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Incandescent light sources have broad emission throughout the visible spectrum. However, the majority of the radiated emission lies in the infrared region. In an incandescent source, a heated tungsten filament emits radiation according to Planck’s law. This mathematical equation defines the output energy of a heated (black-body) light source as a function of wavelength.
Figure 1 shows the predictions of the Planck equation for several filament temperatures over a very broad wavelength range extending from 400 nm in the visible (blue) region to 10,000 nm in the far infrared. Typical temperatures for heated filaments used in lighting applications range from 2000°K for low wattage lamps (1 to 50 W) up to 2800°K for high wattage lamps (300 to 1000 W).
Figure 1: Black-body emissions from the visible to the far infrared region
The highest point of the black body radiation occurs at a wavelength that is defined by the following equation:
 (maximum, nm) = (2898/T)1000
where (maximum, nm) is equal to the peak wavelength of the radiation curve in nanometers (nm) and T is the filament temperature in degrees K. The peak wavelength in the near infrared shifts to shorter, visible wavelengths as the filament temperature increases.
Figure 2 shows the radiated emission for the same set of filament temperatures as Figure 1 except that the wavelength scale now spans the 400 nm to 900 nm wavelength range. The visible (400 nm to 700 nm) portion of the radiant energy decreases dramatically as the filament temperature is reduced from 2800°K to 2000°K. At both filament temperatures the incandescent light source produces a large amount of NIR emission that will interfere with all types of night vision devices
Figure 2: Black-body emissions from the visible to the near infrared region
For low wattage incandescent lamps, a plastic Nightshield filter may be employed. For these lower power lamps, several Nightshield filters are available for green, yellow and red colors. For example, see the data sheet for a yellow (Class B) filter (P/N 26130-001) (PDF, 84 KB).
In contrast, for high wattage lamps (20 to 1000 W) we recommend a colored glass or thin-film coated glass filter. The ambient temperature in the vicinity of the higher wattage lamps far exceeds the recommended operating temperature for plastic filter materials. |
