Active IR vs. Passive Thermal Imaging

[pegasusherd]

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!

 

[mgb_phys]

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.

Yep, normally just a regular Silicon CCD CCTV camera with an ir filter to block visible light. The source is normally just a regular halogen floodlight with an IR (or deep red filter)

For Passive Thermal Imaging, the sensor simply detects IR radiation emitted as heat, and this can vary anywhere from 900 - 14,000 nm

Yes

1) Active IR produces a much clearer image than Thermal Imaging. I assume this is because the wavelength for Active IR is much smaller?

No, it's because the active Ir camera is an ordinary TV camera and the scene is strongly illuminated with an Ir spotlight. Active infrared cameras use a much smaller sensor and are lower sensitivity and higher noise (at least in cheap civilian models) - they are also a lot more expensive, because of the small market.

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?

The Ir emitter is just a light bulb - it is hot 2000deg C - by everyday standards but cold than visible emitters like the sun

3) What exactly is the reason for Active IR?

Generally to have a surveillance camera that can work at night without putting up visible floodlights. Either because these would annoy the neighbors, or you don;t want someone to know they are being observed

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...

Because an object at room temperature emits very little 800nm radiation

when we do Thermal Imaging we're solely concerned with the temperature distribution of the image. Does this make sense?

Yes - sometimes you do active IR because it a good way of finding a hot object against a cold background, especially with obscuration like fog or smoke - that's why it's used by both firemen and tanks

 

[Andy Resnick]

Hello,
<snip>

For Passive Thermal Imaging, the sensor simply detects IR radiation emitted as heat, and this can vary anywhere from 900 - 14,000 nm

My experience in this is remote sensing, which is different than, say, spectroscopy.

Be careful here- different detectors are used for different parts of the spectrum. Silicon will work out to about 1100 nm, after that, the useful IR band is (usually) divided into the mid-IR (3-5 microns) and longwave IR (8-12 microns), the divisions occuing due to the absorption of water vapor. the MWIR and LWIR detectors are different materials and SWIR: InSb, Ge, MgCdTe, etc.

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?

Not nessessarily- it could simply be a signal-to-noise issue.

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?

Most light sources used in this application are non-thermal, so it's difficult to assign a temperature. But living objects usually have a peak emission in the LWIR, while things like engine exhaust peak in the MWIR. Even so, the *total* optical emitted power by a living object is much less than an exhaust.

3) What exactly is the reason for Active IR?

Usually it is used to increase the signal-to-noise ratio. The disadvantage is that the illumination source can be detected, requires power, can be big and bulky, and leads to a more complex optical system than a simple detector. There's some recent work using the SWIR source present inthe upper atmosphere, but again, current detectors are not sensitive enough to obtain a good quality image.

 

[pegasusherd]

Thank you both for these two great responses!