Electrical light-measuring systems
United States Patent Kobayashi ELECTRICAL LIGHT-MEASURING SYSTEMS Inventor: Katsumi Kobayashi, Tokyo, Japan Kabushiki Kaisha Yashica, Tokyo. Japan Filed: May 29, 1973 Appl. No.: 364,738  
 Assignee:  
 Foreign Application Priority Data May 31, 1972 Japan 47-63331 US. Cl 356/226; 250/206 Int. Cl. G01j l/44 Field of Search 356/226, 218; 250/206,  
 References Cited UNITED STATES PATENTS 8/1946 Sweet 356/226 9/1971 Nobusawa 250/214 3,700,337 10/1972 Fujii 356/218 Primary ExaminerRonald L. Wibert Assistant Examiner-Conrad Clark Attorney, Agent, or Firm-Steinberg &amp; Blake ABSTRACT An electrical light-measuring system capable of effectively measuring even relatively small amounts of light. A meter circuit is provided with an indicating meter which indicates the light intensity. This meter circuit is electrically connected with a currentamplifier circuit which controls the current of the meter circuit. This current-amplifier circuit has an input, and a constant-voltage circuit has an output, a photosensitive element being connected between the latter output and input for receiving the light which is to be measured. The meter circuit is capable of being adjusted for adjusting the relationship between the current which flows through the meter and the resistance of the photosensitive element.  
 3 Claims, 5 Drawing Figures PEJENTEE PRZ E SHKET 1 BF 2 E I PRIOR ART mimzumzzm 3,879.138  
 saw 2 If 2 5 1 JA J J5 LARGE LARGE Kz RD R l SMALL k2 SMALL IM LARGE 5mm 1&#34;. LARGE I ELECTRICAL LIGHT-MEASURING SYSTEMS BACKGROUND OF THE INVENTION The present invention relates to electrical lightmeasuring systems.  
  Thus, the present invention relates to electrical exposure meter systems capable of measuring the light intensity at an object which is to be photographed by a camera, for example.  
  Particular difficulties are encountered with conventional light-measuring systems of the above type because of the fact that an extremely feeble current is provided when there is a relatively low light intensity at the object which is to be photographed. Such systems require meters which have movable components, and such components operate in a mechanically improper manner when only a feeble current can be achieved. As a result of these considerations. the conventional systems do not operate uniformly throughout the entire range of light which is measured, and in addition the range of light which can be properly measured is limited to a range which is less than that which is required for making photographs under all lighting conditions which are likely to be encountered.  
 SUMMARY OF THE INVENTION It is accordingly a primary object of the present invention to provide a light-measuring system which will avoid the above drawbacks.  
  In particular. it is an object of the invention to provide a light-measuring system which makes it possible to obtain a relatively large current at a relatively low voltage even under conditions where the object to be photographed is illuminated at a relatively low light intensity.  
  Thus, it is an object of the present invention to increase considerably the range of light which can be measured with systems of the above type.  
  In addition, it is an object of the present invention to provide a system which enables the movable components of a meter to move in a mechanical manner with great reliability even under conditions of low light.  
  In addition, it is an object of the invention to provide a light-measuring system which will eliminate any lack of uniformity in the operation of the system under widely varying lighting conditions.  
  Furthermore, it is an object of the present invention to provide circuitry for accomplishing the above objects and utilizing simple rugged elements which are very reliable in operation and which are capable of operating in such a way that they are not undesirably influenced by such factors as temperature fluctuations.  
  In accordance with the invention, the light-measuring system includes a meter circuit means provided with an indicating meter for indicating the light intensity. A current-amplifier circuit means is electrically connected with the meter circuit means to control the current flowing therethrough. This current-amplifier circuit means has .an input. A constant-voltage circuit means has an output, and between the latter output and the above input there is connected :1 photosensitive means which receives the light which is to be measured. The meter circuit means includes in addition an adjusting means for adjusting the relationship between the resistance of the photosensitive means and the current which flows through the indicating meter.  
 BRIEF DESCRIPTION OF DRAWINGS The invention is illustrated by way of example in the accompanying drawings which form part of this application and in which:  
 FIG. I is a circuit illustrating the prior art;  
  FIG. 2 is a circuit of a light-measuring system in accordance with the present invention;  
  FIG. 3A is a graph illustrating one possible manner in which the relationship between the resistance of a photosensitive means and the current flowing through the meter can be regulated;  
  FIG. 3B is a graph illustrating another possible adjustment of the relationship betweenthe resistance of a photosensitive means and the current flowing through an indicating meter; and  
  FIG. 4 is a wiring diagram of another embodiment of a light-measuring circuit according to the present invention.  
 DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to FIG. 1, there is illustrated therein a light-measuring system as conventionally used In the circuit of FIG. 1 there is a&#39;battery 1 to provide a source of energy. The circuit also includes a photosensitive means 2 which receives the light which is to be measured. Resistances 3 and 4 are provided for correction of &#39;y In addition to the variable resistors 3 and 4 there is a variable resistor 5 for switching purposes, and the circuit includes the indicating meter 6 across which the variable resistor 5 is connected in parallel therewith.  
  As may be seen from FIG. 1, in this conventional light-measuring system, an extremely feeble current is provided when the light has a relatively low intensity since the photosensitive means 2 and the indicating meter 6, which may be a moving coil instrument such as a galvanometer, are connected in series with each other. As a result there is a disadvantage in that the movable components of the meter which are required to move mechanically operate very poorly under conditions of low current. In addition, with this conventional light-measuring system, operation of the switching resistance 5 under regulation of the variable resistors 3 and 4 which correct 7 produces a current stress with the result that the range within which the switching operations may be carried out by the variable resistor 5 is limited to the order of 3.5 EV.  
  With the light-measuring systems of the invention described below it is possible to avoid the above drawbacks of the conventional systems. Thus, with the systems of the invention it is possible to use a relatively intense current at low voltage so as to enable the indicating meter to be properly driven, while smoothing out and facilitating characteristic correction and exposure switching of the photosensitive element which receives the light while enlarging the range of exposure switching.  
  Referring to FIG. 2, a basic embodiment of the present invention is illustrated therein. The light-measuring system of FIG. 2 includes the source battery 11 which is electrically connected with a constant-voltage circuit means formed by the series-connected voltage divider resistance 12 and the constant voltage element 13. A current-amplifier circuit means is formed by the transistor 14 which is emitter-follower connected and which has an input at its base. The junction a between the voltage divider resistance 12 and the constant volt- 3 4 age element 13 forms the output of the constantchanically strong, utilizing a large output current with voltage circuit means 12,13. Between this output a and relatively low voltage. the input formed by the base of transistor 14 there is b When the light intensity at the object to be photoconnected a photosensitive means 15 in the form of an graphed is relatively low, relationship between 1,, and element such as a cadmium sulfide element connected R,, i expressed b in the manner shown in FIG. 2 between the base of the K, transistor 14 and the junction :1. K. .R,,  
  The current-amplifier circuit means 14 serves to conand, when the light intensity at the object is high, this trol the current ofa meter circuit means which is elecrelation is in accordance with the above-mentioned trically connected to the emitter of the transistor 14. to equation (3) as follows:  
 This meter circuit means includes the indicating meter I K. R,, 1r  
 17 which, for example, may take the form of a moving coil instrument such as a galvanometer.  
  In order to adjust the relationship between the resistance of the photosensitive means and the current 15 which flows through the indicating meter 17, a pair of variable resistors are provided in the example of FIG.  
 2, these variable resistors including variable resistor 16 This relationship between I and R may be illustrated by the characteristic curves of FIG. 3A. As may be seen from FIG. 3A, adjustment of the series variable resistor 16 varies K and in a corresponding manner the slope of the characteristic curve. It is possible. therefore, to absorb non-uniformity in the operation of the which is connected in series with the meter 17 between indicating meter 17 and to correct non-uniformity in the latter and the emitter of transistor 14. Thus, the 20 the operation of the photosensitive means 15 based variable resistor 16 forms a series variable resistor. A upon the characteristic by adjustment of the variable parallel variable resistor 18 is connected across the serresistor 16. Adjustment on the order of 1.5 EV is possiies-connected series variable resistor 16 and indicating ble. meter 17 in parallel therewith as illustrated in FIG. 2. c. Variation of K; by adjustment of the variable resis- The driving current which passes through the indicattor 18 displaces the characteristic curve without changing meter can be calculated in accordance with the foling its slope, as is indicated in FIG. 33, since K is the lowing: proportional constant of R Accordingly, it is possible Current 1,, which passes through the photoreceiver to achieve exposure switching such as ASA adjustment 15 is obtained as follows: conversion as well as to correct non-uniformity in this V V IB 2 D n I e o 0 18 16 n,  
  &#39;5 (h 1) Rh 15 i I le parallel displacement caused by operation of the indiwhere V terminal voltage of the constant voltage element eating meter 17 and the photosensitive means 15 by ad- 13, justment of the variable resistor 18. While K can vary V voltage across base-emitter of the transistor 14, infinitely on a theoretical basis, actually it can vary sufh amplification factor of the transistor 14, ficiently to attain exposure switching on an order of be- R,,: resistance value of the photoreceiver element 15, 40 tween 5 and 6 EV. Such a range of exposure switching R R resistance values of the adjustable resistcan be, of course, enlarged by use of suitable means ances 16, 18, respectively, and such as a filter at the operating or working surface of R resistance value of the indicating meter 17. the photosensitive element 15. Ac r ngly. Current n hich passes through the in- FIG. 4 illustrates another embodiment of the invendicating meter 17 iS O tained 8 fOlIOWSI tion where the voltage variation characteristic and the V (h l) (2) i-W t +(h +1)(R +11) 18 p re 16 M Assum d that n. ac) re i) Kh temperature characteristic at a low voltage range (for R,,.+ ,,,+R,,, example on the order of 3 to 2 V) are considered. In  
  Rm this embodiment, the constant voltage circuit means inga eludes the constant voltage element 19 connected in n series with a voltage divider resistance 20. A pair of junctions in the series connection between elements 19 It be readily understood from the latter equation and are respectively connected a pair of resisthat depehds p a Particuizlr Stop Value reia&#39; tors 21 and 22 which as illustrated are in parallel with iive to the photosensitive meahs 15, as is appariaht from each other and electrically connected to the base of a the following? transistor 23 which is in series with the constant voltage Proper Choice of the ampiiiicatioh factor of element 13. The current passing through the constant Sisior l4 Wiii Provide [M of reiativei) high ihiehsiiy P&#39; voltage element 13 is then selected in such a way that proximiiieiy 10 times that which cah be achieved the terminal voltage of constant voltage element 13 is a conventional circuit as shown in FIG. 1). Thus, it is always maintained constant i d d fl f voltage possible to provide an indicating meter which is mevariation at h battery Source 11 The temperature characteristic is compensated by the combination of the constant voltage element 19 and the transistor 23 as well as the combination of the constant voltage element 13 and the transistor 14. In addition. the temperature balance is maintained by use of a thermistor for the resistance 22 in combination with the elements 19, 23 and l3, 14.  
  Thus. in accordance with the invention, as is apparent from the above, it is possible to obtain a large current at a relatively low voltage even when the object which is to be photographed is illuminated with a relatively low light intensity so that it is necessary to operate in a low range of brightness. It becomes possible with the invention to utilize for the indicating meter robust mechanical elements which will move reliably with the required force even when operating at relatively low light intensities, so as a result with the present invention it is&#39; not essential to use extremely delicate components for the meter, as is necessitated by the requirement of operating with conventional systems at relatively low light intensities. in addition, it is possible with the invention to enlarge the range of exposure switching as compared with previously known exposure meter systems. In addition, lack of uniformity in the operation of the system, resulting from operation of the indicating meter and the photosensitive means can be very readily corrected by proper adjustment of the variable resistors. As a result, the features of the present invention are of great practical value.  
  It is to be noted that the constant voltage elements such as elements 13 and 19 may each be formed by a diode, a varistor, or a Zener diode.  
 What is claimed is:  
  1. In an electrical light-measuring system, meter circuit means including an indicating meter for indicating light intensity, current-amplifier circuit means electrically connected with said meter circuit means for controlling the current thereof, said current-amplifier circuit means having an input, constant voltage circuit means having an output, and photosensitive means for receiving the light which is to be measured, said photo: sensitive means being electrically connected between said output and input, said meter circuit including a series variable resistor connected in series with said indicating meter for varying the slope of the relationship between the resistance of said photosensitive means and the current which flows through said indicating meter. and a parallel variable resistor connected across the series connected series variable resistor and indicating meter in parallel therewith for changing the range of said relationship, said current-amplifier circuit means including a transistor having a base connected to said photosensitive means and an emitter connected in series with said series variable resistor and indicating meter.  
  2. The combination of claim 1 and wherein said constant voltage circuit means includes a constant voltage element and a voltage divider resistance connected in series and having therebetween a junction which forms said output connected to said photosensitive means.  
  3. The combination of claim 1 and wherein said constant voltage circuit means includes a first constant voltage element and a transistor connected in series therewith and having in the series connection a junction which forms said output, a second constant voltage element and a voltage dividing resistance connected in series with each other and being in parallel with said first constant voltage element and said transistor connected in series therewith, said second constant voltage element and voltage divider resistance having a pair of junctions in the series connection therebetween, and a pair of resistors respectively connected to said pair of junctions and connected in parallel to each other and to the base of said transistor which is in series with said first constant voltage element.