Optics: measuring and testing – Photometers – Photoelectric
Reexamination Certificate
2000-12-19
2003-01-21
Stafira, Michael P. (Department: 2877)
Optics: measuring and testing
Photometers
Photoelectric
Reexamination Certificate
active
06509963
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a photometric apparatus and, more particularly, to an improved photometric apparatus having plural light sensors for detecting light from different regions at plural luminance measurement points and having means for enhancing accuracy of photometry by increasing the number of luminance measurement points in regions where the respective light sensors have a non-linear characteristic.
2. Description of the Related Art
Conventionally, a photometric apparatus used, for example, as an exposure meter for a camera comprises an incident light measuring unit for measuring an average brightness of light irradiating a subject to be photographed, and a reflected light measuring unit for measuring brightness of a particular portion of the subject.
FIG. 4
shows an example of the above-described photometric apparatus. In the illustrated apparatus, incident light is measured using a white ball
102
arranged on a front surface of a body case
101
. In connection with the measurement of incident light, light is input from a first area corresponding to a relatively large angle of visibility (for example, 30° to 40°) through the white ball
102
arranged on the front surface of the body case
101
and is measured using a first light sensor SPD
1
, such as a silicon diode or the like, arranged behind the white ball
102
to detect the received light. In response to the output of the first light sensor SPD
1
, the average brightness of the surrounding area is measured. That is, brightness of the first area is measured on the basis of an output of the first light sensor SPD
1
. Measurement of reflected light is performed by so-called spot photometry, which uses a lens
103
arranged on a rear surface of the body case
101
to receive light from a second area corresponding to a relatively small angle of visibility (for example, 5° to 10°). For this purpose, a second light sensor SPD
2
, such as a silicon diode or the like, is arranged behind the lens
103
to detect the light received from the second area. In response to the received light, the brightness of a particular portion of the first area, that is, the brightness of the second area, is measured on the basis of output of the second light sensor SPD
2
.
In many cases, photometric devices are provided with a viewfinder
104
which allows the user to visually define the specific portion of the subject for which measurement of reflected light is desired. Normally, a display unit
105
for displaying the results of measurement and a mode switch SWm are also provided on the front surface of the body case
101
where the white ball
102
is provided. The lens
103
may be mounted on the surface of the body case
101
opposite the surface on which the white ball
102
is provided, or the lens may be mounted so that it is rotatable relative to the position of the white ball
102
on the body case
101
.
As described above, the second light sensor SPD
2
used for measurement of reflected light receives light collected from a smaller area than that used for measurement of incident light. Therefore, when performing measurement of incident light and measurement of reflected light under the same ambient luminance, the quantity of light incident upon the second light sensor SPD
2
used for measurement of reflected light is smaller than the quantity of light incident upon the first light sensor SPD
1
used for measurement of incident light. Thus, the photoelectric current output by the second light sensor SPD
2
is also smaller than that output by the first light sensor SPD
1
. As is often the case, identical components are used in parallel light detection circuits to simplify construction and avoid errors due to circuit constants occurring when circuit components of different construction are used. Accordingly, in the case where light sources having the same structure are employed for the first and second light sensors SPD
1
and SPD
2
, and photoelectric current-to-voltage conversion circuits having the same performance characteristics are used to convert photoelectric currents output by the light sensors into voltages, the relationship between input and output becomes that as shown in FIG.
5
. More specifically, when photometry is performed under the same ambient luminance, the output of the photoelectric current-to-voltage conversion circuit associated with the second light sensor SPD
2
used for the measurement of reflected light has a smaller value than that output by the photoelectric current-to-voltage conversion circuit associated with the first light sensor SPD
1
used for the measurement of incident light.
An input/output characteristic of the light sensors will be described below with reference to FIG.
5
. Generally, the relationship between the input (luminance value LV) versus the output (exposure value EV) characteristic of a light sensor used in exposure meters containing a photoelectric current-to-voltage conversion circuit is linear in a predetermined range of luminance values, but tends to become non-linear in its output in areas having higher or lower luminance values than the predetermined range of luminance values. This loss of linearity is usually more pronounced on the low luminance side of the predetermined range. Such tendency is illustrated in the graph shown in FIG.
5
. Due to the above-described difference in the quantity of light incident upon the first and second light sensors SPD
1
and SPD
2
, the photoelectric current output by the second light sensor SPD
2
begins on the low luminance side to become non-linear at a higher luminance value (LV about 6) than that of the first light sensor SPD
1
. Conversely, because a higher intensity light is incident on the first light sensor SPD
1
, the photoelectric current output by the first light sensor SPD
1
begins on the high luminance side to become non-linear at a lower luminance value (LV about 15) than that of the second light sensor SPD
2
. As a result, the predetermined range of luminance values having linearity, for which signal processing is easily conducted, is normally deemed the measurable luminance range of the device.
A method of linear interpolation such as that disclosed for example, in Japanese Patent Laid-Open No. 44018/1992 (which is incorporated herein by reference), has been used for correcting errors in measurement results due to errors in products into design values. Simply explained, the method of linear interpolation disclosed therein comprises the storing of design values (design data) and actual measurement values (actual measurement data) of outputs from a plurality of measurement points, which are preset for the ambient luminance, finding design values of outputs for the measurable area on the basis of the preset values stored by way of linear interpolation in actual use, and correcting actual measurement values in actual use into the design values thus found by linear interpolation, on the basis of characteristic errors between the stored actual measurement values and the design values. For example, with a photometric apparatus having one sensor, photometry is actually performed in brightness at a plurality of predetermined luminance measurement points in a process of manufacture or product inspection, and actual measurement values and design values at that time are stored as correction data in a nonvolatile memory such as EEPROM or the like. During actual use of the device, the correction data stored in nonvolatile memory is used to conform actual measurement values obtained by the photometric apparatus to design values obtained by performing linear interpolation on a plurality of design values in the correction data stored in the nonvolatile memory.
While an example of linear interpolation using one sensor is described above, a method of correcting actual measurement values in actual use into design values is carried out under the same general scheme as described above in the case where two sensors are used. For example, photometry is actually performed
Iwasawa Takeshi
Oda Hajime
Adams & Wilks
Seiko Instruments Inc.
Stafira Michael P.
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