Last updated 4-27-16
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Field Lab 9: Forensic Techniques
Using UV flashlight to detect automotive liquids and household stains.

Fluorescein spectrum

Fluorescein is a common fluorescent dye that is put in many liquids to allow them to glow bright under blue or ultraviolet light.  From the chart we see that fluorescein absorbs blue light the strongest and then glows (emits) a yellowish green light.  That's what fluorescence is.  It's when a substance absorbs a higher energy ligth (shorter wavelength light) and the glows (reflects back) a light of lower energy (longer wavelength).  Fluorescein also will absorb UV light and emit it as yellow-green light.

antifreeze inside of radiator

One use of fluorescein is to mix it with antifreeze. That's why most antifreeze looks bright yellow-green.  If the antifreeze leaks out, the fluorescence of fluorescein makes it easier to see the leak.

Antifreeze fluorescence

Here we see drops of antifreeze more easily because of the fluorescein showing a bright yellow green color.  Sunlight or a ultraviolet (or blue) light makes the fluorescein glow more.

UV dye in freon

A UV dye like fluorescein or one that is similar is added to Freon (auto air conditioning refrigerant).  It's added for the same purpose.  If there is a Freon leak, it is more visible due to the UV dye.

UV dye visible with UV light on condensor

Here we see the UV dye is glowing yellow-green at a point where some Freon that contained the dye leaked out the car's air conditioning condenser (part where Freon gas is cooled so that it turns into a liquid).  Without the dye, it's harder to find these kind of leaks.   

UV dye for oil

Fluorescent dye is also made for oil-based liquids such as engine oil, transmission fluid, power steering fluid, and even gasoline.    The dye can be added to any of these to help find leaks.  Also, some fluids like power steering fluids may already have the dye in them.   Red dye is normally already in transmission fluid to help identify those leaks.  That red dye does not necessarily fluoresce but can be seen as red in ordinary white light.

A) Field Lab 9: Detecting automotive fluid leaks using UV flashlight
inspection of car with UV

This mechanic is using a UV flashlight to look for Freon leaks.   For this lab you will do something similar but no need to get under the car.

Coolant reservoirs At night or in a darkened garage, open up the hood of your car (or friend's car).   Find the coolant (antifreeze) reservoir.  The coolant may be yellow green, orange, or red.   Shine your UV flashlight onto the coolant reservoir.  What color does it seem to glow under the UV light?
UV flashlight on engine

Use your UV flashlight to look around the engine and radiator to see if there are any leaks that light up under the UV light.  Again, it is best to do this at night.   If you do see a leak, report what color does the UV light make the leak.   What liquid do you think it is?

It's possible to have oil or Freon leaks without seeing any glowing dye.   Oils and Freon may or may not have UV sensitive dye; however, nearly all antifreeze solutions have a dye in them.

Drips from car

If the car is leaking any liquid that is on the ground, shine the UV light on them to see if any fluoresce (glow due to UV light).  Report what you see.


B) Field Lab 9: Detecting stains around the home
dog with urine stain UV

UV lights are often used to detect urine stains from cats and dogs.  The top photo is without a UV light and the bottom one shows that UV light causes the stain to fluoresce (glow).  It's not always a strong glow, but it is visible in a darkened room.


UV flashlight on stain

Even in the daytime or with other lights on, a stain can usually be seen with a UV flashlight.  This photo came from an ad by Walmart advertising this UV flashlight for finding stains.

stains under UV light

These stains are circled and are visible due to a UV flashlight.  Some stains that glow are just soap residue.  The floor or carpet may be clean, but soap left over from cleaning a stain, or soap that got spilled can fluoresce.    Laundry soap, especially, has whiteners that will glow under UV light.

At night, check floors, furniture and walls to see if something glows.   Also, in bathrooms or laundry rooms, soap residue on showers and counters will often glow.

Report some of the things you discovered using your UV flashlight.  

C) Field Lab 6: Locating scorpions using UV light
Scorpion glow
A very common use of a UV light is to help find scorpions.  The outer shell of the scorpion has a pigment that absorbs UV light from the Sun.   This protects the scorpion from the damaging UV rays from sunlight.  The shell is fluorescent because it converts the high energy (dangerous) UV light to a low energy (non-dangerous) blue-green light.    This was good for scorpions until people discovered that a UV light makes scorpions easier to find and kill using that fluorescent property of the scorpion's shell.
scorpion under uv light

1) Use your UV flashlight to see if there are any scorpions inside or outside where you live.  They are more commonly found outside on fences and walls.

2) Report if you found any.   I recall one time I went outside and found 10 scorpions around my home.  Most were on the fences.   Once in a great while, I find one inside.   Every month or so I go scorpion hunting to reduce their population.

Either send your results to or print (or write them down) and hand them to Mr. Costello in the lab class.