Image reading apparatus for reading images formed by a light transmission

An illuminator for applying light to an image, a reader for photoelectrically reading the light applied image, an amplifier for amplifying an analog signal outputted from the reader, an A/D converter for converting an analog signal outputted from the amplifier into a digital signal, a logarithmic transformer for logarithmically transforming the digital signal outputted from the A/D converter, a first controller for changing the logarithmic transformation of the logarithmic transformer, and a second controller for changing the gain of the digital signal in accordance with a change of the logarithmic transformation characteristic caused by the first controller.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image reading apparatus, and more 
particularly to a film reading apparatus for reading through light 
transmission the image printed in a silver salt film. 
2. Related Background Art 
It has been proposed heretofore to photoelectrically read images recorded 
in books, magazines and so on by using a CCD image sensor or the like to 
thereafter transmit the images to a remote location, electrically edit 
them, store them in an electronic file or the like, or perform other 
processings. 
It has also been proposed to photoelectrically read images recorded in a 
film such as a 35 mm film, microfilm and so on by using a CCD image sensor 
or the like. The present applicant has already proposed apparatus for 
reading images recorded in a film, for example, as in U.S. Pat. Nos. 
4,674,126, 4,700,237, 4,762,985, 4,837,450, 4,825,065, 4,933,983 and U.S. 
application Ser. No. 419,702 (filed on Oct. 11, 1989). 
An image in a silver salt film includes the information of very broad 
dynamic range as compared with a reflection image of an ordinary printing 
matter or the like. Printing matters has the image density range of about 
0 to 1.5, whereas the transmission density range of a film is about 0 to 
3.0 which is about two times as broad as that of printing matters. 
If an image in a film has a limited density range within the film 
transmission density range (0 to 3.0), it is common that the image only 
within the limited density range is read and converted into image signals. 
This is particularly the case for a negative film. 
Next, a method of variably changing the density range will be described 
with reference to FIG. 1. 
In the graph shown in FIG. 1, the first quadrant illustrates the 
relationship between the density of a subject to be taken with a film and 
the density of the image printed on the film. A curve 31 represents the 
film characteristic which shows that the black and white are reversed. 
The second quadrant illustrates the relationship between the film density 
and an output value obtained by reading the image and performing A/D 
conversion and logarithmic transformation. A straight line 32 indicates 
that the film density 0 to 3.0 is reversed in reading the image. 
The third quadrant illustrates how the output value after logarithmic 
transformation is changed to a signal having only a specific density 
range. 
The fourth quadrant illustrates the relationship between the final image 
signal and the original camera subject density. 
If a subject has a low contrast and flat image, the subject density range 
is restricted to .DELTA.D.sub.0, whereas if a subject has a high contrast 
and large luminance difference, the subject density range is restricted to 
.DELTA.D.sub.0 ". 
The subject density ranges .DELTA.D.sub.0, .DELTA.D.sub.0 ', and 
.DELTA.D.sub.0 " correspond to the film density ranges .DELTA.D.sub.f, 
.DELTA.D.sub.f ' and .DELTA.D.sub.f ". The image of a subject is read by 
restricting the film density range to one of three ranges .DELTA.D.sub.f. 
.DELTA.D.sub.f ' and .DELTA.D.sub.f ". Such change is carried out by using 
one of the straight lines 33, 34 and 35 shown in the third quadrant in 
FIG. 1. 
The changed output signal is represented by one of the straight lines 36, 
37 and 38 shown in the fourth quadrant. 
The output after A/D conversion does not correspond to the film density D, 
but to the film transmittance T. The relationship between the film density 
D and the film transmittance T is given by the following equation (1): 
EQU D=-log T (1) 
The change in image signal by using the straight line 32 in the second 
quadrant shown in FIG. 1 and one of the straight lines 33, 34 and 35 can 
be efficiently performed at the same time by using a logarithmic 
transformer 109 shown in FIG. 2. 
FIG. 2 shows a film reading apparatus employing such efficient processing. 
In FIG. 2, reference numeral 103 represents a film to be read, 101 a lamp, 
and 102 a condenser lens, these elements constituting transmission type 
illumination means which transmission-illuminates the film 103 through the 
Kohler's illumination. Reference numeral 104 represents a focusing lens 
for focusing an image transmitted from the silver salt film 103 onto the 
surface of photoelectric conversion means in the form of a photoelectric 
conversion element 105 such as a charge storage type CCD line sensor, area 
sensor or the like. If a CCD line sensor is used as the photoelectric 
conversion element 105, the silver salt film 103 or the CCD line sensor is 
moved by a driver mechanism (not shown) to read the entire area of an 
image. 
Reference numeral 106 represents an amplifier which amplifies an output 
from the photoelectric conversion element 105 to a predetermined output 
level. Reference numeral 107 represents a sample/hold circuit which 
samples and holds the an output signal from the amplifier 106 on the pixel 
unit basis. Reference numeral 108 represents an A/D converter which 
converts an output signal from the sample/hold circuit 107 into a digital 
signal. Reference numeral 109 represents a logarithmic transformer serving 
as logarithmic conversion means which stores a plurality of logarithmic 
conversion tables for different reading density ranges and operates to 
select a conversion table corresponding to one of the curves 41, 42 and 43 
shown in FIG. 3 in accordance with a density range select signal, i.e., 
address signal (digital value) to thereby logarithmically transform the 
digital signal corresponding to the transmittance T from the A/D converter 
108. The logarithmic transformer 109 outputs a final image signal. 
The curves 41, 42 and 43 shown in FIG. 3 correspond to the lines 33, 34 and 
35 shown in FIG. 1. 
The graph shown in FIG. 3 is used for transformation between digital 
signals. As seen from FIG. 3, the input dynamic range of the curve 41 is 
narrower than that of the curve 43 so that bits are likely to be missed 
during signal transformation, resulting in a problem that the quality of a 
final image may be lowered. 
As a means for solving such a problem, there is known a method wherein 
logarithmic transformation is carried out in an analog fashion by using a 
logarithmic amplifier or the like prior to the A/D conversion. Although 
this method solves the above problem of lowering the final image quality, 
there arises a new problem that the apparatus becomes bulky. 
SUMMARY OF THE INVENTION 
The present invention has been made in consideration of the above problems. 
It is an object of the present invention to provide an image reading 
apparatus capable of properly reading images having a different contrast. 
It is another object of the present invention to provide an image reading 
apparatus capable of reading an image in a proper manner matching a set 
image density range. 
It is a further object of the present invention to provide an image reading 
apparatus capable of properly reading an image recorded in a film. 
The above and other objects and advantages of the present invention will 
become apparent from the following detailed description when read in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
The embodiments of this invention will now be described with reference to 
the accompanying drawings. 
FIG. 4 shows an embodiment of a film reading apparatus embodying the 
present invention. 
In FIG. 4, elements 101 to 109 are similar to those shown in FIG. 2. The 
film 103 is assumed to be a negative film. Reference numeral 113 
represents an operation unit which is acted upon by an operator. The 
operator uses this operation unit to select a contrast of an image to be 
read which has been recorded in the film. Reference numeral 114 represents 
a CPU which outputs an address signal indicative of the selected contrast 
in accordance with the contrast select signal from the operation unit 113. 
Reference numeral 111 represents a memory into which look-up tables have 
been written, the look-up tables being used for outputting an A/D 
reference value corresponding to the address signal sent from CPU 114. 
This address signal from CPU 114 is also supplied to a logarithmic 
transformer 109 to thereby select one of a plurality of logarithmic 
transformation characteristics prepared beforehand in the logarithmic 
transformer 109. 
Therefore, upon selection of a contrast by the operation unit 113, the 
logarithmic transformation characteristic of the logarithmic transformer 
109 is changed and the reference value for A/D conversion is 
correspondingly changed. 
In the manner as described above, the address signal from CPU 114 is 
supplied to the memory 111 which stores look-up tables. An output from the 
memory 111 is passed through D/A converter 112 and converted into an 
analog signal which is applied to the A/D converter 108 as its reference 
voltage for A/D conversion. As the reference voltage becomes small, the 
input signal for A/D conversion is apparently amplified by 
(10.sup.D.sbsp.fc /10.sup..gamma..DELTA.D/2) times. The memory 111 and D/A 
converter 112 constitute conversion amount changing means. 
Next, referring to FIG. 5, a method of changing the density range will be 
described. 
Specifically, assuming that the camera subject density ranges change among 
.DELTA.D.sub.0, .DELTA.D.sub.0 ' and .DELTA.D.sub.0 " corresponding to the 
contrasts to be selected by the operation unit 113, the characteristics 
shown in the third quadrant are changed among straight lines 14, 15 and 
16. The characteristics shown in the second quadrant are correspondingly 
changed among straight lines 11, 12 and 13. The transformation according 
to the straight lines 14, 15 and 16 is digital signal transformation as 
conventional. The transformation according to the straight lines 11, 12 
and 13, however, can be carried out in an analog fashion prior to A/D 
conversion. The change of the straight lines 11, 12 and 13 corresponds to 
parallel motion thereof with respect to logarithmic scale, so that it is 
no less than multiplication of the film transmittance by a predetermined 
magnification factor. Specifically, this can be achieved by changing the 
reference voltage of the A/D converter 108 shown in FIG. 4. This can also 
be achieved by changing the light amount of the illumination lamp, the 
amplification factor of the amplifier, or the storage time of CCD. 
Next, the method of changing the density range will be detailed with 
reference to FIG. 6. 
FIG. 6 illustrates the first and fourth quadrants of the graph shown in 
FIG. 5. The ordinate of the fourth quadrant is represented by a maximum 
digital value 1 which corresponds to 255 is an eight bit digital signal is 
used. 
In FIG. 6, the relationship between the subject density D and the film 
density D.sub.f can be expressed by the following equation (2): 
EQU D.sub.f =-.gamma. (D-D.sub.c)+D.sub.fc (2) 
where D.sub.c is the center value of the subject density range, and 
D.sub.fc is the film density corresponding to the center value D.sub.c. 
The final signal value S can be given by the following equation (3) by 
using the subject density D: 
EQU S=(1/.DELTA.D)(D-D.sub.c)+0.5 (3) 
The relationship between the film density D.sub.f and the final output 
signal S is given by the following equation (4) by deleting D from the 
equations (2) and (3): 
EQU S=0.5-(1/.gamma..DELTA.D)(D.sub.f -D.sub.fc) (4) 
As described previously, since the output from CCD or the like represents 
not the film density D.sub.f but the film transmittance T.sub.f, D.sub.f 
=-log T.sub.f is substituted into the equation (4) which is then arranged 
to obtain the following equation (5): 
EQU S=1-{-(1/.gamma..DELTA.D)log(T.sub.f (10.sup.D.sbsp.fc 
/10.sup..gamma..DELTA.D/2))} (5) 
The equation (5) means that after amplifying the film transmittance T.sub.f 
by (10.sup.D.sbsp.fc /10.sup..gamma..DELTA.D/2) times, a logarithmic 
transformation y=-(1/.gamma..DELTA.D) log x is carried out and then 
reversion of "0" and "1", i.e., reversion of negative and positive is 
carried out. As described previously, the amplification by 
(10.sup.D.sbsp.fc /10.sup..gamma..DELTA.D/2) times is carried out in an 
analog fashion, e.g., by changing the reference voltage of the A/D 
converter 108, and the logarithmic transformation and negative/positive 
reversion are carried in a digital fashion by the logarithmic transformer 
109 using the logarithmic transformation table. It becomes therefore 
possible to alleviate bit missing during digital operation as much as 
possible, independently of the value of .DELTA.D. 
FIG. 7 shows the contents of logarithmic transformation tables prepared in 
the logarithmic transformer 109. The tables have similar contents to those 
shown in FIG. 3. Curves 61, 62 and 63 correspond to .DELTA.D.sub.0, 
.DELTA.D.sub.0 ' and .DELTA.D.sub.0 ". Although bit missing is more or 
less large for the range .DELTA.D.sub.0, it is considerably improved as 
compared with FIG. 3. 
In the above embodiment, a negative film has been used by way of example. A 
reversal film may also be used with similar advantageous effects. 
Specifically, curves 70 to 79 shown in FIG. 8 correspond to the curves 10 
to 19 shown in FIG. 5, and the relationship between the final output 
signal S and the film transmittance T.sub.f can be expressed by the 
following equation (6): 
EQU S=-(1/.DELTA..DELTA.D)log(T.sub.f (10.sup.D.sbsp.fc 
/10.sup..gamma..DELTA.D/2)) (6) 
In accordance with this equation, the transmittance signal T.sub.f is 
amplified by (10.sup.D.sbsp.fc /10.sup..gamma..DELTA.D/2) and then 
subjected to logarithmic transformation. In the case of a reversal film, 
it is not necessary to carry out a process of reversion of 
negative/positive. 
Further, in the above embodiment, the amplification amount at the A/D 
converter 108 has been controlled in accordance with the selected density 
range. Instead of this, the light amount of the illumination lamp 101 may 
be controlled in accordance with the selected density range by using the 
arrangement in FIG. 9. 
In FIG. 9, similar elements to those shown in FIG. 4 are represented by 
using identical reference numerals. The difference from that shown in FIG. 
4 is that a lamp driver 115 for the lamp 101 is controlled in accordance 
with an output from the D/A converter 112 for changing the power to the 
lamp 101. Namely, the logarithmic transformation is changed in accordance 
with a contrast selected by the operation unit 113 to correspondingly 
change the light amount of the lamp 101. It is needless to say that light 
amount values corresponding to selected contrasts are previously written 
in the memory 111. 
Further, instead of controlling the amplification at the A/D converter 108, 
the storage time of CCD 105 may be controlled in accordance with a 
selected density range by using the arrangement shown in FIG. 10. 
In FIG. 10, like elements to those shown in FIG. 4 are represented by using 
identical reference numerals. The difference from that shown in FIG. 4 is 
that a CCD driver 116 is controlled in accordance with an output from the 
D/A converter 112 to change the storage time of CCD (photoelectric 
conversion element) 105. Namely, the logarithmic transformation is changed 
in accordance with a contrast selected by the operation unit 113 to 
correspondingly change the storage time of CCD 105. It is needless to say 
that storage times corresponding to selected contrasts are previously 
written in the memory 111. 
Still further, instead of controlling the amplification at the A/D 
converter 108, the amplification factor of the amplifier 106 may be 
controlled in accordance with a selected density range by using the 
arrangement shown in FIG. 11. 
In FIG. 11, like elements to those shown in FIG. 4 are represented by using 
identical reference numerals. The difference from that shown in FIG. 4 is 
that the amplification factor of the amplifier 106 is changed with an 
output from the D/A converter 112. Namely, by previously writing into the 
memory 111 the data indicative of the amplification factors corresponding 
to contrasts to be selected by the operation unit 113, the logarithmic 
transformation is changed in accordance with a contrast selected by the 
operation unit 113 to correspondingly change the amplification factor. 
As described with the embodiments shown in FIGS. 9 to 11, amplification in 
an analog fashion is possible by changing the light amount of the lamp 
101, the storage time of CCD 105, the amplification factor of the 
amplifier 106, or the like factor, thereby allowing proper processing 
without bit missing. 
In the arrangements shown in FIGS. 9 to 11, an output from the memory 111 
is supplied to various elements via the D/A converter 112. Instead of 
using the D/A converter 112, the digital signal outputted from the memory 
111 may be directly supplied to necessary elements for such control 
operation. 
It is also possible to use a combination of two or more arrangements shown 
in FIG. 4 and FIGS. 9 to 11, thereby achieving finer control operation. 
Further, in the above embodiments, the logarithmic transformation 
characteristic and analog amplification factor (gain) have been controlled 
in accordance with the contrast information entered from the operation 
unit 113 by an operator. The invention is not limited thereto. For 
example, the logarithmic transformation characteristic and analog 
amplification factor (gain) may be automatically controlled in accordance 
with the measured density range of a film image, the measurement of the 
film image being carried out, e.g., by pre-scanning the film image under a 
predetermined condition and forming a histogram of output values from CCD 
105. 
As described so far, without making large the apparatus dimension, it is 
possible to provide a film reading apparatus allowing an improved final 
image quality. 
The present invention has been described in connection with the preferred 
embodiments. The invention is not limited to such embodiments only, but it 
is needless to say that various changes and modifications are possible 
without departing from the scope of appended claims.