Thought this was a pretty interesting video:

Video tutorial: Taking infrared pics with a standard digital camera

Terry O

Terry,

Interesting, but PLEASE do not get this mixed up with thermal imaging! This type of "infrared" light is termed near infrared. Primarily you are looking at reflected light which is otherwise invisible to our eyes. You can see some cool pictures and you can do some useful things with such detectors.

Thermal infrared, on the other hand, sees reflected as well as emitted mid- and/or long wave heat. This is what is associated with many maintenance problems, such as high-resistance electrical heating, hot bearings, differences in fluid flow, etc.

Thanks John,

Why are they using the term "infrared"?

Terry O

quote:

Why are they using the term "infrared"?

Terry -
Until I saw your response to John, I did not think you had confused thermal infrared with the infrared of your original posting.

"Infrared": means below red. The wavelength of infrared light, whether reflected or emitted is longer than the visible region we can see. The wavelength of thermal infrared is dependent on the temperature of the body that is emitting it. The wavelengths gets shorter as the object gets hotter. Eventually, at high temperatures, the wavelength is short enough that it has moved into the visible region. Think of heating an iron rod in a forge. It starts as only visible with available light by reflection, but as it gets warmer it begins to glow, moving from a far violet up through dull orange to white hot as the temperature increases.

The wavelengths generally of interest for thermal infrared are 2 microns up to about 14 microns. At those wavelengths, objects actually emit detectable amounts of infrared radiation at common (ambient up to boiler furnace) temperatures. While shorter wavelengths are emitted by even hotter objects, we don't often deal with them in thermal infrared work, especially for maintenance applications.

The sun, of course, is extremely hot -- lots of visible wavelength emission (makes it real useful for seeing in daytime)as well as a lot of short wavelength or near infrared emissions. These latter infrared wavelengths are not visible to the naked eye (of people) but some visible camera detectors can sense them. The video you linked to takes advantage of this capability by filtering the visible light out before the camera sees it and then using what is left, the near infrared, to display its image.

It is using reflected near infrared, not emitted short wave/mid wave/long wave infrared for the image. Common materials react differently to near infrared than to visible light. Hence they "look" different in the near infrared region than they do in visible light. Some materials appear transparent, hence the flap about the Sony Nightshot cameras a few years ago -- they looked like they could "see" through clothing because they extended into the near infrared.

HTH
Jack

Thanks Jack

I did not confuse the technologies but it seemed strange to me that they call it Infrared Photography - I guess because of my bias for Thermal Imaging.

I appreciate your explanation very much.

Terry O

It is fair to call it Infrared Photography. Essentially this is also what is obtained with infrared film -- reflected light photography in the (near) infrared region. There is actually a lot of this on various photography websites.

IIRC Kodak's infrared film was sensitive in this region. I have had many arguments about its not being thermal infrared. In fact, I have had engineers tell me that 30-40 years ago they used to use film for thermal infrared of buildings, but I think they are mistaken in what they were actually getting.

In addition to "fun" stuff like looking through apparently opaque objects, reflected near infrared does have technical applications. Austin Richards book "Alien Vision" touches on these, I believe. Areas include plant health, black ice detection, and water detection, I think

In any event, your video link does offer some fun.

There have been over the last few weeks offerings on ebay for "X-RAY INFRARED VISION EYE GLASSES THERMAL FLIR HEAT LCD
YOU GOTTA GET THESE... ",(sic) which are at least 99.9% a come on. They do have some images of black on black printing that becomes visible through the glasses. Of course, there is no way they are thermal, or x-ray, or photocathode, but caveat emptor. Their hype makes for interesting reading.

Jack