Author: A/Prof Amanda Oakley, Dermatologist, Hamilton, New Zealand. August 2014.
Wood lamp examination is a diagnostic test in which the skin or hair is examined while exposed to the black light emitted by Wood lamp. Black light is invisible to the naked eye because it is in the ultraviolet spectrum, with wavelength just shorter than the colour violet. The lamp glows violet in a dark environment, because it also emits some light in the violet part of the electromagnetic spectrum.
A traditional Wood lamp is a low-output mercury arc covered by a Wood filter (barium silicate and 9% nickel oxide), and emits wavelength 320–450 nm (peak 365 nm). This was invented in 1903 by a Caltimore physicist, Robert W. Wood.
Modern black light sources may be specially designed BLB fluorescent lamps, mercury vapor lamps, light-emitting diodes, or incandescent lamps. Fluorescent black light tubes have a dark blue filter coating on the tube, which filters out most visible light. There are several models with varying properties. The medical Wood lamp may include:
Examination using Wood lamp involves the following steps.
A Wood lamp is used to identify the extent of pigmented or depigmented patches and to detect fluorescence. Normal healthy skin is slightly blue in colour but shows white spots where there is thickened skin, yellow where it is oily, and purple spots where it is dehydrated. Clothing lint often shines bright white.
A positive result is reported if a pigmentary disorder is more noticeable when examined using the Wood lamp or if fluorescence is noted.
Fluorescence is a coloured glow seen when certain substances such as collagen and porphyrins absorb black light and emit it again at a longer wavelength in the visible spectrum. Items on the skin surface such as fabric, topical medications and soap residue can also fluoresce.
A Wood lamp for skin examination may reveal:
Apart from Wood lamp skin examination, the Wood lamp has other uses.
The black light emitted by a Wood lamp is harmless. The lamp does not emit short wavength ultraviolet B radiation (290–320 nm) so it does not cause sunburn or otherwise damage normal skin.
It is possible that a patient with extreme photosensitivity might develop a rash on skin exposed to black light. However, Wood lamp examination is usually very brief and unlikely to cause problems even in very photosensitive patients.
It is prudent to ask the patient to close their eyes when examining the face.
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