A Woods lamp is a special ultraviolet lamp that contains a nickel oxide filter applied to help limit the maximum wavelength emitted. By using this technique a SANE or other properly trained health worker can more easily see DNA evidence and injury which will fluoresce when exposed to the ultraviolet light. These conditions are not normally visible to the naked eye in normal lighting conditions.  

when used in the course of a sexual assault examination,  researchers found that semen actually is more likely to fluoresce at a wavelength of about 490 nm (rather than the 360 nm of the standard Wood’s lamp).  Use of a standard 360nm woods lamp may detect some false positive for seminal fluid with other materials like and cream, saliva, bacitracin zinc, Surgilube, spermicide and A&D ointment.