Posted by admin on Wednesday, September 8, 2010 · Leave a Comment
i've heard that infra red light starts at about 800nm and also that certain other types of light emitted from bulbs, eg sodium, mercury etc only emit light upoto certasin freqencies. i think tungsten emits lights into the range of infa red (730nm +) but what does it stop at. Im trying to find a infra red pass filter to put in front of a camera to take a photo of a car number plate with its head lights on using an infrared lamp and an infra red filter on the camera. the idea is the filter will block the headlight glare but dont know what frequency filter is required? any help?
In fact, it doesn't stop. As you go to longer and longer wavelengths, the light intensity of a glowing filament grows weaker and weaker, but it never goes to zero. We can categorize the intensity in the infrared as a fraction of the intensity at the main frequency.
The frequency spectrum of a glowing object (like a tungsten filament) matches the blackbody radiation profile. The frequency spectrum depends only on the temperature of the object.
For a 5500 Kelvin object (if that is the temperature of your filament), the peak radiation intensity will be at a wavelength of 526 nanometers (green light). The light intensity at 730 nm, as a fraction of the intensity at 526 nm, is 79.5%. So, there is still a substantial amount of near-infrared light being emitted (infrared starts at about 750 nm, and goes to about 1 millimeter).
Here are the intensities (relative to the peak intensity) at various other wavelengths:
800 nm -- 61.0%
900 nm -- 48.2%
1000 nm -- 38.0%
1100 nm -- 30.1%
1200 nm -- 24.0%
1300 nm -- 19.2%
As you can see, most of the infrared radiation is concentrated in the near-infrared. So, if you got a filter that selectively absorbed radiation from 700 nm to 1500 nm, you'd eliminate almost all the infrared, with little impact on the visible frequencies.
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