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Infrared FAQ's

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Infrared FAQ's

What is IR?

IR refers to ‘infrared’, which is the name for the part of the electromagnetic spectrum between about 0.7 micro-meters (“microns”) out to about 100 microns.
All materials with a temperature above ‘absolute zero’ (O Kelvin = -273.3 Celsius) emit infrared light. The strength of infrared emissions (also called ‘thermal’ emissions) from a given target is a function of the temperature of the target, the wavelength of the infrared light, and the radiative properties of the target.

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What is Infrared Imaging?

'Infrared imaging’, like visual imaging, is the collection, recording, and displaying of light from a scene. However, ‘infrared’ refers to light with longer wavelengths than that of visual light. Infrared imaging is also referred to as thermal imaging; these terms are taken to be identical. Infrared thermal imaging shows the thermal patterns emitted from, or reflected off of a target, and as such, it does not require visual light.

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How does IR 'see' in the dark?

How does IR ‘see’ in the dark?
Since infrared imaging uses light in a different portion of the spectrum from what we see with our eyes, IR cameras are insensitive to the amount, or lack, of visible light. IR imagers sense thermal energy that is emitted from, reflected off of, or transmitted through the target. Therefore, “darkness” (as we perceive it) does not stop an IR imager from acquiring an image.

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Can an enemy detect that I am using an IR camera?

Certain systems IR systems require active infrared illumination; these systems usually operate in the NIR (Near Infrared) region of the spectrum. The active IR illumination could then be detected by an enemy, if that enemy were equipped with similar equipment. However, MWIR (Mid-Wave Infrared) and LWIR (Long-Wave Infrared) detectors do not require active illumination of any sort; these detectors are sensitive enough to detect the energy emitted from human or vehicle targets. Since there is no active illumination used with these systems, they are undetectable by an enemy.

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How reliable are infrared cameras?

Modern infrared cameras tend to be very reliable, particularly LWIR (Long-Wave Infrared) systems, which generally do not require cryogenic cooling. While these systems often use TE (Thermoelectric) cooling, TE coolers have no moving parts and tend to be very reliable. Many LWIR cameras are completely solid-state, and have no moving parts whatsoever.

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Does IR 'see' in color?

No. Since an IR imager is sensing light in a different part of the spectrum from what our eyes can see, the term “color” (in the way that it is commonly used) has no meaning. All IR cameras actually image grayscale intensities, i.e., a measure of the “brightness” of the image. Some imaging systems then apply ‘false coloring’ to the image, to help the user interpret the relative ‘temperature’ of the infrared image.

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How do infrared cameras actually measure temperature?

The principles of infrared technology are rooted in Plank’s Law, which describes the way that objects emit electromagnetic energy as a function of temperature and wavelength. However, in practice, radiometric (i.e., quantitative) IR temperature measurements are made by comparing the output (for example, a change in voltage or resistance) of a detector with a calibration table. Since the response from the detector is a function of many things (detector material, the electronics of the signal processing system, materials & coatings of the optics in the light path, etc.), the calibration table is unique to each system.

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How accurate are infrared measurements?

This depends on many factors, including the detector being used, operator skill, knowledge of the material properties of the target, and even the temperature of the target itself. Depending on all of these factors, temperatures can be measured to accuracies of fractions of a degree C, or to as wide of a range as 5-50 degrees C. Radiometric systems are designed to meet specific needs, with the understanding that greater accuracy requires greater effort and cost. For a vast array of infrared applications (such as security and night navigation), radiometry is not needed. For these kinds of applications, simply detecting that something in the field of view is warmer (or colder) than its surroundings is sufficient.

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What is the difference between accuracy and resolution?

Accuracy is the ability of the system to properly measure the actual quantity (in this case, temperature) in question. This term really only has meaning with regards to a radiometric system. A system will not be radiometrically accurate unless it is calibrated properly.
On the other hand, resolution (sometimes called precision) applies to both radiometric and non-radiometric systems. Resolution refers to the ability of the system to detect or “resolve” a small difference in the scene. This can be thought of as a measure of the ‘sensitivity’ of the imaging system.

Thermal resolution is often described in terms of the NETD (Noise Equivalent Temperature Difference), or the MRTD (Minimum Resolvable Temperature Difference). Both of these criteria refer to the ability of the system to detect a small change in temperature of an object in the field of view.

Spatial resolution refers to the ability of the system to spatially discriminate an object in the filed of view. Since this is clearly affected by the optics in front of the detector, the spatial resolution of the detector itself is sometimes (casually) specified by the size of the pixel array, since in general, more pixels improve the spatial resolution of the system. However, other factors (fill factor of the array, diffraction limits, etc) can limit or at least play a role in the spatial resolution of the detector.

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