Thermal imaging & vision systems

radiance measurement

object is 100m, or a kilometre, or even farther away, it’s important to know how passing through that much atmosphere changes the radiance measurement. Fortunately, there is software available to

deal with such changes. Developed by the US Air Force, MODTRAN (for moderate resolution atmospheric transmission) allows users to model the atmospherics. They can then use that model to estimate how much the air is affecting their readings and adjust their measurements accordingly, leading to more accurate results.

Warm vs. Cold Filters

It is not only the detector that is involved in radiance measurements. Filters also have an important role to play. Researchers may want to characterise a spectral signature in a specific waveband, based on the spectral response of the detector that will be later used to identify the object. To do that, they want to block out other wavelengths. They may also wish to block light from one part of what they are looking at — the flame from an explosion, for instance — to see something with a different radiance that would otherwise be obscured. Filters can also help to image gas plumes or pick out laser beams that show up at particular wavelengths. Having the right filters can give a camera enough spectral flexibility to accommodate many different applications. Filters come in two basic types, warm and

cold. In cooled cameras, the filter is placed inside the dewar, along with the detector, which is cooled to liquid nitrogen temperatures to reduce the thermal noise coming from the camera itself. Because the filter is also cooled to the same

Instrumentation Monthly April 2019

temperature, it does not emit any background radiation that would throw off the measurement. In highly sensitive applications, a cold filter is preferable. The downside is that because it is built

into the camera, there is no ability to swap it out for another filter at a different waveband. For that reason, users sometimes opt for warm filters, which are usually installed in a filter wheel outside the dewar. Researchers who want to block out different wavebands at different times might prefer a warm filter. But warm filters have the drawback that

they emit their own radiance, even in the waveband they are designed to block. Additionally, IR photons that come through the filter can bounce off surfaces inside the camera and be reflected by the back of the filter into the detector. The filter then becomes its own source of background noise. Generally, warm filters are best at imaging objects that are much brighter than the background, such as a very hot gas or an explosion that needs to be imaged in a specific waveband.

multiple speCtra

Sometimes, users want a multitude of filters to perform multispectral measurements, and they feel having many filters provides optimum flexibility. That may be true, but it comes at a price — the user may end up with a poor-quality image because of background noise from the warm filter. In one case, a customer was interested in

making measurements in a few bands between 1.5 and 3µm and a few more between 3 and 5µm. The question became: was the best choice a broadband camera capable of detecting from

1.5 to 5µm with a set of warm filters to select the desired wavelengths? On examination, it turned out such a camera would have very poor performance. It would also be difficult to compare different images in a high-speed measurement of an explosion they wanted to study, because in the time it would take to switch between filters, the scene would have evolved. Instead, the customer chose to have two

separate cameras, each with a cold filter. One had a filter for the 1.5 to 3µm band, the other for the 3 to 5µm band. That vastly improved the signal-to-noise ratio, resulting in superior measurements. It also allowed the researchers to synchronise the two cameras so they knew that the images from each were taken at the same time, letting them look at different aspects of the explosion in two wavebands simultaneously.

a range oF Considerations

Measuring radiance is the central aim of researchers using IR cameras on the military test range. To make the most accurate measurements, they have to be sure the camera is calibrated for radiance, and not simply use a standardised, possibly inaccurate, conversion from temperature. It is also important to take into account the impact of atmospherics on radiance measurements, and to use the appropriate software to compensate for atmospheric effects. Finally, they need to consider which filters are best for their particular application and decide whether the flexibility of warm filters is worth the tradeoff in accuracy, or whether a cooled filter would be preferable.

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