Forensic Ultraviolet Lights in Clinical Practice: Evidence for the Evidence

By Carter-Snell, C.; Soltys, Kathleen | Canadian Journal of Police and Security Services, June 2005 | Go to article overview
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Forensic Ultraviolet Lights in Clinical Practice: Evidence for the Evidence

Carter-Snell, C., Soltys, Kathleen, Canadian Journal of Police and Security Services


Ultraviolet lights are used in clinical forensic practice, particularly with sexual assault victims. Despite their widespread use, there is little data on either the best wavelengths of light for detecting semen stains, or on the sensitivity and specificity of the wavelengths to semen. In the descriptive literature there are varying examples of the colour of stains under UV lights, however this is not linked to particular wavelengths or circumstances. Much of the related research has been conducted on fabric samples rather than skin or with different types of lights. In this article a review of the literature on ultraviolet lights is provided, along with the results from two preliminary studies.

"Cindy is an 18 year-old female who came to Emergency for examination and treatment following a sexual assault. Her treatment options were reviewed and she chose to have a full physical examination, but did not wish the police to be involved. She cited shame and embarrassment, mainly because she had been quite intoxicated at the time and had little recall of the event. She did recall the male ejaculating on her thigh at one point and again on her chest. During the examination it was noted that she had fluorescence in both these areas, but they were not swabbed for DNA analysis given her request that police not be involved. The following day, Cindy reconsidered and reported to police about the incident. Her case went forward to court based on the results of the examination and the interviews. The sexual assault nurse examiner was asked to testify about her findings, including the stains she saw. A critical question, however, involves what the examiner knows about the ability of the light to detect or rule out certain stains, particularly semen. How confident could she be regarding the type of stain she saw?"

Unfortunately, the "evidence for the evidence" in sexual assault practices has not always been established beyond the laboratory. Ultraviolet lights and alternate light sources have been used in forensic laboratories and at crime scenes for years to look for blood and body fluid stains. Recently, more portable versions of these lights have been used with varying degrees of success in clinical practice with victims of violence. Some forensic laboratory scientists have expressed frustration with their use by clinical staff, citing high rates of unnecessary swabs being taken which turn out to be negative for semen or DNA, or perhaps other fluids which are not of interest (e.g., hair gel) when sent for analysis.

Clinically, some lights allow many substances to fluoresce including hair gel, lubricant and other innocuous substances, while other lights only make semen and sometimes saliva fluoresce. In the clinical literature there is confusion with terminology between ultraviolet lights, Woods' lamps and alternate light sources. Furthermore, the descriptive literature is inconsistent regarding what positive fluorescence looks like, ranging from greenish yellow or blue to white. There is very little clinical literature as to the recommended wavelength of lights for stains on skin, nor what the stain would look like under various wavelengths. This information is vital in order to educate practitioners appropriately. The purpose of the present research is to attempt to clarify the use of ultraviolet lights for sexual assault teams prior to purchasing new lights. It is also hoped that the results will inform and guide further understanding of what it is we see when fluorescence is noted.


In order to understand the need for further research we first describe fluorescence of stains and the role wavelengths play in the ability to view fluorescence, including prior research on the forensic identification of various stains.

Ultraviolet Wavelengths

In clinical practice we have come to call every ultraviolet light either a "Wood's Lamp" or an alternate light source (ALS).

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Forensic Ultraviolet Lights in Clinical Practice: Evidence for the Evidence


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