Trap blackbody

Details Trap blackbody Trap blackbody

2001-08-30

2003-12-16

Meier, Stephen D. (Department: 2853)

Radiant energy

Radiant energy generation and sources

With radiation modifying member

C250S353000, C250S351000, C250S505100

Reexamination Certificate

active

06664553

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to thermal targets, and in particular to thermal targets with a simple construction and high emissivity that simulates a blackbody.
BACKGROUND OF THE INVENTION
Thermal targets are used as radiation sources, primarily for calibrating infrared detectors, which detect electromagnetic radiation in the infrared range, approximately 2 &mgr;m to 1000 &mgr;m. It is important to accurately calibrate an infrared detector in order for the detector to provide reliable information about the temperatures corresponding to the electromagnetic radiation detected by the detector. Because infrared detectors are subject to shift and drift, it is necessary to periodically recalibrate infrared detectors while in use.
A thermal target's temperature and emissivity needs to be known as accurately as possible in order for a thermal target to be used as a calibration reference. Accurately knowing a thermal target's temperature and emissivity establishes a baseline that correlates a known temperature to an amount of radiation. Such correlation allows an infrared detector to detect the amount of radiation and know what temperature that detected amount of radiation represents. Subsequently, when the infrared detector detects infrared radiation, the baseline radiation amount and corresponding temperature are used to calculate the temperature of the object emitting the infrared radiation. Radiated output, E, from an object is calculated using a modified Stefan-Boltzman equation:
E=&egr;&sgr;
4
4
  (equation 1)
where &egr; is the calculated emissivity for the object, &sgr; represents the Stefan-Boltzman constant, approximately 5.670×10
−8
W/m
2
-K
4
, and T is the temperature of the object.
Using equation 1, any object could be used to calibrate an infrared detector if the temperature and the emissivity of the object are known. However, determining an object's emissivity depends upon the temperature of the object, the surface characteristics of the object, the wavelength of the emitted radiation, the viewing angle of the object, and other factors and is therefore difficult to determine. Relying upon an object's determined emissivity is not precise enough for accurately calibrating an infrared detector.
For accurate calibration of an infrared detector a thermal target that closely simulates a blackbody, i.e., a theoretical object that is a perfect absorber and emitter of radiation, i.e., &egr;=1, is used. Use of a blackbody simulator simplifies calculation of the total radiation emitted, E, from a thermal target by relying upon accurate determination of the thermal target's temperature and the Stefan-Boltzman law;
E=&sgr;T
4
  (equation 2)
where &sgr; represents the Stefan-Boltzman constant, approximately 5.670×10
−8
W/m
2
-K
4
and T is the temperature of the thermal target.
The Stefan-Boltzman law accurately predicts the total radiation emitted for a blackbody at a known temperature. Because the use of thermistors and other temperature measuring devices allow accurate measurements of a thermal target's temperature to within plus or minus 0.1 Kelvin, a blackbody simulator with a high emissivity, i.e., &egr; greater than 0.99, provides the reliable correlation of detected radiation to temperature needed for calibrating an infrared detector. The better the emissivity of the blackbody simulator, i.e., the closer &egr; is to 1, the more accurate an infrared detector calibrated using the blackbody simulator can be.
It would appear that a blackbody simulator could be any black object. Such a simulator has been used in the past, but correspondence to a true blackbody has been poor. Many blackbody simulators are formed by creating a cavity in a core material, the cavity forming an aperture on one side of the core. The aperture is used to simulate a flat blackbody, having the shape and size of the aperture. Particular cavity shapes are chosen to cause multiple reflections of radiation within the cavity, and eventual absorption by the cavity walls of the majority of the electromagnetic energy entering the aperture.
Thermal targets simulating black bodies employ multiple reflections in some sort of a cavity to enhance their effective emissivity. If the cavity surface coating of a blackbody simulator has an emissivity above 0.7, then most cavity shapes in commercial use in blackbody simulators result in a device in which the on-axis emissivity of the aperture exceeds 0.99. Primary differences between thermal targets are the shape of the internal cavity, and the finish on the surfaces inside the cavity. Other important parameters are the size ratio of aperture to volume, weight, complexity, uniformity of emissivity, and robustness.
Since thermal target designs attempt to maximize emissivity, one measure of the quality of a thermal target is how many nines the thermal target is capable of. An emissivity of 0.9, one nine, is usually unacceptable. Many thermal targets work in the range of 0.985 to 0.995, two nines. For accurately calibrating infrared detectors, an emissivity of at least 0.999, three nines, is required to ensure accurate measurements by the infrared device. Some laboratory grade instruments have emissivities in the four nine range.
A blackbody simulator with a large cavity and a very small aperture emits almost none of the radiation that enters the aperture because of the high ratio of volume to aperture size of the cavity, i.e., emissivity is in the three to four nine range. A high ratio of cavity volume to aperture size results in blackbody simulators with a large volume and a comparatively small aperture. Although such blackbody simulators have a high emissivity, they are often heavy, bulky, and not easily transported, making them ill suited for many applications requiring a blackbody simulator.
An additional disadvantage of high volume, small aperture blackbody simulators is that the aperture provides only a small field of blackbody radiation, making alignment of an infrared detector with the blackbody simulator critical. For example, in an airborn or spaceborn infrared detection system utilizing blackbody simulators to periodically calibrate the detector, jolts or bumps that occur during flight can misalign the detector from the blackbody radiation field, thereby disabling calibration of the detector during in-flight use. Decreasing the size of the volume, or increasing the size of the aperture, results in a blackbody simulator that is less heavy and less bulky, and/or has a larger blackbody radiation field, however, the emissivity is also lowered because the lower ratio of volume to aperture size allows more incoming electromagnetic radiation to escape the blackbody simulator.
Another drawback to current blackbody simulators is polarization of electromagnetic radiation that escapes the blackbody simulator. Such polarized output results in lower emissivities because some forms of polarized light have a lower emissivity than other forms of polarized light.
A further disadvantage of current blackbody simulators is that complex surfaces on the inside of a volume, although useful for preventing incoming light from escaping through the aperture, are difficult to manufacture and are often fragile. During mobile use of infrared detection systems, i.e., in-flight or in combat situations, the fragile surfaces may become damaged and reduce the emissivity of the blackbody. Such a reduction in emissivity causes the infrared detector in the system to become less accurate and less able to distinguish objects in cluttered infrared fields, i.e., where an object's temperature is close to the temperature of its surroundings. Complex surfaces also result in random reflection of incoming light, which causes non-uniform emissivity for the blackbody simulator.
Considering the above drawbacks of current blackbody simulators, there exists a need for a thermal target that closely approximates a blackbody with a low ratio of volume to aperture and high emissivity. There exis

Affiliated with

Also associated with

Rating

Trap blackbody does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Trap blackbody, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Trap blackbody will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3113849