Semen fluoresces optimally under light with a wavelength of 420 or 450 nm. This is emitted from alternative lighting sources (ALS) rather than the standard Wood's lamp.
Factors affecting the fluorescence seen with semen:
(1) amount of semen present
(2) intensity of the light source, which is affected by the distance of the source from the stain
(3) the surface, with detection on skin difficult
(4) viewing through orange goggles or with an orange barrier screen on the light source
A little training improves the accuracy of the method, especially in distinguishing semen from other materials that may fluoresce such as urine, hand cleaner and hand cream According to Nelson and Santucci, use of ALS by a physician trained to use the light source is 100% sensitive and 83% specific for detection of a semen stain.
A stain should be confirmed with an additional test (p30, acid phosphatase, etc) before concluding that a stain is semen.
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