Hello, I am trying to understand the difference between Active IR and Passive Thermal Imaging. First, I'll start off with what I DO understand, and finish with what's confusing me: I understand that in Active IR, objects are illuminated with Near IR (NIR) light (695-1000 nm) and then the reflections from that illumination is captured using a camera sensitive to that part of the spectrum. For Passive Thermal Imaging, the sensor simply detects IR radiation emitted as heat, and this can vary anywhere from 900 - 14,000 nm Questions: 1) Active IR produces a much clearer image than Thermal Imaging. I assume this is because the wavelength for Active IR is much smaller? If so, using thermal imaging, would you get a clearer image for a very hot object (high freq IR emittance) than a very cold object (low freq)? 2) Since Active IR emits high frequency IR light, does that necessarily mean that the Active IR emitter is very hot? I would imagine that it's not, but I would also imagine that colder things emit lower freq IR and hotter things emit higher freq IR. Explain? 3) What exactly is the reason for Active IR? Minus the illumination of the scene, active IR simply collects NIR radiation (high frequency IR) to form an image. Passive Thermal imagine simply collects IR (high and low freq) to form an image. Thus, they are both just collecting IR radiation to form an image. What necessitates illuminating the scene in Active IR? Why can't you simply thermally image the NIR being emitted? My GUESS is that a cold body simply does not reflect enough NIR to produce a clear image... only low freq IR. And with active IR, you get a very clear image, not because the object is very hot and emitting NIR, but because the object is simply reflecting NIR. So... my guess is that we do Active IR when we're solely concerned with a clear image, and when we do Thermal Imaging we're solely concerned with the temperature distribution of the image. Does this make sense? Anything to add? Thanks in advance for your comments and thoughts!