Light integrating circuit for use in a light measuring device which is accurate for both low and high light values

A light measuring circuit including a light sensing device for sensing light and producing a signal representative of the light sensed, a compressing circuit for compressing the signal, a capacitor having one terminal connected to a predetermined voltage, a charging circuit responsive to the compressing circuit for charging the capacitor, a detecting circuit for responding to the voltage level of the capacitor, a switch for shorting the capacitor when closed for initiating charge of the capacitor by opening, and a delay circuit coupled to one of the charging circuits and the switch for delaying the onset of charge of the capacitor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to light measuring devices, and particularly to 
light integrating circuits for converting quantities of light into 
electrical signals. The invention is especially directed to light 
measuring devices for use in cameras. 
2. Description of Related Art 
The dynamic range of a conventional light integrating circuit, that is a 
light measuring device using an integrating circuit, is too small to 
respond efficiently to changes in the amount of light impinging thereon. 
For example, if the voltage of such a device varies between 18 mV and 2V, 
the ratio of the highest voltage to the lowest voltage is 2.sup.7. This is 
the equivalent of only 7 steps in a camera aperture, or more generally, 
only 7 steps in the APEX value system. 
When a camera uses through the lens (TTL) light metering to adjust a flash 
unit, the light integrating circuit of the measuring system requires a 
dynamic range of 10 APEX steps just to accommodate film sensitivities from 
ISO 6 to ISO 6400. Ten more APEX steps are necessary for changing the 
value of a photographic lens from F 1.0 to F 32. Thus, changing the film 
sensitivity and aperture value independently requires a dynamic range of 
20 APEX steps. 
Consequently, conventional light integrating circuits used with flash 
devices to control the amount of light shed on the object, have a dynamic 
range which is too narrow for this purpose. 
The dynamic range of a light integrating circuit may be widened by 
logarithmically compressing the photo current produced in the light 
measuring photo sensor, such as a photodiode. In this manner, each 
doubling of the amount of light striking the photodiode results in a 
linear increase in integrated voltage. Thus, a change in output voltage 
from 18 mV to 1.8V, presents 100 doublings, or a dynamic range of 100 APEX 
steps. 
However, it has been discovered that light integraters using logarithmic 
compression are accurate only for large light quantities and becomes less 
accurate at smaller light quantities. 
SUMMARY OF THE INVENTION 
It is an object of the invention to improve light integrating circuits. 
It is another object of the invention to avoid the aforementioned 
deficiencies. 
It is yet another object of the invention to furnish a light integrating 
circuit which is accurate when measuring low light values as well as 
higher amounts. 
According to a feature of the invention, these and other objects of the 
invention are attained in a light integrating circuit which compresses the 
signal from a photosensor and charges an integrating capacitor beyond a 
given value, by delaying the time from the start of measurement that the 
charge exceeds the given value so as to delay the introduction of the 
logarithmic rise from the given value. 
According to an aspect of the invention, the objects are attained in a 
light integrating circuit in which a signal from a light sensor element is 
compressed and charges an integrating capacitor connected to an emitter of 
a transistor, by precharging the integrating capacitor to a predetermined 
inverse voltage so that the compressed charge must first remove the 
precharge before rising logarithmically. 
According to another feature of the invention, a voltage source produces 
the precharging current and a switching arrangement separates the voltage 
source from the capacitor at the start of the light integrating operation. 
According to another feature of the invention, in a light integrating 
circuit which compresses the signal from a photo sensor and changes the 
charge on one integrating capacitor on the basis of the compressed signal, 
a detector disables the operation of the integrating capacitor while 
conducting its own integrating operation and enables the operation of the 
integrating capacitor only after the detectors integration reaches a 
predetermined value. 
According to another aspect of the invention, in a light integrating 
circuit which compresses the signal from a photo sensor and changes the 
charge on an integrating capacitor on the basis of the compressed signal, 
a plurality of parallel connected expanding transistors respond to the 
compressed signal and changes the charge on the integrating capacitor. 
According to other aspects of the invention, the circuit forms part of a 
camera, a flash unit, or photographic system. 
Other objects and advantages of the invention will become evident from the 
following detailed description when read in light of the accompanying 
drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The invention will best be understood by comparison with the two 
conventional flash systems shown in FIGS. 1 and 2. In the flash system of 
FIG. 1, a conventional light integrating circuit detects the amount of 
light reflected by an object. The light reflected from the object strikes 
a photodiode 1 to produce a photoelectric current which is stored by a 
capacitor 2 when a switch 3 is opened upon initiation of the flash. A 
comparator 4 compares the terminal voltage of the capacitor 2 with the 
voltage of a reference voltage source 5 to detect the amount of light 
striking the photodiode 1. When the value of the charge across the 
capacitor 2 reaches the voltage of source 5, the comparator 4 operates a 
switch 6 in series with the flash tube 7 of the flash unit so as to 
control the current from a storage capacitor 8. 
As previously stated, if the voltage produced by the reference source 5 
extends from 18 mV to 2V, ratio of the highest voltage to the lowest 
voltage is 2V/18 mV=2.sup.7. This allows only seven steps of APEX value 
representation. As previously mentioned, at least 20 APEX steps are 
necessary. 
FIG. 2 shows another flash arrangement with another conventional light 
integrating circuit. Here, the dynamic range is widened by logarithmically 
compressing the photo current produced in the photodiode 1. An operational 
amplifier 10 senses the photocurrent through the photodiode 1. A 
transistor 11 responds to the operational amplifier 10 by charging the 
capacitor 2, which is now in the emitter circuit of the transistor 11, 
when the switch 3 is open. In FIG. 2, a logarithmically compressing diode 
12 in the feedback loop of the operational amplifier 10 widens the dynamic 
range of the photocurrent by logarithmically compressing it. However, this 
circuit operates accurately only in response to high amounts of light and 
not to smaller amounts. 
The deficiencies of circuits such as those in FIG. 2 will be recognized 
from the following: 
If the photo current flowing through the photodiode 1 is ip, the base 
voltage Vb of the transistor 11 is represented by 
EQU Vb=(KT/q) ln (ip/is) (1) 
where 
is=Inverse saturation current diode 
K=Boltzmann coefficient 
T=Absolute temperature 
q=Charge of an electron 
If the emitter current of the transistor 11 is i(t), the terminal voltage 
Vc of the capacitor 2 is represented as follows: 
##EQU1## 
The value i(t) is determined by the voltage Vb-Vc between the base and the 
emitter of the transistor 11 and represented as follows: 
EQU i(t)=is exp [(q/KT) (VB-Vc)] (3) 
In conclusion, the terminal voltage Vc of the capacitor 2 is represented as 
follows: 
##EQU2## 
From known constants, .alpha. is about 26 mV for ordinary temperature. When 
Q is much larger than 1, the value of Vc increases 18 mV (=0.026 ln 2) 
each times Q doubles. That is, when the amount of light incident on the 
photodiode doubles, the terminal voltage Vc of the capacitor 2 increases 
18 mV. 
Hence, changing the output voltage of the reference voltage generator 5 
from 18 mV to 1800 mV, produces the dynamic range of 100 APEX steps. 
Equation (4) representing the terminal voltage of the capacitor of the 
light integrating circuit shown in FIG. 2 includes a (Q+1) term. Thus, as 
shown by curve "a" in FIG. 3, when Q is substantially greater than 1, a 
logarithmic relation prevails between the amount of light incident on the 
photodiode and the terminal voltage of the capacitor. However when Q is 
not sufficiently greater than 1, the logarithmic relation between the 
amount of light incident on the photodiode and the terminal voltage of the 
capacitor no longer holds. 
For example, as shown by curve a in FIG. 3 and column a of the table in 
FIG. 4, when Q is sufficiently large and doubles, the terminal voltage of 
the capacitor increases 18 mV, while when Q is near 1 and doubles, for 
example, as Q changes from 2 to 4, the terminal voltage of the capacitor 
increases only 13.2 mV. On the other hand, generally as the light 
adjusting level of the flash unit increases one APEX step, the standard 
level representing the light adjusting level increases to a predetermined 
value. Consequently, where the light emission of the flash light device is 
stopped, if the light adjusting level is low when the light integrating 
circuit is used as the light adjusting circuit of the flash unit, that is, 
if the level of the light reflected by the object from the flash light 
device only reaches a very low level, a logarithmic relation between the 
amount of light incident on the photodiode and the terminal voltage of the 
capacitor does not obtain. Hence, a large error exists in the light 
adjusting level, and a proper flash exposure cannot be obtained. 
In the disclosed systems embodying the present invention and containing 
light integrating circuits, elements having the same reference numerals as 
those of FIGS. 1 and 2 denote like parts, and their explanations are 
omitted. In FIG. 5, which illustrates an embodiment of the invention, a 
switch 21 is normally closed before a light measurement and opens in 
synchronism with the start of a light measurement. A voltage V1 connected 
to the non-inverting terminal of the operational amplifier 10 applies its 
voltage to the terminal B of the capacitor 2 while a voltage V2, which is 
less than the voltage V1, applies its potential to the terminal A of the 
capacitor 2 through the closed switch 21. Hence, while the switch 21 is 
closed, the capacitor 2 is charged to an inverse voltage -(V2-V1). After 
opening of the switch 21, the capacitor 2 is charged by the emitter 
current of the expander transistor 11. 
The switch 21 is constructed to be closed upon closing of the flash unit's 
power switch (not shown) and to be opened either by a signal source 25 
which produces the signal that actuates the light emission of the flash 
unit, or the signal synchronous with the time the camera shutter starts to 
run. In any case, the switch is constructed to close before the start of 
the light measurement by the light integrating circuit, and opened upon 
start of the light measurement by the light integrating circuit. 
In operation, when the switch 21 is closed before the start of the 
operation of the light integrating circuit, the terminals A and B of 
capacitor 2 are set at voltages V2 and V1, respectively, so as to charge 
the capacitor to a voltage difference of V1-V2 or -(V2-V1). 
When the switch 21 is opened, the expander transistor 11 begins to charge 
the capacitor 2 with current that is established by the operational 
amplifier 10 and the diode 12 compressing the photocurrent produced by 
light striking the photodiode 1. 
Because the voltage at the terminal A of the capacitor 2 is pre-set at the 
potential V2 for the start of the light measurement, and the capacitor 2 
has been charged with the voltage difference -(V2-V1) across its 
terminals, opening of the switch 21 at the start of light measurement 
causes the voltage at the terminal A first to rise to the level V1. 
Hence, the voltage at the terminal A starts at a value less than the 
reference value V1, and follows a curve such as the curve b in FIG. 3. A 
more detailed representation of the curves a and b appears in FIG. 6. 
Here, the curve b virtually follows an ideal logarithmic curve c even in 
the range in which the amount of light Q is quite low. Consequently, 
applying light adjustment on the basis of APEX values to the present 
invention, namely increasing the voltage of the reference voltage source 
by a predetermined value every time the light adjustment level is doubled, 
reduces the error to a very small value. 
In this embodiment, the value V2 is set 54 mV lower than the reference bias 
level V1. The voltage V2 may be further lowered where it is necessary to 
make the relationship between the terminal voltage of the capacitor 2 and 
the incident light approach an ideal logarithmic relation. 
According to this embodiment of the invention, a light measuring circuit 
integrates the amount of light incident upon the light sensing element by 
charging a capacitor connected to the emitter of an expander transistor 
with current representing logarithmically compressed and expanded light 
striking the light sensing element. Charging means charge the capacitor to 
a predetermined voltage whose polarity is inverse of the emitter current, 
before the light integrating operation. This makes the relationship 
between the terminal voltage of the capacitor and the light striking the 
light sensing element more closely approximate a logarithmic relationship 
even in the range in which the amount of light striking the light sensing 
element is small. Hence, the incident light can be detected more 
accurately. Furthermore, even using a photodiode having a small current 
output as a light sensing element, namely one with a small PN junction 
area, a logarithmic relationship exists between the incident light and the 
voltage at the terminal of the capacitor. Thus, the light sensing element 
can not only be made small, but its cost may be low. 
Column b of FIG. 4 illustrates the voltages achieved by various values of 
light by the circuit of FIG. 5 at low levels. It will be noted that an 
increase from 2 to 4 produces a voltage doubling of 18 mV. 
FIG. 7 illustrates another embodiment of the invention. Here, again, parts 
having reference numerals the same as those in FIGS. 1, 2, and 5, 
represent like elements. The signal source 25 open the switch 22 in 
synchronism with the start of a light measurement. Although the switch is 
shown as mechanical, it goes without saying that it can be replaced with a 
semiconductor switch. 
A capacitor 20 is connected to the collector of the expander transistor 11. 
A comparator 23 detects the charge level of the capacitor 20 and goes high 
when the capacitor 20 charges to a predetermined level Vc. A switch 31 
opens when the level of the output of the comparator 23 goes high. This 
switch can also be replaced with a semiconductor switch in the same manner 
as the switch 22. 
Although the signal source 25 opens the switch 22 in synchronism with the 
start of a light measurement, it may also open the switch 22 with the 
start of light emission of the flash device when the present invention is 
applied to the light adjusting circuit of a flash unit. Furthermore, the 
source 25 may be synchronized with the start of a shutter curtain run. 
Before the light integrating circuit starts its operation, both of the 
switches 22 and 31 are closed to discharge both of the capacitors 2 and 
20. When the light integrating circuit starts to operate either in 
response to a signal for starting the operation of the flash unit or a 
signal which starts the shutter running, the switch 22 opens and the 
operational amplifier 10 and diode 12 compress the photocurrent produced 
by the light striking the photodiode, while the transistor 11 expands the 
latter signal. 
The collector current through the transistor 11 charges the capacitor 20 
until the charge exceeds a predetermined value. Then the level of the 
output of the comparator 23 goes high, and this opens the switch 31 and 
allows the emitter current of the transistor 11 to charge the capacitor 2. 
When the voltage of the capacitor 2 exceeds the reference voltage of the 
reference voltage source 5, the output of the comparator 4 goes from low 
to high and the light control circuit 6 stops the light emission of the 
flash discharge tube 7. 
The relation between the terminal voltage of the capacitor 2 and the amount 
of light striking the photodiode is shown by the curve b in FIG. 3. 
Because the voltage of the capacitor 20 does not reach the predetermined 
value until a predetermined amount of light strikes the photodiode 1, the 
output of the comparator 23 remains low so that the switch 31 remains 
closed. This short circuits the capacitor 2 and its terminal voltage 
remains zero. 
When the predetermined amount of light strikes the photodiode 1, the 
voltage of the capacitor 20 reaches the predetermined value and the output 
of the comparator 22 goes high. Hence, emitter current of the transistor 
11 starts to charge the capacitor 2. Consequently, the terminal voltage of 
the capacitor 2 is zero, as shown by the curve b in FIG. 3, until the 
amount of light striking the photodiode 1 reaches the predetermined value 
and rises along the logarithmic curve according to the amount of light 
striking the photodiode. 
This structure overcomes the shortcomings of conventional light integrating 
circuits where the relationship between the amount of light striking the 
light sensing element and the terminal voltage of the capacitor is not 
logarithmic in the range in which the amount of light striking the light 
sensing element is small. Also, it similarly has the advantages of the 
embodiment in FIG. 4. 
FIG. 8 illustrates yet another embodiment of the invention. Here, elements 
having the same reference numerals as in earlier figures represent like 
elements. In FIG. 8, transistors 11' and 11" have bases, emitters, and 
collectors common with those of the transistor 11. According to an 
embodiment of the invention, more than two such transistors may be added 
to the transistor 11. Instead of providing a plurality of transistors, 
whose bases, emitters, and collectors common with those of the transistor 
11, a transistor whose junction area between its base and emitter is a 
plurality of times as large as a junction area between the base and the 
emitter of a diode-connected transistor 13 in the feedback loop of the 
operational amplifier 10 may be provided to achieve the same effect. 
In operation, the photocurrent produced by the photodiode 1, which is 
current-to-voltage converted, and compressed by the transistor 13 which is 
diode-connected in the feedback path of the operational amplifier 10, is 
expanded by each of the transistors 11, 11' and 11" and charges the 
capacitor 2. Thus, the terminal voltage V(t) of the capacitor 2 can be 
represented as follows: V(t)=.alpha. ln (nQ+1) 
where n is the number of transistors whose bases, emitters, and collectors 
are connected in common with those of the transistor 11. 
If n is set to satisfy nQ&gt;&gt;1 in the above equation, namely, if a proper 
number of transistors whose bases, emitters, and collectors are common 
with those of the transistor 11 are provided, the terminal voltage V(t) of 
the capacitor 2 can be approximated as follows: V(t)=.alpha. ln (nQ), and 
the approximate logarithmic relationship shown by curve d in FIG. 6, and 
column c in the table of FIG. 4 prevails. 
The circuit of FIG. 8 will best be understood from considering FIG. 6, and 
the table of FIG. 4. In FIG. 4, column a represents the relation 
Vc=.alpha. ln(Q+1) the amount Q of incident light and the terminal voltage 
Vc in a conventional circuit, and column b represents the ideal relation 
Vc=.alpha. lnQ (in which the voltage Vc always increases a given value 
when the amount Q doubles whatever the amount Q may be) between the amount 
Q of incident light and the terminal voltage Vc. 
As can be seen from column a, in a conventional circuit, when the light 
amount Q doubles from 1 to 2, the terminal voltage increases 
28.6-18=10.6(mV). When the amount Q increases from 2 to 4, the voltage 
increases by 41.8-28.6=13.2(mV), and when the amount Q increases from 4 to 
8, the voltage increases 57.1-41.8=15.3(mV). Therefore, the increasing 
amounts (increments) of the terminal voltage are 10.6, 13.2 and 15.3, 
which are far from the increase by a constant value. 
In the present invention, as shown in column c, when the light amount Q 
increases from 1 to 2, the terminal voltage increases 57.1-41.8=15.3(mV). 
When the light amount Q increaes from 2 to 4, the voltage increases by 
73.7-41.8=16.6 (mV) and when the light amount increases from 4 to 8, the 
voltage increases by 90.0-73.7=17.2 (mV). Thus, the increments of the 
voltage for doubling of the light amount Q are 15.3, 16.6 and 17.2 which 
approximate a constant value of increase and provide a relation very close 
to the ideal relation in curve c of FIG. 6. 
In the circuit of FIG. 8, as shown by column c of FIG. 4, the voltage 
generated by the signal source 5 is set to correspond to the number of the 
transistors (11, 11', 11"). Also, because the terminal voltage generated 
in correspondence to the amount Q of incident light is higher than that in 
the conventional circuit, as shown by column a, it is possible to prevent 
the terminal voltage from being significantly influenced by noise. 
FIG. 9 shows the invention as embodied in a camera. Here a camera C 
includes a body B and an interchangeable lens L. In the body B, an 
otherwise conventional operating system OS for the camera C includes a 
light integrating circuit L1 which controls a flash unit F. The light 
integrating circuit L1 constitutes any one of the circuits in FIGS. 5,7, 
or 8, and operates in the manner described with respect to those figures. 
The invention is based on the recognition that if a logarithmic relation 
Vb=lnQ exists between the amount of incident light Q and the terminal 
voltage Vb of the capacitor, the terminal voltage increases by a specific 
constant value each time the amount Q doubles. In other words, when the 
terminal voltage Vb increases by the specific constant value, it can be 
assumed that the amount Q increases two times. 
By contrast, in a conventional light measuring circuit, the relation 
Vb=ln(Q+1) prevails between the amount Q of incident light and the voltage 
Vb so that when the amount Q doubles, the terminal voltage Vb of the 
condenser does not always increase by a constant value. In other words, it 
is impossible to assume the amount of incident light has increased twice 
even if the terminal voltage has increased a constant amount. 
The invention may also be embodied as shown in FIG. 10. Here, elements 
having the same reference numerals as in earlier Figures represent like 
elements. In FIG. 10, the capacitor 2 is connected to a voltage -V10, 
below ground, such as -54 millivolts. The switch 3 is connected across the 
capacitor 2. 
In operation, the switch 3 short-circuits the capacitor 2. Hence, current 
through the transistor 11, arising from light striking the photosenser 1 
and compressed by the diode 12 and amplifier 10, charges the capacitor 2 
so that its lower terminal is at the negative voltage V10. When the source 
25 opens the switch 3 at the start of a measurement, compressed current 
through the transistor 11 charges the capacitor 2 from a level -V10 below 
ground so that the voltage at the non-inverting terminal of the amplifier 
4 does not exceed ground level until a predetermined delay. Accordingly, 
the voltage follows the essentially logarithmic curve b in FIG. 3 and b in 
FIG. 6, and produces the results in column b of FIG. 4. According to an 
embodiment of the invention, the structure of FIG. 10 is used in the 
camera of FIG. 9. 
While embodiments of the invention have been described in detail, it will 
be evident to those skilled in the art that the invention may be embodied 
otherwise without departing from its spirit and its scope.