Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Reexamination Certificate
1999-02-04
2001-04-24
Allen, Stephone B. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
C250S228000, C356S236000
Reexamination Certificate
active
06222172
ABSTRACT:
BACKGROUND
Many different image sensors are known in the art. In most cases, the basic function of an image sensor is to produce an electrical response indicative of the intensity of light illuminating its picture elements, or pixels. Each individual pixel contains a light detector, which produces an electrical signal by converting the photons of the incident light to electrons, and accumulating these electrons for a certain period of time. This period of accumulation is often called the integration time, because the process of accumulating light-generated electrons, or photoelectrons, is equivalent to integrating the light intensity over time. Typically, the pixel is exposed to light for its entire integration time, in which case the exposure time and integration time are identical. It is possible, however, to make the exposure time shorter than the integration time by turning off or blocking the light for some part of the integration time.
Generally, the number of photoelectrons collected in the pixel depends on the length of the exposure time and on the intensity of the incident light during that time. A change in one quantity is typically indistinguishable in its effect from a change in the other—both change the amount of light absorbed by the pixel, the number of photoelectrons that are generated, and consequently the electrical response of the pixel. In the particular case when the light intensity is constant throughout the exposure time, the pixel response is simply proportional to both the light intensity and the length of the exposure.
Complete testing of an image sensor requires measuring its various physical characteristics, including the response to different amounts of light. Determination of the dynamic range of the sensor requires varying the amount of light over a comparable, or wider, dynamic range. In most cases, it is difficult to do so by varying the light intensity while keeping the exposure time constant. Light sources typically work well only within a narrow range of output intensities, or change their spectral characteristics when their output intensity is changed. Moreover, the variation of their output intensity within the available dynamic range is typically nonlinear. An alternative to changing the light source output is to use an external light intensity attenuator. The throughput of such a device usually cannot be continuously and precisely varied. In contrast, it is relatively easy to control the exposure time of the sensor with high precision. As stated above, changing the duration of the sensor's exposure to a constant light level causes a proportional change in its response. Hence, varying the exposure time instead of the light intensity has often been the preferred method of measuring such sensor parameters as dynamic range, linearity, signal-to-noise ratio, and conversion gain.
Traditional incandescent light sources have large thermal inertia. This slows their response to power supply interruptions. Therefore, it is difficult to effectively vary the time of the sensor's exposure to light from such a source by interrupting the operation of the source. Typically, a more accurate exposure control is achieved by fast shuttering of the continuously emitted light.
SUMMARY
The present specification describes a light source which does not have these drawbacks. This light source uses light emitting diodes with controllable output parameters. Light-emitting diodes are orders of magnitude faster in their response to voltage-supply interruptions than incandescent light sources. Typically, a light-emitting diode has a rise-and-fall time below 1 microsecond. This makes it possible to precisely control the sensor's exposure time.
According to a preferred mode, an LED array is powered with a periodic rectangular voltage waveform whose duty cycle is digitally controlled. A system is described herein which allows achieving a three-decade dynamic range of exposure time for integration times below a thirtieth of a second.
REFERENCES:
patent: 4232971 (1980-11-01), Suga
patent: 4720190 (1988-01-01), Peterson
Fossum Eric R.
Waligorski Gregory
Allen Stephone B.
Fish & Richardson P.C.
Photobit Corporation
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