Optics: measuring and testing – Photometers – Photoelectric
Patent
1987-01-28
1988-12-27
Howell, Janice A.
Optics: measuring and testing
Photometers
Photoelectric
2503361, 25037001, G01J 144
Patent
active
047937041
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATION(S)
This United States application stems from PCT International Application No. PCT/SE 86/00325 filed July 4, 1986.
BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to an electric circuit in an instrument for detecting ultra-violet radiation, visible light or infrared radiation, comprising a photo-diode of silicon or germanium, a precision resistor for measuring, via which generated photo-electric current from the photo-diode generates a voltage, and a voltmeter with low leakage current for measuring the voltage generated.
The present invention has as an object to render possible a structural design, which is less expensive, simpler and safer than the ones known and commercially available.
The invention is characterized in that the precision resistor and calibrated voltmeter each are connected in parallel with the photo-diode.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in the following with reference to the accompanying drawings, in which
FIG. 1 shows schematically a circuit diagram for a known photometric circuit,
FIG. 2 shows the current-voltage characteristic of the photo-diode in the forward direction, and
FIG. 3 shows a circuit diagram according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The cicuit shown in FIG. 1, comprises a photo-diode F and a voltmeter connected in parallel therewith. Before the voltmeter an operational amplifier O and a resistance R.sub.A are connected. The resistance can be exchanged with resistances of a different size for changing the measurement ranges. The circuit acts in such a way, that the photo diode F at exposure to the radiation E generates in its backward direction a photo-electric current I.sub.F. The photo-electric current I.sub.F is passed on via the amplification resistor R.sub.A of the operational amplifier O. The generated voltage is indicated by the voltmeter V. This voltage is proportional to the radiation E provided that the resistance trimmer T is adjusted so that
As this active amplification system is temperature-sensitive, the adjustment must be made manually or automatically at considerable cost.
In FIG. 2 the current-voltage characteristic of the photo-diode is shown in the forward direction, i.e. opposed to the direction of the photo-electric current generation. It appears from FIG. 2 that at low voltage V in the forward direction the diode acts as an insulator. The resistance dV/dI is very high, near the origin of the coordinates. This applies especially to silicon diodes.
FIG. 3 shows the circuit according to the present invention. The voltmeter and the measurement resistor R.sub.A both are connected in parallel with the diode F. The measurement resistances can be exchanged automatically for changing the measurement ranges. The resistor R.sub.L illustrates the resistance in the barrier layer of the photo-diode in the conducting direction, i.e. slope dV/dI of the curve in FIG. 2.
I.sub.F is the photo-electric current in the backward direction of the diode generated by the radiation E. I.sub.L is the leakage current generated by the voltage V where
The leakage current is in the forward direction. I.sub.V is the very low leakage current through the modern voltmeter V. The magnitude for for the leakage current is one pico-ampere.
When the generated voltage V across the resistor R.sub.A is limited to the amplitude range .DELTA.V in FIG. 2, below which the backward resistance R.sub.L can be regarded as much higher than R.sub.A, therefore the leakage current I.sub.L is much less than the photoelectric current I.sub.F, and when I.sub.V can be regarded as much lower than I.sub.F, it applies that I.sub.F -I.sub.V -I.sub.L is approximately equal to I.sub.F and, thus, the generated voltage is approximately
The voltmeter is adjusted so that maximum deflection is attained at a voltage equal to or lower than .DELTA.V. Calibration with a known radiation E can be carried out directly by adjustment of the voltmeter when R.sub.A is chosen so that I.sub.F .times.R.sub.A i
REFERENCES:
patent: 3113220 (1963-12-01), Goulding et al.
patent: 3379094 (1968-04-01), Bertram
patent: 3381147 (1968-04-01), Abraham
patent: 3418473 (1968-12-01), Blue
Wallmark et al, "Optoelektronik," (in Swedish), published 1981 by Studentlitteratur (Lund), p. 81.
Howell Janice A.
Rauchholz William F.
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