Radiant energy – Photocells; circuits and apparatus – Photocell controls its own optical systems
Patent
1901-06-03
1993-01-19
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Photocell controls its own optical systems
250221, 250214B, 351210, G01J 132
Patent
active
051809079
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to measuring intensity variations of light, visible and/or not visible to the human eye, preferably not visible, such as IR-light. The invention is particularly intended for use in a system for measuring eye movements but is not limited to this field of application.
In measuring the amount of light or other similar radation, light sensitive detectors as photo diodes or photo transistors can be used. These elements are usually not linear, i.e. the delivered electric signal is not directly proportional to the amount of incident light upon the detector surface. This means that the curve of the corresponding electric signals, e.g. a voltage E, has to be determined as a function of the amount of incident light O. In this way a precise measurement of the amount of light could be obtained. However, this procedure is complicated, and in automatic use the detector has to be supplemented with an electronic calculating device, e.g. a microprocessor, to deliver the desired proportional electric signal.
These measurements are normally performed when recording the eye movements of a person. In this procedure, generally, infrared light is emitted from a light emitting element placed in front of the eye and the reflected light is detected by an infrared sensitive light detecting element. The total amount of light incident upon the person's eye is composed of the ambient light and the light reflected by the eye. This reflected light is caused essentially by the light emitted by the light emitting element. When the intensity of the ambient light changes, however, the light detecting element has to be recalibrated to compensate it. The above mentioned lack of linearity in most commercially available photo sensitive elements makes this recalibration complex.
To overcome the above mentioned difficulties, the invention purposes a process and a device for measuring variations in light intensity, the characteristics of which are set out in the appended claims.
Thus, it is suggested that an extra light emitting element is arranged to emit some extra light directly into said light sensitive element. This extra light is controlled to give the light sensitive element a predetermined, stable operational point. In this way the variations of light intensity are always taken from a fixed point on the characteristic curve of said light sensitive element.
In practical use it is supposed that the incident light is composed of background light, which is almost constant or changing relatively slowly, and rapidly changing light variations. By suitable means, e.g. electronic circuitry, some of the background light and the injected extra light is controlled to be constant. This can most easily be done when said light variations are known to be equal to zero in predetermined time intervals. If the light detector output is changed from one interval to another, the injected light is changed to keep this output signal constant.
Instead of maintaining a stable operational point for the light detector it can, by means of the extra light injected by the extra light emitting element, be arranged in such a way that the measuring process always is performed for light values on a linear portion of the characteristic curve of the light sensitive element. In periods where the rapidly changing light variations are known to be equal to zero, the injected extra light is changed stepwise with steps of known sizes. By comparing the output signals delivered by the light sensor, to the value obtained without the injected light it can easily be seen if the output values either are proportional to the total amount of light incident upon the light sensitive element or if they at least are located on a linear portion of the characteristic.
In an embodiment of the invention the light sensitve element is substantially planar, e.g. rectangular, and the first light emitting element is then placed adjacent to and facing one edge or side, preferably the long side, of the light sensitive element.
When using the invention for measuring eye movemen
REFERENCES:
patent: 4260882 (1981-04-01), Barnes
patent: 4273999 (1981-06-01), Pierpoint
patent: 4720189 (1988-01-01), Heynen et al.
patent: 4812642 (1989-03-01), Hasegawa et al.
Ober Jan K.
Udden Per
Nelms David C.
Shami K.
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