Method to detect radiation and measure its angle of incidence an

Details Method to detect radiation and measure its angle of incidence an Method to detect radiation and measure its angle of incidence an

1989-07-20

1991-04-23

Nelms, David C.

Radiant energy

Photocells; circuits and apparatus

Photocell controlled circuit

356141, H01J 4014

Patent

active

050102443

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a method for detecting radiation and measure its angle of incidence and a detector arrangement to carry out the method. The background for the invention is that in certain electromagnetic radiation, for example laser light of different wavelengths from UV to IR, there occurs an irregularity, speckledness, that is among other things caused by turbulence in the atmosphere. The speckledness varies with a frequency of up to 100 Hz. Experiments that have been carried out have shown that this phenomenon, that means a great variation in the irradiance of the radiation that falls onto a detector, gives rise to problems during the registration of a single or a few short pulses of radiation. On one hand the probability of detection can decrease considerably and on the other hand difficulties to reliably determine the angle to the source of radiation with high resolution can arise.


SUMMARY OF THE INVENTION

These problems are, however, solved by the invention by giving it the design that is evident from the following claims.


BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in more detail with reference to the accompanying drawing where
FIG. 1 shows a detector arrangement according to the invention and
FIG. 2 shows a detector sensitive to the angle of incidence according to the invention.


DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The basis for the idea of the invention is that the probability of detection by trying to detect the incident speckled radiation in a number of points at such a great distance from each other that the correlation between the irradiance received by the different detectors is very small increases, at the same time as the direction of the radiation in relation to a reference direction by means of specially designed detectors that consists of two detector elements 1,2 is determined. The detector elements have their detector planes in a certain angle to each other and are placed so close to each other that the correlation between the irradiance of the radiation towards them is high. Popularly speaking both detector elements measure towards the same speckle in the speckled radiation.
The correlation between the irradiance detected by the detector elements 1,2 depends on the mounting distance between the two detector elements. The wavelength, the illumination distance and the atmospheric condition also play a part here. As an example the irradiances are completely uncorrelated if the detector elements 1,2 are mounted at a distance of more than about 5 cm, if the wavelength is 1 .mu.m and the illumination distance is 2000 m. (This is calculated from ##EQU1## .lambda.=the wavelength and L=the illumination distance). When the distance is about 4 mm the correlation between the signals from the detector elements are about 0.9, which is a minimum value if one is to get useful results by the invention. The example is valid for a moderately turbulent atmosphere.
Already by using two detectors placed at such a distance that the irradiance towards them is essentially un-correlated, a much smaller risk is obtained for both detectors to get into a dark spot in the incident radiation and not detect it. The probability of detection increases with more detectors receiving un-correlated radiation.
Depending on the application the detectors can be placed along a line or over an area. The mutual distance should then be about ##EQU2## with a limitation of L to 2000 m. In a possible application on a mast of a ship it can be suitable to place the detectors above each other, more or less in a line. In certain other cases it is conceivable, however, that another thing, for example another mast blocks all detectors in a certain direction, so that an installation over an area is preferred. A possible, simple arrangement with good effect is shown in FIG. 1 and comprises four detectors a, b, c and d placed in the corners of a square. In the case with the radiation of the wavelength 1 .mu.m the side of the square can

REFERENCES:
patent: 3229102 (1966-01-01), Spencer et al.
patent: 3448273 (1969-06-01), Webb
patent: 3992099 (1976-11-01), Laughlin
patent: 4611914 (1986-09-01), Homma
patent: 4769531 (1988-09-01), Malek
patent: 4868378 (1989-09-01), Biverot

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