.8 to 1.2 has generally been established to provide 
laboratory services improved in terms of commercial value. However, if a 
microfilm is used for a long time or exposed for a long time in some 
maintenance/handling processes, or if a master microfilm having 
photographed images is used to obtain to duplicate films, the transmission 
density of the original microfilm is changed (it usually tends to 
decrease). An arrangement for enabling user operation in such a case is 
therefore desirable. There is therefore a problem of density control 
operability and image reproducibility with respect to the image output 
operation of outputting an enlarged film image to a printer. 
In FIG. 13, the region (3)-(4) corresponding to a film density range of 1.0 
to 1.2, the region (4)-(5) is set to cover a film density range of 1.2 to 
about 1.4, and the region (5)-(6) is set to cover a film density range of 
1.4 or higher. The region (2)-(3) is set in correspondence with a film 
density range of 0.8 to 1.0, and the region (1)-(2) is set in 
correspondence with a film density range of 0.6 to 0.8. Printing images 
with fidelity in a film density range of 0.6 to 1.4 is thereby enabled 
while considering ease of density adjustment. 
This embodiment has the same effects as the second embodiment. It is also 
arranged to obtain gamma correction curves further improved in 
reproduction fidelity and in adaptability for user's preference, and to 
achieve high gradation performance and high reproducibility even if the 
density of the film varies as described above. 
In the above-described embodiments, image processing is performed by 
interposing the density correction circuit 4 between the edge enhancement 
circuit 5 and A/D converter 3. To enhance the edge level and to improve 
image sharpness while further increasing the degree of fidelity, a gamma 
correction circuit may be provided for operation after edge enhancement. 
The error diffusion method has been described as an example of a pseudo 
half-tone processing method, but this is not exclusive. Any method can be 
used so long as a gradation effect can be achieved. 
As described above, in a digital reader printer, optimum .delta. curves are 
selected for any of negative and positive films and the quantity of light 
from the light source is changed according to the selected curves. It is 
thereby possible to easily obtain a copy having an improved gradation 
effect from a negative or positive film. Further, a photographic image and 
a character image can be discriminated from each other to form an image 
according to the .delta. characteristic of the original image. It is 
therefore possible to obtain a high-gradation printed image from a 
high-.delta. photographic microfilm. 
A laser printer as disclosed in U.S. Pat. No. 4,700,237 may be provided as 
the printer to obtain a copy from the disclosed image reader, or a 
different type of printer may be used. The gamma correction circuit need 
not comprises a look-up table embodied in ROM, PROM, EPROM, etc. but could 
equally comprise a processor (e.g. an MPU) operating in accordance with a 
stored algorithm. The light level signal EN/EP could be provided by a 
different ROM or a processor, in dependence on the same image control 
signals. The means for supplying the image processing control signals SEL, 
N/P and character/photograph could be an automatic control circuit for 
judging an appropriate signal value, additionally or alternatively to the 
user control panel 12. 
The invention could be used to read paper originals, for example, as well 
as film. 
While the present invention has been described with respect to what 
presently are considered to be the preferred embodiments, it is to be 
understood that the invention is not limited to the disclosed embodiments. 
To the contrary, the present invention is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims. The scope of the following claims is to be 
accorded the broadest interpretation so as to encompass all such 
modifications and equivalent structures and functions.