Obviously visible semen stains can eventually be washed off, but do they stay in the fabric? Like would a black light still show signs of a stain even if what's visible to the naked eye isn't visible anymore? And if so, is this permanent?

Thanks!

Hi! here is a link to an about page with information about what flouresces under a black light:

http://chemistry.about.com/cs/howthingswork/f/blblacklight.htm

"Many body fluids contain fluorescent molecules. Forensic scientists use ultraviolet lights at crime scenes to find blood, urine, or semen (all fluorescent)."

However, if the fabric was washed enough, it wouldn't be detectable under the black light, as the fluorescent molecules would not be present in enough quantity. I doubt it is permanent.

They have found semen in expensive hotels using a black-light - obviously they wash their sheets - so I guess it would stay after a few casual washes.

Very simply -- all alternate light sources (e.g. black lights) are pretty much useless. When we receive a submission where an investigator/nurse indicates that they used such a tool, we pretty much disregard them.

Yes, some semen does fluouresce, but some doesn't. What's more, a tonne of other chemicals and natural fluids (including saliva) also fluoresce under black light.

The only reliable tests for semen are chemical or microscopic tests, that are performed in the lab. I might be going a little too far by saying an ALS is completely useless, since in the field investigators don't have much to go on, but they are weak prelim exam tools only.

And I am pretty sure that washed semen stains would NOT fluoresce. The only component left after laundering is the sperm cells themselves (or some of them -- this is how we get the DNA profile), but the fluorescent chemicals would be all gone.

Hope that helps.

Yes it can be used to find various stains that may not be visible for various reasons, But depending on the size of the semon if any lab results would be able to be done is another issue,

But even these will wash out at some point

But this is just one tool used in investigations, and all good investigators will try to use any methoid available to determine what happened in a crime scene, if this evidence gives you an idea of what happened, it can be helpful, but also you can not always date such evidence as to an exact day, esp on carpet or bedding or the wall. So it is just one tool of many used in solving various crimes.

And a good police investigator never ( or should never) disregard any evidence no matter how slight.