Photography – Exposure control circuit – Having log transformation circuit
Reexamination Certificate
2000-03-13
2001-11-27
Perkey, W. B. (Department: 2851)
Photography
Exposure control circuit
Having log transformation circuit
C356S222000, C396S234000
Reexamination Certificate
active
06324344
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improvement on a light measuring device having an integration-type light measuring sensor.
2. Description of Related Art
Some of known exposure computing devices use multiple-divided integration-type light measuring sensors arranged to logarithmically compress an electrical signal from a light receiving part and to output the compressed signal as a light measurement output measured by each pixel. To have one and the same specific output value from all the pixels of the light measuring sensor for a field having uniform distribution of luminance which varies, the exposure computing device of this kind is arranged to make level correction and gain correction on each pixel of the light measuring sensor and to compute an exposure value on the basis of these correction values and the output values of the light measuring sensor. A memory which is provided for storing correction values is arranged to store the level correction value and gain correction value for each of pixels. Assuming that a storage capacity of 1 byte is necessary per pixel for storing each of the level correction value and gain correction value, in the case of a light measuring sensor which is divided into six parts, i.e., pixels, for example, a storage capacity necessary for correction of all pixels is “2 bytes×6=12 bytes”.
To broaden the dynamic range of the integration-type light measuring sensor, the integration period of the light measuring sensor is sometimes varied according to changes taking place in luminance of a field. Therefore, in order to obtain a highly reliable exposure value, it has been necessary to correct the sensor outputs for a plurality of integration periods. Then, in the case of the above-stated light measuring sensor divided into six parts, if this sensor is arranged to be capable of coping with five different integration periods, the memory which stores sensor-output correction values must be arranged to have a storage capacity of “12×5=60 bytes”.
As described above, the conventional exposure computing device is arranged to use a multiple-divided integration-type light measuring sensor, to be provided with correction values for making the output of the light measuring sensor into specific values suited for various luminance values, and to carry out an exposure computing operation by using the correction values as applicable. In a case where the number of dividing the light measuring sensor increases and the different integration periods are many in number, an enormous storage capacity becomes necessary for adequately correcting the output of the light measuring sensor. For example, with a memory capacity required for storing a gain correction value per pixel assumed to be 1 byte, if a light measuring sensor divided into 100 pixels is arranged to be capable of coping with three different integration periods, as shown in
FIG. 13
, the memory must be arranged to have a storage capacity of “100×1=100 bytes” for gain correction.
Further, with a memory capacity required for storing a level correction value per pixel assumed to be 1 byte in this case, as shown in
FIG. 14
, the memory must have a storage capacity of “100×3=300 bytes” for level correction. Then, a total storage capacity of the memory required for all correction values becomes 400 bytes.
The conventional arrangement also requires many processes for computing and storing the sensor correction values in the memory within an exposure computing part. A very long period of time thus becomes necessary for adjusting the outputs of the light measuring sensor.
BRIEF SUMMARY OF THE INVENTION
It is one object of the invention to provide a light measuring device for a camera or the like arranged to be capable of solving the problems of the conventional arrangement described in the foregoing.
To attain the above object, in accordance with one aspect of the invention, there is provided a light measuring device, which comprises an integration-type light measuring sensor having a plurality of pixels and arranged to logarithmically compress a signal of light received by each pixel and to output the signal as a light measurement value for each pixel, a memory arranged to store an average of gain correction values for all pixels of the light measuring sensor, a deviation-from the average of gain correction values obtained for each pixel of the light measuring sensor, a level correction value for each pixel of the light measuring sensor obtained for a reference integration period, and an average of level shift amounts of all pixels of the light measuring sensor caused by a change of integration period, and a computing circuit arranged to compute, from the level correction value for each pixel and the average of level shift amounts of all pixels stored in the memory, a level correction value for each pixel of the light measuring sensor for an integration period other than the reference integration period, so that, as compared with a case where a memory is arranged to store a level correction value for each pixel of the light measuring sensor with respect to every one of different integration periods, the above arrangement effectively permits reduction in storage capacity of the memory.
The above and other aspects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.
REFERENCES:
patent: Re. 32376 (1987-03-01), Fukuhara et al.
patent: 5602615 (1997-02-01), Muramatsu et al.
Canon Kabushiki Kaisha
Perkey W. B.
Robin Blecker & Daley
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