UV sensor

Details UV sensor UV sensor

1999-04-28

2001-08-21

Epps, Georgia (Department: 2873)

Radiant energy

Invisible radiant energy responsive electric signalling

Ultraviolet light responsive means

Reexamination Certificate

active

06278120

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to ultraviolet sensors and, more particularly, to a sensor design which virtually eliminates solarization effects on optical components and photodetector.
2. Background Description
Ultraviolet (UV) radiation has been long known to produce solarization in materials. Solarization presents a major problem in UV measuring instruments because it changes the transmission characteristics of the measurement light path and hence, changes the reading produced by the instrument.
The degree of solarization is a function of, among other factors, how many UV photons strike the material. Since there is a finite probability that a single photon striking a piece of material will cause a physical change in the material, one key to reducing solarization is to reduce the number of photons striking the material in question. However, UV measurements require a finite number of photons striking a unit area to make a measurement.
The problem is a long standing problem. One attempt to solve the problem is disclosed in U.S. Pat. No. 5,514,871 to Hayes et al. The Hayes et al. UV sensor comprises an attenuating aperture, filter and photodetector serially disposed in the path of the UV radiation. If the light source covers a large viewing angle, the light source will be attenuated by the ratio of the size of the aperture divided by the height of the light source. However, if the light rays are collimated (parallel to each other), the aperture will pass the rays with unattenuated intensity. While the unattenuated rays would form an image only the size of the aperture, they would be unattenuated and the associated intensity would be sufficient to solarize the filters and detector in the area where the spot fell. Thus, the Hayes et al. approach works only for large area sources.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a UV sensor design that virtually eliminates solarization effects on optical components and the photodetector.
It is another object of the invention to provide a UV sensor design that allows the use of optical components, particularly short wavelength UV filters, which would otherwise be unusable because of susceptibility to moisture and external gases that would attack the filters if they came into contact with them.
It is a further object of the invention to provide a UV sensor design which is effective for both broad area and collimated light sources.
According to the invention, there is provided a technique for reducing the number of photons, striking a unit area of any of the materials in the path and sensitive to UV light, to a level which does not produce appreciable solarization even over very long periods of time. The UV sensor has as the first optical element in the viewing path a UV transmitting, extremely low solarization window. The back surface of this window is frosted to produce some diffusion of the UV rays. A metal disc with a small aperture is the next optical element and reduces the total amount of energy admitted to the optical measurement system. A second frosted UV transmitting, extremely low solarization window further diffuses the light beam that has passed through the aperture. The UV light from this second window travels toward to two UV filters and is further dispersed to produce low intensity level UV light which impinges on a silicon photodetector.
The UV sensor, according to the present invention, is effective for both broad area and collimated light sources. The back surface of the first UV transmitting, extremely low solarization window is frosted so that broad area image, semi-collimated or collimated rays are uniformly scattered over an 180° angle. Only a portion of the original, scattered rays pass through the aperture. The attenuated rays which pass through the aperture strike the second window whose back surface is also frosted to produce additional scattering. This scattering and aperture attenuation produces an aggregate attenuation of the light rays which reduces the intensity to a level and uniformity of distribution that will not produce solarization on the filter andlor the photodetector.


REFERENCES:
patent: 5497004 (1996-03-01), Rudolph et al.
patent: 5514871 (1996-05-01), Hayes et al.

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