Low-temperature blackbody radiation source

Details Low-temperature blackbody radiation source Low-temperature blackbody radiation source

1998-10-13

2001-05-15

Berman, Jack (Department: 2881)

Radiant energy

Radiant energy generation and sources

With radiation modifying member

C250S338500, C250S338100

Reexamination Certificate

active

06232614

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to a blackbody radiation source operable at low temperatures.
Sensors such as imaging focal plane array sensors are calibrated prior to, and sometimes during, service. In one calibration procedure, the sensor is placed into a facing relation with a calibration source which produces a standard condition. The standard condition typically includes a simulated background field comparable with the background field expected in service for the wavelength range of the sensor but without any target present. The sensor is operated while facing the calibration source. From the data collected in the calibration procedure, the operability of, zero values for, and/or scaling factors for individual pixel sensor elements of the array are determined and stored for later use.
An important class of sensors includes infrared sensors for use in space or in viewing space from earth. The relevant background for the calibration of such sensors is a blackbody radiation source having a low temperature of 200®K. or less. One blackbody radiation source previously used for this calibration procedure has been a bath of liquid nitrogen or a flat metal container whose exterior is painted black and which is filled with liquid nitrogen. This type of source yields somewhat unpredictable equivalent radiometric temperatures. Reflections from the ambient environment may interfere with the measurement of the source. Additionally, this source is limited to a single source temperature and lacks the flexibility required for many applications.
There is a need for an improved low-temperature blackbody radiation source for use in calibrating sensors and in other applications. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a low-temperature blackbody radiation source which is operable for the calibration of sensors and other applications. The blackbody radiation source is operable at temperatures below 200®K. It is effective in reducing ambient radiometric reflections that otherwise interfere with the calibration performed using the blackbody radiation source. The aperture of the source may be made quite large, as may be required for large sensors. The equivalent radiometric temperature of the blackbody radiation source, in terms of spectrally integrated radiance, may be varied over a range of temperatures.
In accordance with the invention, a blackbody radiation source comprises an insulated enclosure having a viewing aperture defining a line of sight therethrough, and a heat sink within the insulated enclosure. The insulated enclosure preferably has a bottom, a top, and a lateral side, and the viewing aperture is through the lateral side of the insulated enclosure. The heat sink is preferably a pool of a liquefied gas lying around or below the level of the viewing aperture. The heat sink preferably has a temperature of no greater than the boiling point of nitrogen, and most preferably is a pool of liquid nitrogen. The blackbody radiation source further includes a viewing surface in thermal contact with the heat sink but having an unobstructed view through the viewing aperture, wherein the viewing surface is inclined to the line of sight through the viewing aperture. The viewing surface has a high-emissivity, diffuse surface over the radiation wavelength range of interest, with an emissivity that is preferably greater than about 0.97.
The viewing surface is desirably constructed as at least a portion of one side of the inner surface of a closed box having an outer surface in contact with the heat sink. The closed box is positioned inside the insulated enclosure, and the heat sink such as the pool of liquefied gas lies between an inner surface of the insulated enclosure and the outer surface of the closed box. The viewing surface is preferably inclined such that the viewing aperture lies within an acute angle formed between the viewing surface and a horizontal plane.
By making the substrate of the viewing surface of a relatively thin piece of a high thermal conductivity material such as aluminum, liquid oxygen from the atmosphere condenses on and flows over the viewing surface. The movement of the liquid oxygen prevents the buildup of condensed water on the viewing surface
In one embodiment, the blackbody radiation source includes a heater whose radiant output is directed toward the viewing surface but which itself is not within the line of sight and therefore is not itself directly measured by the sensor being calibrated. The viewing surface is cooled from its back side by conduction from the heat sink, and is radiatively heated on its front side by the heater. Although the temperature of the viewing surface stays approximately constant at the temperature of the heat sink, radiant energy from the heater is reflected back from the viewing surface, through the aperture, and to the sensor being calibrated. By adjusting the power level of the heater, the amount of reflected energy reaching the sensor may be controllably varied, producing a condition wherein the equivalent radiometric temperature of the viewing surface source may be controllably varied. The ability to achieve a range of radiometric temperatures near that of the heat sink is an important advantage of the invention.
In operation, a sensor to be calibrated is placed into a facing relation with the aperture of the insulated enclosure and thence into a facing (but inclined) relation with the inclined viewing surface inside the enclosure. The blackbody radiation source is brought to the desired temperature state. The sensor is operated, and calibration is performed against the blackbody field.
The present invention thus provides an important advance in the field of blackbody radiation sources for use in the calibration of sensors and other applications. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.


REFERENCES:
patent: 4999498 (1991-03-01), Hunt et al.
patent: 5399856 (1995-03-01), Sandridge et al.
patent: 5602389 (1997-02-01), Kato et al.

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