Hello,

I'm curious to know what the light is called, and a bit more
information on it (links would be great).  I've seen these lights on
TV, where someone used it to detect a blood stain in a hotel room.  It
wasn't visible by the naked eye, however when you shine this "blue
light" over the area you can see it.

Thank you!!
Dan Gessler

Hi Dan Gessler,

The short answer to your question would be UV (ultra violet) light but
what they might use on a TV show is not going to be as fined tuned as
the equipment used in a crime investigation. The chances are good that
what you have observed is a "black light", which can be either a
fluorescent or incandescent bulb, coated to allow only the long-wave
UV-A light and some blue and violet visible light pass through, or, an
LED Blue Light.

BLACK LIGHT:

What is the difference between a black light and a normal light bulb?:
http://ask.yahoo.com/ask/20030305.html

How Black Lights Work:
http://science.howstuffworks.com/black-light1.htm

What Glows?
"Forensic scientists use them to analyze crime scenes. To pick out
fingerprints, for example, they often dust with fluorescent dye under
a black light. This makes it easier to pick the fingerprints out from
surrounding dirt. Black lights can also identify semen and other
bodily fluids that naturally fluoresce."
http://science.howstuffworks.com/black-light2.htm

"Ultra violet light proves itself time and again in forensic
investigation - from searching for blood traces, fingerprints and
footprints to the discovery of subcutaneous bruising on the body,
which may not be visible under any other conditions or light sources."
http://www.uv-light.co.uk/applications/fluorescent_inspection/revelation/forensic_evidence.html

Forensic Light Source Applications:
"A forensic light source is made up of a powerful lamp containing the
ultra-violet, visible and infrared components of light."
"The dried body fluids will actually glow under the light source
illumination. Although the body fluids will fluoresce under an
ordinary UV black light, many articles on which you would find them
including clothing and sheets will also glow and deter their
detection...Although blood does not glow in the visible range, it has
a unique color band (wavelength) under which the blood stain will
darken to enhance its contrast by approximately 4 times."
http://www.crimescope.com/march%2015/Applications.htm

UV Marking & Detection
http://www.glowshop.com/en-gb/dept_93.html

LED LIGHT:

LED's (light emitting diodes) have been used by the automotive
industry as indicator lights for years, and are now available as a
light source for use by ... forensic scientists."
http://www.theledstore.com/ledfaq.html

How Light Emitting Diodes Work
http://electronics.howstuffworks.com/led1.htm

How Light Emitting Diodes Work
http://electronics.howstuffworks.com/led2.htm

"Blue is also used in forensics for finding bodily fluids at crime
scenes. A great tool now with DNA technology."
http://www.cases4less.com/more_about_led_lites.html

Additional Links:

How Luminol Works:
http://science.howstuffworks.com/luminol1.htm

How Investigators Use Luminol:
http://science.howstuffworks.com/luminol2.htm

Bloodstain Terms:
http://www.peelpolice.on.ca/FIS/Blood-Terms.html

Bloodstain Patterns:
http://www.peelpolice.on.ca/FIS/Blood-Pat.html

What is UV Light:
http://www.uv-light.co.uk/what_is_uv.html

I hope this helps. If you have any questions, please post a
clarification request before rating my answer.

Thank you,
hummer


Google Search Terms Used:

uv blue light blood
"blood detection" "blue light"
"uv light" forensics
"uv light" "detecting blood"
blood fluorescence "blue light"

Thank you!  Exactly what I was looking for.

Thank you, Dan Gessler, I appreciate all and I'm happy to hear I was
on the right trail.  Sincerely, hummer

Don't forget about Luminol-- which is also used to detect blood stains
at crime scenes.  The reaction of luminol with blood results in the
production of light rather than color.  If an area is thought to
contain blood traces, luminol is sprayed on the entire area-- then,
the lightning must be darkened, and traces of light are looked for. 
The great thing about luminol is that it does not tamper with or
affect the chemical composition of any evidence.  It's possible that
you saw this being used as well!

-Rebekah