Photometry device that detects and compensates for an inversion abnormality

A photometric circuit performs photometry on an object field using an accumulating photometric element, such as, for example, a CCD. An accumulation controller controls the photometric circuit. An abnormality sensor determines whether an abnormality, such as, for example, the inversion effect of the photometric output of the photometric circuit has occurred. The accumulation controller sets the accumulation time of the photometric element for the next round of photometry to a minimum value, for example, when it has been determined by the abnormality sensor that an abnormality has occurred. This allows the photometric circuit to return to normal operation as quickly as possible.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to photometry devices that measure the brightness of 
an object, and more particularly, to photometry devices suitable for use 
in automatic exposure control of a camera. 
2. Description of Related Art 
FIG. 13 is a block diagram showing one example of a prior art photometry 
device (Japanese Laid-Open Patent Application No. 57-131178, which 
corresponds to U.S. Pat. Nos. 4,479,062 and 4,544,848). A saturation 
sensor 52 senses whether the output of a photometric circuit (which 
includes a photoelectric conversion element 51) is saturated. When 
saturated, sensor 52 switches a switch 53, which reverses the output of a 
comparator 54. This causes a clock generator 55 to generate clock pulses 
such that the accumulation time of the photoelectric conversion element 51 
is shortened. 
The photometry device described above detects whether the output of the 
photoelectric conversion element 51 is saturated using the saturation 
sensor 52. However, the saturation sensor 52 cannot determine whether the 
output is normal or abnormal. For example, depending upon the construction 
of the photoelectric conversion element, when 100 (or more) times the 
allowable amount of light enters the photoelectric conversion element, the 
output passes through a state of saturation and then begins to decrease, 
resulting in the so-called inversion effect. When this inversion effect 
occurs, because the output value becomes less than in the state of 
saturation, the photoelectric conversion element 51 continues to output 
erroneous photometric information without the saturation sensor 52 being 
able to sense the abnormality of the output. 
SUMMARY OF THE INVENTION 
Embodiments of the invention aim to provide a photometry device that can 
determine whether the output from a photometric circuit is normal or 
abnormal, and can return the photometric circuit to a normal output by 
performing a specified processing upon detection of the abnormality. 
In order to achieve the above and other objects, embodiments of the 
invention include a photometric circuit that performs photometry on an 
object field using an accumulating photometric element, e.g., a CCD. A 
controller controls the photometric circuit, and an output determination 
circuit (i.e., an abnormality sensor) determines whether the output of the 
photometric circuit is normal or abnormal. In particular, when the 
abnormality sensor determines that the output of the photometric circuit 
is abnormal, the controller performs specified processing that differs 
from the processing used for normal operation of the photometric circuit. 
In a preferred embodiment, the abnormality sensor determines whether 
inversion of the photometric circuit output has occurred. 
In one example, the controller sets the accumulation time of the 
photometric element for use during the next round of photometry to a fixed 
value when the output of the photometric circuit is determined to be 
abnormal. This fixed value can be a minimum value, i.e., the shortest 
possible accumulation time that can be used with the photometric element. 
Additionally, when the photometric circuit has an output gain switching 
function (i.e., the gain of the photometric circuit can be switched), the 
controller sets the output gain to low gain when the output of the 
photometric circuit is determined to be abnormal. 
When the photometric element has a light-blocked pixel output, in which one 
or more of the photoelectric converters of the photometric element are 
optically blocked from receiving light (so that they generate a so-called 
dark current) the abnormality sensor determines whether the output is 
normal or abnormal based on the value of the light-blocked pixel output. 
For example, the abnormality sensor can determine whether the output is 
normal or abnormal based on comparing the light-blocked pixel output to a 
reference output that has a fixed output value that is not dependent on 
the accumulation time. The value of the photometric output also can be 
used in the determination.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Photometric circuit 12 is a circuit that performs photometry on an object 
field using an accumulating photometric element 9, which can be, for 
example, a CCD. Photometric circuit 12 outputs photometric data to a 
brightness computation circuit 14, an abnormality sensor 18, and an 
accumulation control circuit 19 after the photometric data is converted to 
numeric values by an A/D converter 13. 
The abnormality sensor 18 determines, based on the most recent photometric 
data, whether the so-called abnormal saturation has occurred, which causes 
an inversion effect on the photometric circuit 12. The output of 
abnormality sensor 18 is input into the accumulation control circuit 19. 
One method of determining whether abnormal saturation has occurred is 
explained in more detail below, for example, with reference to FIG. 8. The 
accumulation control circuit 19 determines the accumulation time and 
photometric circuit gain that is to be used for the next round of 
photometry based on information provided by the A/D converter 13 and by 
the abnormality sensor 18, and controls the photometric circuit 12 based 
on these determinations. One method of determining the accumulation time 
and photometric circuit gain for the next round of photometry is explained 
in more detail below, for example, with reference to FIGS. 9, 11 and 12. 
The brightness computation circuit 14 computes the absolute brightness 
value of the object field based on the output from the A/D converter 13 
and on data stored in a lens data memory 15, which stores data concerning 
the attached lens. The output of brightness computation circuit 14 is 
input into an exposure computation circuit 16. The exposure computation 
circuit 16 computes the appropriate exposure values based on data from the 
brightness computation circuit 14 according to any number of techniques. 
The output of exposure computation circuit 16 is provided to an exposure 
controller 17. The exposure controller 17 senses the full depression of a 
release button (not shown), and then drives a quick return mirror 2, a 
diaphragm 10, and a shutter 11 based on the appropriate exposure value set 
by the exposure computation circuit 16, so as to perform exposure of the 
film. 
In the preferred embodiment, the A/D converter 13, the brightness 
computation circuit 14, the exposure computation circuit 16, and the 
abnormality sensor 18 are all implemented using a microprocessor 
(henceforth, microcomputer) 20. The processing performed by the 
microcomputer 20 is explained in more detail below. It is, however, within 
the scope of the invention to perform the functions of these components by 
other means. For example, the invention can be implemented using a 
plurality of separate dedicated or programmable integrated or other 
electronic circuits or devices (e.g., hardwired electronic or logic 
circuits such as discrete element circuits, or programmable logic devices 
such as PLDs, PLAs, S, or the like). A suitable programmed general 
purpose computer, e.g., a microprocessor, microcontroller or other 
processor device (CPU or MPU), either alone or in conjunction with one or 
more peripheral (e.g., integrated circuit) data and signal processing 
devices can be used to implement the invention. In general, any device or 
assembly of devices on which a finite state machine capable of 
implementing the flow charts shown in the figures can be used as a 
controller with the invention. 
FIG. 2 is a block diagram of the optical system of a camera according to 
the preferred embodiment. After the luminous flux has passed through a 
photographic lens 1, it reaches the eye of the photographer after 
reflecting from a quick-return mirror 2, and passing through a diffusion 
screen 3, a condenser lens 4, a pentaprism 5, and an eyepiece lens 6. 
Meanwhile, after a portion of the luminous flux has been diffused by the 
diffusion screen 3, it reaches the photometric element 9 after passing 
through the condenser lens 4, the pentaprism 5, a photometric prism 7, and 
a photometric lens 8. 
FIG. 3 illustrates the partitioned state of the photometric element 9 
mapped onto the object field. The photometric element 9 includes an 
accumulating sensor such as, for example, a CCD (charge-coupled device), 
and can perform photometry by partitioning nearly the entire object field. 
In the present example, the photometric element 9 is vertically 
partitioned into twelve sections and horizontally partitioned into twenty 
sections, totaling 240 regions. 
FIG. 4 is a block diagram representing in more detail the internal 
construction of the photometric element 9 according to the preferred 
embodiment. The photometric element 9, which includes an accumulating 
element such as a CCD, is equipped with three types of registers. The 
first type of register includes an OPB register, which does not have a 
light-receiving element. The second type of register includes a first 
charge injection register and a second charge injection register. The 
third type of register includes twelve H (horizontal) registers that have 
light-receiving elements so as to perform photometry on the object field. 
Twenty independent pixel signal outputs are connected to each of the 
registers mentioned above. The photometric outputs, etc., pass via these 
registers through a V (vertical) register that is placed at a 90.degree. 
angle relative to the H register, and are output to an output circuit 22. 
After the output circuit 22 has converted the output from the V register 
into a voltage, it magnifies the signal by, e.g., 1 (during L gain) or by, 
e.g., 4 (during H gain), and outputs a signal having a constant timing to 
the output terminal. The gain of the output circuit 22 can be switched by 
the microcomputer 20. 
The OPB register has a photoelectric conversion element that is blocked 
optically. That is, the photoelectric conversion element of the OPB 
register does not receive light. Thus, the photoelectric conversion 
element of the OPB register outputs only the dark current that is 
generated during accumulation of the photometric element 9. This 
information commonly is used to compensate for the dark current 
characteristics of the photoelectric conversion elements. The output of 
the first and second charge injection registers is used mainly when 
performing compensation on the output of the photometric element 9. The 
first and second charge injection registers always output a fixed charge 
regardless of the brightness of the object field. An explanation of the 
method of output compensation is omitted since it is not directly related 
to the present invention. The first and second charge injection registers 
have differing amounts of injected charge. 
FIG. 5 illustrates the signals output from the photometric element 9 of the 
preferred embodiment. The output of the photometric element 9 is obtained 
by reading in the Vout signal, which is synchronized with the trailing 
edge of a timing signal (SYNC). The first three pulses are so-called dummy 
output that does not include information from the photometric circuit 12, 
and this voltage level Voref (about 4V) becomes the reference level of the 
photometric element 9. Following Voref is the OPB output Vopb, the first 
and second injection charges V1 and V2, and the outputs Vox! 
(x=1.about.12) of registers H1.about.H12. These signals are output for 
each pixel as a difference from the reference level Voref. This comprises 
one series of signals for use by the V register. The signal component of 
each output is output in the 0V direction with the reference level being 
set to 0. A full signal component is set by subtracting its pixel signal 
levels from the reference level. For example, when the reference level=4V 
and the signal output level=3V, the pixel signal component is 1V. One 
screen of output (i.e., information for one screen of light data--a screen 
being shown in FIG. 3) is completed by repeating this process for 20 
series (in this example where the CCD includes a matrix of 12.times.20 
light-receiving components). 
FIGS. 6A-6E show the light intensity and wave forms output from the 
photometric element 9. FIG. 7 shows the relationship between the 
photometric output and the dynamic range of the photometric element 9. 
FIG. 6A shows the state in which the light intensity is weak and the output 
is not saturated. In this case, the dynamic range of the output, as shown 
at (B) of FIG. 7, enters the state in which only the Vopb portion of the 
dark current output is subtracted from the total dynamic range (the total 
dynamic range being shown at (A) of FIG. 7). 
When the light intensity increases, as shown in FIG. 6B, the value of Vopb 
becomes greater due to the transmission of charge to the light-blocked 
pixels. This causes the signal component to become smaller as shown at (C) 
of FIG. 7. From this state, the charge transmission to the light-blocked 
pixels becomes even greater as the accumulation time is lengthened, and 
the usable photometric output becomes smaller as shown by the dotted lines 
due to the value of Vopb becoming greater. This is known as the inversion 
effect. 
When this inversion effect has occurred, because the output becomes smaller 
as the accumulation time is lengthened, a proportional relationship 
(between Vopb and the signal component) is not maintained as is the case 
for the usual accumulation time and output. In this case, if the usual 
control of the accumulation time is performed, the accumulation time is 
extended more and more in an (unsuccessful) attempt to increase the 
output, until, ultimately, the maximum accumulation time is reached. 
When the light intensity is further increased from that shown in FIG. 6B, 
the entire dynamic range of the output ends up being taken to Vopb, and 
the usual photometric output becomes impossible to output, as shown in 
FIG. 6C. At this time, the output is saturated in spite of the photometric 
output being 0. This is called a state of abnormal saturation. Also, when 
the light becomes even stronger, the voltage rises even when the output is 
clipped to Voref and, as shown in FIG. 6D, the voltage immediately before 
the value of the Vopb output becomes Voref or higher. In compensation, the 
value of the Vopb output becomes smaller than during (C) as shown at (D) 
of FIG. 7. 
As the light intensity becomes even stronger, finally, even the Vopb output 
becomes 0 as shown in FIG. 6E. This state is shown as (E) in FIG. 7. In 
this manner, because there are various stages of output abnormalities, it 
is difficult to determine the state only by looking at the size of Vopb. 
For example, in both FIG. 6A and FIG. 6D, because the value of the Vopb 
output itself is smaller than the size of the dynamic range shown as (A) 
in FIG. 7, it is impossible to determine whether the output is normal or 
abnormal. 
In fact, when Vopb increases up to the vicinity of the dynamic range, as 
shown in FIG. 6C, because the dynamic range of the usual photometric 
output becomes extremely small, it is estimated that it will be difficult 
to obtain an effective photometric output. Also, as shown in FIG. 6E, when 
in the completely saturated state, discrimination is impossible because 
the V1 and V2 outputs become 0 and are not output, as compared to the 
state of FIG. 6A. 
In order to avoid the undesirable situations described above, according to 
one aspect of the invention, the accumulation time and/or gain is 
controlled in a manner that is different from the usual manner of control. 
FIG. 8 is a flow chart showing a program performed by the microcomputer 20. 
When the camera release button (not shown) is half depressed, the camera 
power supply is switched ON, and the program is executed. 
In step S101, accumulation by the photometric element 9 is performed, and 
the photometric output is read out after completion of the accumulation. 
The accumulation time and gain used during this accumulation are 
determined either by the step S107 or by the step S106 performed during 
the previous round of photometry. Alternatively, when the photometry is 
the initial photometry performed after the initialization of the power 
supply, because the steps S107 and S106 have not yet been passed, 
accumulation is performed using default values, for example, by setting 
the gain to L and the accumulation time int to 1 mS. In step S102, Vopbmax 
is computed from the photometric data read out. As shown in FIG. 4, there 
is one OPB register, and therefore for 1 screen there are 20 values for 
Vopb. Vopbmax is the maximum value among these 20 values. 
Whether Vopbmax is greater than Vov is determined in step S103. Vov 
represents the overflow voltage of the photometric element 9, and it is 
the basis of determining whether there is abnormal saturation. Because Vov 
is set close to the maximum value in which the output of the photometric 
element 9 is linear, an abnormal situation may exist when the output is at 
Vov or higher. The base value for Vov is about 3.4V. This is mainly for 
the purpose of determining the state shown in FIG. 6C. When the dynamic 
range differs according to whether the gain of the photometric element is 
set to H or L, it is desirable to prepare multiple values for Vov 
according to the gain, and to use the Vov value that corresponds to the 
gain that was used during accumulation. 
When Vopbmax&gt;Vov in step S103, it is determined to be an abnormal 
saturation state, and flow advances to step S106. When the result of step 
S103 is negative, flow advances to step S104. In step S104 the maximum 
value Vomax of the 240 photometric outputs is computed. 
In step S105, it is determined whether the second injection charge output 
V2 is less than Vn and whether Vomax is less than Vn. Referring to the 
prior description of FIG. 4, there are also 20 values of the second 
injection charge output, but because these are fundamentally equivalent 
values, an arbitrary V2 value may be selected and used. Alternatively, the 
average of the 20 V2 values may be taken. As another alternative, the 
maximum value or the minimum value of the 20 V2 values may be taken. When 
there is no memory and computation time, the method of taking an arbitrary 
V2 value is suitable. When stable values are desired, the method of taking 
the average of the 20 V2 values is suitable. Also, the method of using the 
maximum value is effective when it is desired to make it less likely that 
abnormal saturation will be detected. Conversely, the method of using the 
minimum value is effective when it is desired to make it more likely that 
abnormal saturation will be detected. Vn is the value of the output noise 
level of the photometric element 9. Because there may be outputs below the 
noise level even when there is abnormal saturation, step S105 is effective 
in preventing such abnormal saturation from being missed. As above, when 
the noise level differs according to the gain of the photometric element 
9, it is desirable to prepare multiple values of Vn according to the 
different gains and to use the Vn value that corresponds to the gain that 
was used during accumulation. 
When step S105 is affirmative, flow advances to step S106, where processing 
appropriate for abnormal saturation is performed. That is, the next round 
accumulation time int is set to int.sub.-- min and the gain is set to low 
gain (L) in step S106. Here, int.sub.13 min is the minimum accumulation 
time that is able to be set. Thus, when abnormal saturation occurs, the 
probability of being able to avoid the state of abnormal saturation in the 
next round of photometry is increased by setting the parameters to be used 
for the next accumulation such that the output of the photometric element 
9 becomes its smallest. 
When the result of step S105 is negative, computation of the next round 
accumulation time is performed assuming a normal state, in step S107. This 
method is explained in more detail below. 
In step S108, the absolute brightness value is computed using the obtained 
data and the open aperture stop of the attached lens, and the proper 
exposure for the object field is computed by any commonly known method. In 
step S109, it is determined whether the release button (not shown) is 
fully depressed. When fully depressed, in step S110 exposure of the film 
based on the proper exposure values is performed. Flow then advances to 
step S111. Flow also advances to step S111 when the release button is not 
fully depressed. In step S111, it is determined using a half depression 
timer whether a specified time has elapsed after release of the half 
depression. If the half depression is continuing or is within the 
specified time, flow returns to step S101 and the processing is repeated. 
If the timer has expired, the program ends. 
FIG. 9 is a flow chart of a subroutine that sets the accumulation time int 
and the amp gain for the next round of photometry. This subroutine is 
called and executed when step S107 in FIG. 8 is executed. Before this 
subroutine is called, because at least one round of photometry has been 
performed after the initialization of the power supply, the immediately 
preceding photometry data remains in a memory (not shown) inside the 
microcomputer 20. 
In step S201, Vomax is determined in the same manner as in step S104 of 
FIG. 8. Alternatively, the value of Vomax set in step S104 may be stored 
in memory and used as is. In step S202, it is determined whether 
Vomax&lt;Vov, that is, whether the photometric data is in an overflow state. 
When the photometric data has not overflowed (step S202 is affirmative), 
flow advances to step S203 and the next round accumulation time is 
computed by usual processing, which will be explained in more detail below 
with reference to FIG. 11. If the photometric data has overflowed, flow 
advances to step S204 and the next round accumulation time is computed by 
an overflow processing routine, which will be explained in more detail 
below with reference to FIG. 12. 
Next, it is determined in step S205 whether I.sub.-- CUT=1, that is, 
whether a photometry prohibiting interrupt signal has occurred, such as 
occurs if the release operation occurs during photometric accumulation, 
etc. The release operation occurs, for example, when the photographer 
presses the release button to the fully depressed position, causing the 
quick release mirror to move to the up position. When I.sub.-- CUT=0, that 
is, when a photometry prohibiting interrupt signal has not occurred, flow 
advances to step S206, and parameters are set such as shown in equations 1 
and 2: 
EQU int.sub.-- L.sub.-- max=intx0 (1) 
EQU int.sub.-- H.sub.-- min=intn0 (2) 
Here, int.sub.-- L.sub.-- max and int.sub.-- H.sub.-- min are parameters 
that are used to determine whether to switch the gain of the photometric 
circuit 10. Specifically, by comparing the length of the next round 
accumulation time set in step S203 or in step S204 with the two values 
above (int.sub.-- L.sub.-- max and int.sub.-- H.sub.-- min the gain of the 
photometric circuit 10 may be switched in the next round of photometry to 
the high gain (H) or to the low gain (L). The specific processing is 
explained in more detail starting with step S208. 
Meanwhile, when I.sub.-- CUT=1, that is, when a photometry prohibiting 
interrupt signal has occurred, flow advances to step S207, and parameters 
are set such as shown in equations 3 and 4: 
EQU int.sub.-- L.sub.-- max=intx1 (3) 
EQU int.sub.-- H.sub.-- min=intn1 (4) 
Thus, different values are assigned to int.sub.-- L.sub.-- max and 
int.sub.-- H.sub.-- min based on a determination as to whether an 
interruption of the accumulation occurred in the previous round of 
photometry. The significance of these differences will become clear from 
the following description. 
In step S208, it is determined whether the present gain setting of the 
photometric circuit 10 is L, that is, low gain, and whether the next round 
accumulation time int is greater than int.sub.-- L.sub.-- max. If the 
result of step S208 is affirmative, that is, the gain is L and the 
accumulation time is longer than int.sub.-- L.sub.-- max, flow advances to 
step S209. In step S209, the accumulation time for the next round of 
photometry is set by dividing the present accumulation time int by VL as 
shown in equation 5, and the gain is switched to H. 
EQU int=int/VL (5) 
Here, VL is the gain constant of the photometric circuit 10. Specifically, 
VL represents how many times larger the H gain output is relative to the L 
gain output. FIG. 10 illustrates the processing of this case. The 
processing of step S209 corresponds to switching from A to B (when 
I.sub.-- CUT=0) in FIG. 10, or switching from A' to B' (when I.sub.-- 
CUT=1). Referring to FIG. 10, the gain will be switched (from L to H) and 
the accumulation time (int) will be shortened only when, for an 
accumulation using a gain of L, the accumulation time has a length of A or 
greater when there was not an interruption. When, however, an interruption 
occurred, the gain will be switched (from L to H) and the accumulation 
time will be shortened when the accumulation time had the shorter duration 
of A' (or a value greater than A'). Thus, the threshold for switching the 
gain from L to H and for reducing the accumulation time is lowered when an 
interruption occurs. This increases the likelihood that the next round of 
accumulation will take place in a time period that is short enough to 
avoid the occurrence of an interruption. 
When the result of step S208 is negative, it is determined in step S210 
whether the present gain setting of the photometric circuit 10 is H, that 
is, high gain, and whether the next round accumulation time int set in 
step S203 or in step S204 is smaller than int.sub.-- H.sub.-- min. When 
the result of step S210 is affirmative, that is, when the gain is H gain 
and the accumulation time is short, flow advances to step S211, where the 
next round accumulation time int is multiplied by VL as shown in equation 
6, and the gain is switched to L. 
EQU int=int.times.VL (6) 
Also, referring to FIG. 10, the processing of step S211 corresponds to 
switching from C to D (when I.sub.-- CUT=0), or switching from C' to D' 
(when I.sub.-- CUT=1). In this manner, when the gain is H and int is 
small, the better signal to noise characteristics of the L gain are 
utilized by switching the gain to L and making int longer. As can be 
appreciated from FIG. 10, when the gain is H, the threshold for switching 
the gain from H to L and for increasing the accumulation time is decreased 
when an interruption occurs. Thus, lower values of int (the accumulation 
time) are permitted when interruptions occur, which minimizes the 
possibility of subsequent interruptions occurring. Also, when switching 
the gain between H and L, as shown in FIG. 10, due to hysteresis, stable 
control becomes possible even in spite of photometric scattering and cases 
when the brightness has varied minutely in the vicinity of the switching 
points. 
Thus, when I.sub.-- CUT=1, that is, when a photometry prohibiting interrupt 
has occurred during photometric accumulation, the duration of the next 
round accumulation time is made shorter than the usual case (when there is 
no interrupt) by shifting the switching points (i.e., the values of 
parameters int.sub.-- L.sub.-- max and int.sub.-- H.sub.-- min in the 
shorter direction of the accumulation time. This enables the next round of 
photometry to be performed immediately after the photometric prohibition 
has been released because when a photometric prohibiting interrupt has 
occurred, it indicates that the release state of the camera, that is, the 
exposure state has been entered due to the release button being fully 
depressed. In particular, when the film supply mode is a high-speed 
advancing mode, once the first exposure has finished, the mirror has gone 
down, and the photometric prohibition has been released, a second exposure 
may begin and another photometric prohibition signal may occur. In order 
to avoid such an occurrence, the present process makes the time spent in 
photometry shorter than usual in the first round of photometry that occurs 
after release of the photometric prohibition signal (i.e., by reducing the 
accumulation time). From the second round of photometry onward, after 
release of the photometric prohibition signal, because the newest 
photometric data already is obtained and the first round (after occurrence 
of an interruption) has finished (and there very likely was not an 
interruption of the previous accumulation), the process returns again to 
the usual state (e.g., using a gain of L), which has a better signal to 
noise ratio. However, due to hysteresis, for example, it is also likely 
that the subsequent rounds will use accumulation times that are shorter 
than the accumulation time that led to a photometry interruption. 
Accordingly, it is likely that these subsequent rounds will not be 
interrupted, even though they are performed at a gain of L. 
Next, in step S212, the determination of equation 7 is performed. 
EQU int&lt;int.sub.-- min (7) 
Here, int.sub.-- min is the minimum accumulation time of the photometric 
circuit 10. That is, in step S212 it is determined whether the int that 
has been set for the next round of photometry is smaller than the minimum 
value. If the result of step S212 is affirmative, the next round int is 
set to int.sub.-- min in step S213. When the result of step S212 is 
negative, the determination of equation 8 is performed in step S214. 
EQU int&gt;int.sub.-- max (8) 
Here, int.sub.-- max is the maximum accumulation time of the photometric 
circuit 10. That is, in step S214 it is determined whether the value of 
int set for the next round is greater than the maximum value. When the 
result of step S214 is affirmative, the next round int is set to 
int.sub.-- max in step S215. 
The typical values of each parameter in the above processing are shown 
below. 
EQU int.sub.-- min=10 .mu.S (9) 
EQU int.sub.-- max=100 mS (10) 
EQU intx0=80 mS (11) 
EQU intn0=10 mS (12) 
EQU intx1=40 mS (13) 
EQU intn1=5 mS (14) 
EQU VL=4 (15) 
FIG. 11 is a flow chart of a subroutine for computing the next round 
accumulation time in the usual accumulation time processing. This 
subroutine is called and executed when step S203 of FIG. 9 is executed. 
First, it is determined in step S301 whether the gain of the photometric 
circuit 10 is H. If the gain was L, it is determined in step S302 whether 
Vomax is less than VnL, with flow advancing to step S305 if the result of 
step S302 is affirmative, and to step S304 if negative. Here, VnL is the 
noise level of the photometric output in the case of L gain. If Vomax is 
at or below VnL, it indicates that all the photometric outputs were at or 
below the noise level. 
In step S301, if the gain was H, it is determined in step S303 whether 
Vomax is smaller than VnH, with flow advancing to step S306 if the result 
of step S303 is affirmative, and to step S304 if negative. Here, VnH is 
the noise level of the photometric output in the case of H gain. If Vomax 
is at or below VnH as with VnL, it indicates that all the photometric 
outputs were at or below the noise level. 
In step S304, the next round accumulation time when Vomax was greater than 
the noise level is computed using the equation 16 shown below: 
EQU int=int'.multidot.Vagc/Vomax (16) 
Here, Vagc indicates the target level of Vomax of the photometric data in 
the next round of photometry, and is set to a somewhat lower value than 
the saturated output voltage of the photometric circuit 10. The standard 
values are saturated output voltage=3.4V and Vagc=3V. Also, int' is the 
accumulation time of the previous round of photometry. 
In step S305, the next round accumulation time int is set when Vomax was at 
or below the noise level during L gain, and is computed using the equation 
17 below: 
EQU int=int'.multidot.VovL/VnL (17) 
Here, VovL is the saturated output voltage at L gain; its standard value is 
about 3.4V as described above, and the standard value of VnL is about 40 
mV. 
In step S306, the next round accumulation time int is set when Vomax was at 
or below the noise level during H gain, and is computed using the equation 
18 below: 
EQU int=int'.multidot.VovH/VnH (18) 
Here, VovH is the saturated output voltage at H gain; its standard value is 
about 3.4V as described above, and the standard value of VnH is about 160 
mV. 
FIG. 12 is a flow chart of the subroutine for setting the next round 
accumulation time int when the Vomax of the photometric output of the 
previous round has overflowed. This subroutine is called and executed when 
the step S204 of FIG. 9 is executed. First, in step S401, the number of 
overflowed regions among the 240 photometric regions is determined. In 
other words, it is determined how many areas of the output are at or above 
VovL if the gain was L, or at or above VovH if the gain was H. The 
resulting value is assigned to the variable ovf. 
Next, it is determined in step S402 whether ovf is less than px/16. Here, 
px is the number of photometric regions, and in this case, px=240. If the 
result of step S402 is affirmative, flow advances to step S403, where int 
is made int'/2, that is, 1/2 the value of the previous round. If the 
result of step S403 is negative, it is determined in step S404 whether ovf 
is less than px/8, and if so, in step S405, int is set to int'/4, that is, 
to 1/4 the value of the previous round. If the result of step S404 is 
negative, it is further determined in step S406 whether ovf is less than 
px/4, and if so, in step S407, int is set to int'/8, that is, 1/8 the 
value of the previous round. If the result of step S406 is also negative, 
it ends with int=int'/16, that is, 1/16 the value of the previous round. 
While this invention has been described in conjunction with specific 
embodiments thereof, it is evident that many alternatives, modifications 
and variations will be apparent to those skilled in the art. Accordingly, 
the preferred embodiments of the invention as set forth herein are 
intended to be illustrative, not limiting. Various changes may be made 
without departing from the spirit and scope of the invention as defined in 
the following claims.