Scanning type optical microscope

In order to make it feasible to obtain a differential phase image having a uniform brightness and good quality, a scanning type optical microscope comprises: a laser light source; an objective lens for collecting, onto an object under observation, a light beam emitting from the laser light source; a scanning light deflector disposed between the laser light source and the objective lens; a light detector comprised of a plurality of photoelectric converters receiving a light from the object and being separated into two sections; a signal processing circuit for calculating the difference between the signals coming from the two sections of the light detector to thereby obtain a differential phase signal; and adjusting means for adjusting the differential phase signal with a signal varying with an image height and being synchronous with the scanning of the light beam.

BACKGROUND OF THE INVENTION 
(a) Field of the invention 
The present invention relates to a scanning type optical microscope which 
is based on the system for conducting the scanning of an object with a 
beam of light. 
(b) Description of the prior art 
There have been proposed scanning type optical microscopes which are of 
many advantages over ordinary microscopes that images of good contrast are 
obtained because of the absence of scattered lights coming in various 
other directions than from the picture elements per se which are the 
target for observation, or that special and effective images can be formed 
by relying on such techniques as the confocal method or the differential 
phase difference method, or further that various kinds of physical 
phenomena can be imaged such as OBIC (Optical Beam Induced Current) 
images, photo-acoustic images, etc. 
As the scanning system employed in the scanning type optical microscopes, 
there is the system that the specimen for observation is mechanically 
moved for scanning, and the system that a laser beam spot per se is moved 
to scan the stationary specimen. In case of the system arranged so that 
the observation is performed while mechanically moving the specimen, 
however, there are the drawbacks that the specimens are limited to only 
those having a small size and a light weight or those which are fixed to 
inhibit their movement caused by vibrations developing from the scanning 
operations, and further that the scanning cycles cannot be set at a much 
high rate. In view of these drawbacks encountered in the prior-art such 
microscopes, the inventor has proposed, in his U.S. Pat. No. 4,734,578, a 
scanning type optical microscope which, while being of the system to move 
the laser beam spot, is capable of forming such special images as obtained 
from the confocal technique, the differential phase difference technique, 
etc. and which allows the observation of specimens of any kind. 
Description will be made hereunder with respect to this unique system by 
referring to FIGS. 1 to 5. 
This scanning type optical microscope is so arranged that, by introducing 
into the optical microscope, the system that the surface of the specimen 
is scanned by deflecting the light beam with a light-deflector, a good 
convenience of its handling is secured as in the ordinary microscopes 
while retaining a high degree of resolving power. Also, by setting the 
light-deflector at the position of the pupil in the scanning optical 
system, the optical axis in the scanning system can be held constant even 
when the scanning by the light beam is performed by the light deflector. 
And, along therewith, by arranging, in case of detection of the 
transmitted light, the detector at a position conjugate with the pupil, it 
is made feasible to utilize the informations occurring at the pupil in 
case of off-axial light rays also. Whereby, it has been made possible to 
conduct an observation by a mere manipulation of a switch of the electric 
circuitry even in case of such a special microscopy as mentioned above. 
FIG. 1 is an illustration showing the arrangement of the scanning optical 
system and the detector of the above-mentioned prior art which takes the 
pupil into consideration. A light beam 1 coming from a laser source which 
is considered equivalently as the spot light source transmits through a 
beam splitter 2 and incides onto a first light-deflector 3. This 
light-deflector 3 is disposed at a position conjugate with the pupil 5 of 
an objective lens 4. In case no deflection of light is performed, the 
light beam 1 advances along an optical axis 6. In case light deflection is 
performed, i.e. in case scanning is performed by the light beam 1, it 
should be noted that, since the light-deflector 3 is provided at the 
position of the pupil, the direction of the light beam 1 is rendered to 
the coincident with that of an off-axial principal ray 7, and the center 
of the light beam 1 also is rendered to be coincident with the off-axial 
principal ray 7. Next, in each of these cases, the light beam passes 
through pupil relay lenses 8 and 9 and incides onto a second 
light-deflector 10 which is disposed also at the position of the pupil. 
When this light-deflector 10 is to perform the scanning only in the 
direction X among the two-dimensional scanning, the first-mentioned 
light-deflector 3 will undertake the scanning in the direction Y. By the 
employment of a light-deflector which is capable of performing the 
deflection of light in both directions X-Y, therefore, the provision of a 
single light-deflector is enough. The light beam which thus scans in two 
dimensions by the two light-deflectors 3 and 10 is caused to incide onto 
the pupil 5 of the objective lens 4 by means of a pupil projection lens 11 
and a focusing lens 12. An off-axial light beam which is formed by the 
light-deflectors 3 and 10 also has its direction and center coincident 
with the off-axial principal ray 7. Therefore, this off-axial light-beam 
also impinges exactly onto the pupil 5 of the objective lens 4. And, these 
light beams develop, on a specimen 13 by the objective lens 4, a dot-like 
spot of light which is restricted by diffraction. By performing scanning 
in both directions X and Y by the light-deflectors 3 and 10, a 
two-dimensional scanning of the specimen 13 by the dot-like light is 
carried out. 
In case the light which has transmitted through the specimen 13 is 
observed, the light is collected by a condenser lens 14 and the resulting 
light is detected by a detector 15. This detector 15 also is disposed at 
the position of the pupil. Accordingly, off-axial lights appear always at 
a same position, whereby it is possible to prevent the adversary effects 
caused by, for example, uneven sensitivity of the detector 15. Also, the 
area for the installation of the detector 15 can be greatly reduced. 
Furthermore, in case a differential type detection is performed, the 
detector 15 is formed with two detector-constituent devices 15a and 15b, 
and they are disposed symmetrically relative to the optical axis 6. In 
this case, setting is made to establish the condition that, even in the 
event of an off-axial light, the center of the beam stays coincident with 
the off-axial principal ray, whereby the detector-constituent devices 15a 
and 15b assume symmetrical positions relative to the off-axial principal 
ray also. Thus, it is possible to perform a precise differential type 
detection. 
Also, in case detection is conducted with the reflection light coming from 
the specimen 13, the light beam which has been reflected at the specimen 
13 transmits through the objective lens 4 and its pupil 5, and further 
passes through a focusing lens 12, and is focused once. This focal plane 
is the one which is used in ordinary optical microscopes to observe an 
image. Furthermore, the light beam is caused to return to the 
light-deflector 10 by the pupil projection lens 11. In this way, the 
reflection light beam returns to the beam splitter 2 by tracing back 
exactly the same course as that taken by the beam of light when it 
initially incided onto the specimen. Therefrom, the reflection beam is 
derived by the beam splitter 2 to become a detection beam 16. Since the 
reflection beam has returned after passing through the light-deflectors 10 
and 3, an off-axial scanning will not affect this detection beam 16 in any 
way. The detection beam 16 is then squeezed into a dot-like form by a 
light-collecting lens 17. Therefore, by the provision of a pin-hole 18 at 
the position where the beam is squeezed into a dot-like form, and by 
performing a detection by means of a detector 19 which is located 
rearwardly of the pin-hole 18, it is possible to obtain a flare-free image 
of a higher resolution than offered by an ordinary microscope. It will be 
needless to say that a normal image can be obtained even where the 
pin-hole 18 is not provided. Also, by the provision of a black dot-like 
light-blocking member at the position where the light beam is squeezed 
into a dot form, it is possible to easily observe a dark-field image. 
Also, by constructing the detector 19 with two detector-constituent 
devices 19a and 19b, and by disposing them at positions of the expansion 
of the light beam in symmetrical fashion relative to the optical axis, it 
is possible to conduct a differential type observation. Here, it will be 
needless to say that the signal supplied from the detector 19 can be 
converted to a visible image by such an indicator as a CRT. 
Next, description will be made hereunder in further detail with respect to 
the need for considering the position of pupil in case of the optical 
system and the detection system for scanning with a light beam. FIG. 2 
shows the instance wherein the light-deflector 3 is not provided at the 
position 20 of pupil in that region of the light-deflector 3 and of the 
pupil relay lens 8 of FIG. 1. When the incident beam 1 is deflected by the 
light-deflector 3, the center 21 of this deflected light beam is not 
coincident with the off-axial principal ray 7 which is determined by the 
objective lens 4. This indicates that the off-axial light beam does not 
precisely incide onto the objective lens 4. In FIG. 3, numeral 22 
represents the pupil of the objective lens 4. It is shown that the center 
of this pupil 22 is either the optical axis 6 or the off-axial principal 
ray 7. In this case, when the light-deflector 3 is provided at a position 
conjugate with the pupil, the scanned off-axial light beam coincides with 
the off-axial principal ray 7, and it precisely incides onto the pupil 22 
of the objective lens 4. In contrast thereto, when the light-deflector 3 
is not provided at the position of the pupil, the center 21 of the light 
beam is not coincident with the off-axial principal ray 7, so that the 
expansion 23 of the light beam becomes as shown in FIG. 3, and this 
expansion of light beam will be subjected to vignetting without exactly 
impinging onto the pupil 22. In this case, by arranging the incident beam 
to be a large light beam like the expansion 23', there will not arise a 
shortage of the amount of light, but nevertheless this is not appropriate 
for utilizing the pupil informations. 
Next, description will be made of the instance wherein the detector is not 
provided at the position of the pupil in the detection of the transmitted 
light. In FIG. 4, the light beam is projected in a dot form onto a 
specimen 25 by an objective lens 24, and the transmitted light beam is 
detected by detectors 27 and 28 which are disposed symmetrically relative 
to an optical axis 26. In case of the system for conducting the scanning 
by moving the specimen, the light beam is always located on the optical 
axis. Therefore, it is always possible to perform the differential type 
detection. However, in case of scanning with a light beam by means of a 
light deflector, there occur off-axial lights. Therefore, unless the 
detectors are provided at the position of the pupil, the positions of the 
detectors 27 and 28 will not become symmetrical relative to the off-axial 
principal ray 29. As shown practically in FIG. 4, the off-axial principal 
ray 29 is produced on the detector 28. Accordingly, it is not possible to 
obtain an accurate differential image. From the foregoing description, in 
the scanning type optical microscope using the system of conducting the 
scanning with a light beam, there is the necessity for setting a 
light-deflector at the position of the pupil of the optical system and for 
providing a detector also at the position of the pupil. By so arranging, 
special microscopy can be accomplished easily, and also there can be 
obtained an image of a high resolution. However, as is apparant from the 
foregoing description, it should be noted that, in case detection is 
performed with a reflection light, this reflection light again passes 
through the light-deflector, so that there is no restriction on the 
position of the detector. 
The above-mentioned example of the prior art is so arranged that, also in 
the system of scanning with a laser beam spot by the provision of a 
scanning means (light-deflector) and a detector at the position of the 
pupil, there can be obtained an accurate differential phase image. In 
case, however, the microscope to which this sysem is applied is an 
ordinary optical microscope, it is the usual case that the position of the 
pupil of the objective lens differs depending on the magnification or the 
type of the objective lens. Therefore, in case the scanning means and the 
detector are set in accordance with the position of the pupil of a given 
objective lens, there would occur the instance, when a different objective 
lens is used, that the detector becomes displaced from the position of the 
pupil. Also, there could occur a displacement of the position of the pupil 
arising from the setting error of the detector or from the restrictions of 
placement of the detector. For this reason, the amount of light of the 
light beams incident to the two detectors intended for the detection of 
differential phases, e.g. the detectors 15a and 15b of FIG. 1, will vary, 
respectively, depending on the height of the image as shown in FIGS. 5A 
and 10B. 
Let us here consider the case that, for example, the two detectors have 
their sizes which are substantially larger than the size of the projected 
pupil. Assuming that the radius of the pupil as p, the amount of 
displacement of the pupil from the optical axis as .delta., and the amount 
of light when there is no displacement of pupil as 
1. Then, the amount of light f(.delta.) will become: 
##EQU1## 
Here, when the sum of the output signals of these two detectors is 
calculated to obtain a normal image, these two signals cancel out each 
other so that there will develop no change in the amount of light 
attributable to the image height. In case, however, it is intended to 
obtain a differential image, the difference between the output signals of 
the two detectors is calculated. Thus, the change in the amount of light 
due to the image height will become doubled. In this latter case, there 
arises the inconvenience that, when, for example, the direction of the 
boundary line between the two detectors is set normal to the direction of 
the horizontal scanning, i.e. in case arrangement is given so as to obtain 
a differential image in the direction of the horizontal scanning, the 
brightness will differ on the left-hand side of the picture field from the 
right-hand side thereof. 
Such a difference of brightness between the areas on the two sides of the 
picture field will not cause a substantial problem in case the image 
height is small or where the contrast of the differential image is not 
stressed. In case, however, the image height is big or in case the 
contrast of the differential image is to be stressed, the abovesaid 
unevenness of the brightness will consitute a substantial problem. For 
example, there could arise such an inconvenience that the left-hand margin 
of the picture field is excessively bright so that the details of the 
differential image are lost, whereas the right-hand margin is too dark and 
nothing can be observed. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide a 
scanning type optical microscope incorporating the above-described system, 
which is capable of securing a uniform brightness at all points on the 
entire picture field, whereby an excellent differential phase image can be 
obtained. 
According to the present invention, the scanning type optical microscope is 
of the arrangement comprising: a light source; an objective lens for 
collecting the light beam emitting from the light source onto an object; 
light beam scanning means disposed between said light source and said 
objective lens; a detector comprised of a plurality of photo-electric 
converters receiving the light coming from the object; a signal processing 
circuitry connected to said detector for calculating the difference 
between the signals coming from each of the divided two sections of said 
detector to thereby obtain a differential phase signal; adjusting means 
connected to the signal processing circuitry to use a signal varying with 
an image height, that is, a position where the object is irradiated by the 
light beam, and being synchronous with the scanning of the light beam to 
thereby adjust the differential phase signal, whereby to cancel out, by 
the adjustment signal, the bias component of the differential phase signal 
which varies with the image height. 
This and other objects as well as the features and the advantages of the 
present invention will be apparent from the following detailed description 
of the preferred embodiments when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Prior to describing the embodiments of the present invention, the principle 
for obtaining a differential phase image will be explained. 
On page 203 of "Proceeding of SPIE" vol. 232, issued in 1980, T. Wilson et 
al state that a differential phase image can be detected in the scanning 
type optical microscope. Now, for the sake of simplicity, one dimensional 
image is considered. The intensity I(x) of an image due to a partial 
coherent focusing is indicated, in general, as follows. 
##EQU2## 
wherein: T(m) represents Fourier conversion function of the transmittibity 
of an object; and 
C(m;p) corresponding to the transmission function of the optical system. 
When the sensitivity of the detector is assumed to be D(.xi.), and when the 
pupil function of the optical system is assumed to be P(.xi.), then 
##EQU3## 
wherein: f represents the focal distance of the optical system; and 
.lambda. represents the wavelength of light. 
Here, when an object having a weak contrast is taken up, it is enough to 
consider only C(m;0). Accordingly, assuming D(.xi.) as the sensitivity of 
the split detector-constituent devices, the difference in the output 
signals from these respective detector-constituent devices will be 
considered, and we shall get a C(m;0) in such a configuration as mentioned 
in FIG. 6. The fact that the function of transmittibity has such a 
configuration indicates that a differential of phase of an image can be 
obtained. Also, by utilizing the sum of the output signals coming from the 
respective detector-constituent devices, a normal image is obtained. As 
stated above, there is the feature that a differential image or a normal 
image can be obtained selectively only by the selection of use of the 
difference in the output signals or the sum of these signals. 
Here, in case it is intended to obtain such a differential image as stated 
above, a displacement of the position of pupil as mentioned earlier in 
this specification will result in the development of a change in the 
amount of light due to the height of the image. 
Now, let us here assume that the boundary of the two detector-constituent 
devices is displaced by 2.tau. from the optical axis. Then, the image 
intensity I.tau.(x) will become 
##EQU4## 
and there is developed an overlying of a differential image upon a normal 
image. Here, the first member of the right-hand term of the 
above-mentioned Equation (4) represents a differential phase image, 
whereas the second member thereof represents a normal image. Therefore, 
the resulting image will be one formed with a normal image superposed on 
the differential phase image at the rate of 2(.tau.m+.tau.p). In case the 
amount of displacement of the boundary between the two 
detector-constituent devices from the optical axis is small, the normal 
image signal will be only very trifle, so that the image which is obtained 
may be safely considered as being nothing else but a differential phase 
image. FIG. 7 shows the transmission function C(m;0) in the 
above-mentioned instance. Also, a differential phase image is required 
mostly in case of observation of a phase object. Therefore, usually, it is 
often the case that a normal image signal constitutes a mere bias 
component for a differential phase image. 
What becomes the problem, therefore, is the unevenness in the amount of 
light when a differential phase signal is utilized. However, by the 
addition of such an offsetting component as will cancel out the unevenness 
of the amount of light to the difference signal intended for the 
differential phase signals, relying on the characteristic that an 
electrical offset is an offset of the electric signal subsequent to image 
focusing and that this does not affect the image focusing in any way, it 
is possible to uniformalize the brightness of the entire picture field. 
Therefore, the present invention provides for the arrangement that an image 
having a uniform brightness is obtained by the addition of an adjustment 
(brightness uniformalizing) signal (offsetting component) in synchronism 
with the scanning to the differential signal of the two 
detector-constituent devices which varies in accordance with the image 
height, that is, a position where the object is irradiated by the light 
beam, as to cancel out the bias component of this differential signal. 
Description will be made hereunder in further detail with respect to the 
present invention based on an embodiment thereof by giving reference to 
FIGS. 8 to 10. 
FIG. 8 shows a scanning type laser microscope using two AODs 
(Acousto-Optical Deflectors) serving as light deflectors, i.e. the light 
beam scanning means. Numeral 41 represents a laser source. Its light is 
shaped into an appropriate light bundle by a beam expander 42 including a 
spatial filter 43 (notes: this is intended to render the light from the 
laser source into a single mode beam, and for example, a pin-hole is 
employed for this purpose). The light passes through a beam splitter 44 
and incides onto an AOD 45 (for vertical direction), and is caused to 
impinge onto a next AOD 48 (for horizontal direction) by pupil relay 
lenses 46 and 47, and the resulting beam transmits through a pupil 
projection lens 50 and a tube lens 51, and impinges onto an objective lens 
52. And, the light which has passed through a specimen 53 is converted to 
a signal by means of a collector lens 54, two detector-constituent devices 
55, 56 and amplifiers 57, 58. Also, the reflection light coming from the 
specimen 53 travels backwardly along the same course which the incident 
light has followed, and is reflected by the beam splitter 44 to be formed 
into a signal by a detector lens 59, two detector-constituent devices 60, 
61 and amplifiers 62, 63. Numeral 64 represents a manually operable 
controller; 65 and operation panel; 66 a signal processor; 67 a CRT; 68 a 
frame memory; 69, 70 driver circuits for AODs 45, 48, respectively. 
It should be noted here that the boundary between the detector-constituent 
devices 55, 56 and the boundary between the detector-constituent devices 
60, 61 are each normal to the horizontal scanning direction, and that the 
differential signal represents the differential in the horizontal 
direction. 
The signal processing circuit 66 has an arrangement as shown in FIG. 9. 
Numerals 71, 72 represent buffer amplifiers which receive image signals 
supplied from the detector-constituent devices 55, 56 or 60, 61 through 
the amplifiers 57, 58 or 62, 63. Numeral 73 represents an adder of two 
signals for obtaining a normal image; 74 an analog switch which determines 
a code for the subtraction between the two signals; 75 a subtracter for 
obtaining a difference signal to obtain a differential image; 76 an 
adjustment signal generating circuit which generates an adjustment signal 
in synchronism with a horizontal synchronous signal coming from the 
controller 64 (notes: herein this circuit generates a saw-tooth wave), to 
thereby adjust its magnitude; 77 an adder for adding the adjustment signal 
to the difference signal; 78, 79 contrast-adjusting amplifiers having 
variable gains and offsetting functions, respectively, and serve to adjust 
the contrast thereof. It should be noted here that, in the differential 
signal processing circuit, adjustment of contrast is done after an 
adjusting, i.e. after uniformalizing the brightness of the entire picture 
field. Hence, there is no need to change the amount of adjustment, i.e. 
the amount of the uniformalized brightness. Numeral 80 denotes an analog 
switch for selecting the signal which is to be displayed; 81 a buffer 
amplifier for adding, to an image signal, the synchronous signal intended 
for video purpose and supplied from the controller 64 to form a composite 
video signal; and 82 a buffer amplifier for outputting the image signal 
exactly as it is. 
FIG. 10 shows the manner of adjusting the signal. FIG. 11 shows a specimen, 
i.e. an object 53 requiring observation. In this Figure, the hatched 
portion 53a shows the region having a same transmittibity as that of its 
surrounding area but having a different refractive index from said 
surrounding area, whereas the region 53b of the obliquely crossed lines 
indicates the region having a same refractive index as that of its 
surrounding area but a lower transmittibity than said surrounding area. An 
instance wherein this object 53 is scanned as illustrated (notes: the 
broken lines indicate the returning lines) will be described below by 
giving reference to FIG. 10. "83" represents a horizontal synchronous 
signal. "84", "85" represents signals coming from two detector-constituent 
devices 55, 56 or 62, 63, respectively. As shown in FIGS. 5A and 5B, a 
bias component is superposed on the image signal in accordance with the 
image height (horizontal scanning). Numeral "86" represents a sum signal 
for a normal image, in which the bias component has been cancelled out and 
an altogether normal image is obtained. It should be noted here that, as a 
matter of course, the hatched region 53a in the object 53 has a 
transmittibity equal to that of its surrounding area, so that this portion 
does not appear in the normal image signal. Numeral "87" represents a 
difference signal for the differential phase image obtained by subtracting 
the signal "85" from the signal "84". By this signal subtraction, the 
differential phase signal increases its magnitude, but at the same time 
therewith the bias component also is doubled in magnitude. Also, there 
remains a normal signal only for the amount 2(.tau.m+.tau.p). Therefore, 
when such an adjustment signal as the one "89" (an offsetting signal) is 
added to the signal "87", there is obtained a differential phase signal as 
indicated by "90". Accordingly, it becomes possible to secure a uniform 
brightness at all points on the picture field, and thus there can be 
obtained an excellent differential phase image. 
Description has been made above with respect to the instance wherein 
detection is performed by the use of a reflection light coming from the 
specimen or object 53. It is needless to say that detection can be made 
also with the light which has transmitted through the specimen 53. In this 
latter case, the transmitted light is received by the detector-constituent 
devices 57, 58. However, the output signals from these devices are 
inputted to the signal processor 66 as shown by the chain line in FIG. 8, 
and they are processed in a manner similar to the instance using the 
deflection light, and are adjusted. That is, in the embodiment shown in 
FIG. 8, both the signal processing circuit and the signal adjusting means, 
i.e. the controller 64, the operation panel 65, the signal processor 66, 
the CRT 67, the frame memory 68 and the driver circuits 69, 70 are 
arranged so as to be used in common for the detection conducted with the 
reflection light and also for the detection conducted with the transmitted 
light, so as to be utilized in alternative fashion. In contrast thereto, 
FIG. 12 shows the embodiment wherein there are employed signal processing 
circuit and signal adjusting means both of which are independent from 
those used in the instance wherein detection is conducted with the 
reflection light, relative to the instance wherein detection is performed 
with the transmitted light. In this latter embodiment also, the 
construction and the function of these circuit and means are the same as 
those described above, and accordingly like parts described already are 
assigned with like reference numerals, and their description is omitted. 
In this instance embodiment, it is possible to use only the signal 
adjusting means in common as indicated by the dotted line in FIG. 12. This 
arrangement is suitable for making a simultaneous observation of such a 
specimen as having both a light-reflecting region and a light-transmitting 
region. 
It should be noted here that a similar effect can be obtained also from 
multiplication and division of the adjustment signal, in addition to the 
addition and subtraction of such a signal. Also, as a matter of course, 
the signals "84" and "85" may be adjusted independently in advance. 
It should be noted here also that, while there is shown in this embodiment 
an instance wherein adjustment is performed manually by utilizing the 
controller 64, arrangement may be provided to perform an adjustment in 
such a way that while preliminarily conducting an observation of a uniform 
specimen, a signal of non-uniform amount of light is inputted in the 
computer so that, for example, using the inverse number of this signal as 
the adjustment coefficient, and by dividing with this adjustment 
coefficient the signal produced at the time the specimen is observed 
actually, or that, using the signal per se which comes from the uniform 
specimen to serve as the adjustment data, and by subtracting at a certain 
constant rate this data from the signal produced from the actual 
observation of the specimen. 
Also, as described earlier, a difference signal is provided as an image 
signal such that a differential phase image and a normal image are 
superposed one upon the other at a rate dependent on the image height 
(meaning: the positional displacement of the boundary of the 
detector-constituent devices which divide the pupil into two sections). 
Therefore, by subtracting the normal image signal due to the sum signal 
from the difference signal at the above-mentioned rate, to retain only the 
perfect differential phase component, and further by performing the 
so-called shading adjustment in accordance with the developing output 
intensity, there can be made a perfect adjustment. 
It should be noted that, as shown as "90" in FIG. 10, according to the 
above-described adjustment system, the normal image signal complying with 
the displacement of the boundary line of the two detector-constituent 
devices from the optical axis is mingled in the differential phase image 
signal. In case this is obstructive, the normal image signal "86" is 
amplified or diminished into an appropriate magnitude in advance, and then 
it is subtracted from the signal "90", whereby there can be obtained a 
differential phase image signal "91" which has been adjusted more 
perfectly. FIG. 13 shows an example of the signal processing circuit 66 
for carrying out such an adjustment. In this instant embodiment, like 
reference numerals are assigned to those circuit-constituent parts similar 
to those shown in FIG. 9. That is, arrangement is provided so that a 
portion of the output from an adder 73 is inputted, along with the 
difference signal "90", to a subtracter 93 via a variable gain amplifier 
92. The variable gain amplifier 92 is operative so that its gain is 
controlled by such a signal which varies with the image height as the 
saw-tooth signal which is synchronous with, for example, a horizontal 
synchronous signal, so that the output thereof will become such one as 
indicated by "88" in FIG. 10. By subtracting this signal from the 
difference signal "90" by the subtracter 93, there is produced a 
differential phase signal which has been adjusted perfectly.