Question

Can you make an ultra-violet light source by painting a regular incandescent light bulb with black paint?

Asked by: John Graham

Answer

Incandescent light bulbs, like every heated object, produce a wide band or spectrum of wavelengths. The shape of the spectrum is primarily determined by the temperature of the filament, and to a small extent by the material composing the filament, which is usually tungsten. The shape of the spectrum shows the amount of power being radiated versus the wavelength or color of the light.

The higher the temperature of the filament, the more power is radiated at shorter wavelengths, including ultraviolet. The color temperature of a typical household incandescent light is about 3000 Kelvin or 4940 °F. At that temperature, most of the power is being radiated in the infrared, with a peak near 965 nm. A very small percentage of the total power is also radiated in the ultraviolet range, below 400 nm.

Black Litht BulbIn order to create an ultraviolet source from an incandescent light, it is necessary to remove or filter out the bulk of the power being produced in the infrared and visible wavelengths. Some paints that appear black in visible light may, in fact, be significantly transparent to ultraviolet light. These paints appear black to us because they absorb most of the power in the light that our eyes can see.

Although the amount of power available from this homemade UV light is small, it does make sense. However, watch out that the paint doesn't burn since it is absorbing the bulk of the power from the light. Also, the paint can be transparent to some of the infrared power and nearby objects may 'feel' the heat.

Note from the editor: You are advised to just go to our local hardware or electronics store and buy a professional 'black light' bulb instead of making your own which can be dangerous.

Answered by: Scott Wilber, President, ComScire - Quantum World Corporation

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