search.noResults

search.searching

dataCollection.invalidEmail
note.createNoteMessage

search.noResults

search.searching

orderForm.title

orderForm.productCode
orderForm.description
orderForm.quantity
orderForm.itemPrice
orderForm.price
orderForm.totalPrice
orderForm.deliveryDetails.billingAddress
orderForm.deliveryDetails.deliveryAddress
orderForm.noItems
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.


FLIR Systems www.flir.com 47


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80