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Finding Forensic Evidence Using Lights

Updated on December 13, 2012

Debunking the Myths

Let's face it. We've all seen CSI, Bones and a host of other forensic TV shows where investigators are able to find evidence anytime and any place they like. The reality is that it's (unfortunately) not as easy as TV makes it out to be. This lens exists to help people understand how to use forensic flashlights and headlamps, which are also referred to as forensic light sources (FLS) or alternate light sources (ALS).

I promise to keep things as simple as possible here for ya. I won't discuss lasers here as they can be extremely expensive and generally aren't portable.

Forensic blue ALS and orange goggle
Forensic blue ALS and orange goggle

3 Things You'll Need

Forensics is a science and we'll need the right tools and conditions in order to visualize evidence.

There are 3 absolute requirements.

1) It must be dark (little ambient light)

2) Must use the correct wavelength

3) Must use the correct filter or camera lens

Fluorescent dyes or powders (like Luminol) help a great deal. when looking for fingerprints.

Darkness (little ambient lighting)

Pretty straightforward....the darker it is, the better we'll see evidence. Ambient lighting, a fancy name for background lighting in a room, diminishes the amount of fluorescence or absorption we see.

Evidence can easily be missed if there is too much ambient light.

Visibile LIght Spectrum
Visibile LIght Spectrum

The correct wavelength is dependent upon what evidence you're looking for

There is much debate regarding the best wavelength(s) to use. Reasons for this include varying air density around the globe, differences in how people were trained and user's personal preferences. Evidence in the dry Arizona air will keep differently than evidence in the more humid southeast. In addition, the ethnicity of a person along with the background the evidence rests on will also contribute to different wavelengths being preferred in different scenarios. For example, 450 nm (blue light) might serve you well to find blood at Scene A while 470 nm might be better at finding blood at Scene B.

We select the appropriate wavelength(s) based upon the evidence being sought. Wavelengths are expressed in nanometers (nm) and there is a range of wavelengths associated with each color. The ranges I include below are arbitrary. I'm limiting the colors below to the most commonly used in Forensics. Blue and UV light are the two commonly used.

UV light (200 to 420 nm)

Used to find bruising, serological evidence (semen, saliva, urine), general crime scene search (hairs, fibers, etc.)

Blue light (450 to 475 nm)

Used to find blood spatter, GSR (gun shot residue), general crime scene search (hairs, fibers, etc.)

Cyan (a blue/green color in the 480 to 500 nm range)

Used to find latent prints (AKA fingerprints)

Green light (510 to 540 nm)

Used to find latent prints (AKA fingerprints)

IR light (770 to 2,500 nm; AKA near-IR)

Used to find bruising, GSR (gun shot residue), dried blood on dark clothing

orange goggles
orange goggles

Choosing the right filter or goggle

Filters block out the light that is not absorbed by the evidence. Forensic filters are either camera lenses, goggles or plastic sheets of a particular tint. The filter we use is dependent on what evidence we are looking for along with the wavelength we're using. If you have a camera, use a colored filter. If you're an investigator without a camera, use colored goggles.

Red, orange, yellow and clear filters are used in forensics. Orange and red are the most useful.

Yellow and orange filters are used with blue, UV, green and cyan light.

A red filter is only used with green light.

A clear is only used with UV light.

IR light is a little tricky. No filter or goggle will yield results. A forensic camera with infrared capabilities is required in order to visualize any evidence. Camera manufacturers like Fuji (IS Pro) and Nikon (D90) make such models.

Recommended Operational Distance

Forensic light sources vary greatly in price....pen lights can be as little as $10 USD while high-end lasers can cost upwards of $30,000 USD. Some of the newer LED light sources on the market represent the best value and most cost between $20 and $800 per wavelength. The more powerful the light, the farther you can be from evidence and see it.

You'll need to be very close (within 1 ft) to the evidence to see it with a weak light source. These can still work well when looking into a small, controlled area like a computer keyboard. A very powerful light source can allow you to be 30 ft away from the evidence and still see it. These are best used in larger, uncontrolled scenes.

Some manufacturers, like FoxFury, list their recommended operational distances (RODs).

Scout Series: ROD = 2 ft

Command Series: ROD = 3 ft

PRO Series: ROD = 12 ft

HammerHead Series: ROD = 10 ft

MF Series: ROD = 20 ft

Nomad Series: ROD = 30 ft

Base your buying decision on how much light is needed.

Wavelength specificity

This issue is given little attention but it's important to note. We're talking here about how accurate/specific a wavelength is. The scientific name for this is spectral brightness. So much emphasis is put on the exact wavelength of a light source but the wavelength specificity is often ignored. A more precise wavelength means more of the desired colored light will directed at the evidence, which translates into better observed fluorescence/absorption (and less wasted light).

Some older halogen lights have a wavelength specificity of plus or minus 50 nm. A 470 nm light with that specificity would therefore be putting out everything from 420 nm (in the UV spectrum) to 520 nm (in the green spectrum). Like a bell curve, most of the light would still be blue (between 450 and 490 nm) but a considerable amount (20% or so) of light will not be and will therefore be in the non-usable range.

Newer LED light sources are more accurate and some even have specificity of plus or minus 3 nm. That same 470 nm would put out between 467 and 473 nm, which is exclusively blue. More light would therefore be absorbed, which would make it easier to see evidence and take pictures.

Fingerprints on a soda can

Here is a Coca-Cola can processed with a fluorescent dye. A 525 nm (green) light source was used. This image was captured using a camera with an orange filter over the lens. The fingerprints are easy to spot because it's dark and correct wavelength/filter pair were used.

Fingerprints on a gun

Here are fingerprints (latent prints) on a 9mm pistol magazine. Like the picture above, a fluorescent dye was used and the image was captured using a camera with an orange filter over the lens.

Detecting drugs with lights

Forensic light sources can also be used to locate narcotics. Different narcotics fluoresce/absorb differently. If you shine blue light at cannabis (assuming it's dark and you're using an orange filter), it will appear reddish-brown to the human eye and pinkish to the camera. There are some false positives so this type of investigation is a presumptive search only and not definitive.

Hands-Free vs. Hand-Held Light Sources

Each has its benefits but both will get the job done. We'll list the pros and cons of each below:

Hands-Free Light Sources

These include headlamps and utility lights that can be clipped on to the investigator.

Pros:

Provides two free hands for multi-tasking (taking notes, lifting, etc.)

Reduces cross contamination

Provides light wherever you look

Can sometimes give a wider beam angle

Cons:

Not as powerful

Tend not to be as focused

Extra weight on head

Harder to switch wavelengths

Hand-Held Light Sources

Flashlights are far more commonly used in forensics.

Pros:

More powerful

Light tends to be more focused

Easier to change wavelengths

Cons:

1 hand is devoted to holding light

Possible cross-contamination if light placed on floor, counter

Lights in Forensic TV Shows

Forensic lights are seen in just about every episode of forensic TV shows like CSI and Bones. Sometimes large lights are shown in the laboratory while smaller lights are shown in the field and lab.

Blue lights typically shows up better on TV compared to ultraviolet. The FoxFury HammerHead 470 nm (blue light) ALS flashlight is pictured above in a scene from the hit TV show Bones.

How It All Works

Larger forensic evidence can typically be found without needing a light source. Light sources can help locate smaller evidence including biological fluids, hairs, fibers, etc. that are hard to see with the naked eye. When done properly, light emitted from the correct light source enables evidence to be seen through a filter (lens, goggles). This section gives a quick and simple overview of the light physics involved.

Light emitted from a light source is either absorbed by the evidence or scattered (reflected back to the user, etc.). Filters (lenses, goggles) block the scattered light and allow us to see only the absorbed light (fluorescence)...provided we're using the correct wavelength and lens for the given type of evidence.

Practically speaking, say we use a 395 nm UV light source to look for hair. We aim the light where we think the evidence could be. UV gives off a purple/violet looking light. Hair would absorb some of the UV light and re-emit it at a longer wavelength (higher than 395 nm). In this instance, hair will appear green or yellow when seen through an orange filter. Green (~520 nm) and yellow (~570 nm) are longer wavelengths than UV (~395 nm). The filter blocks out the UV light and allows the evidence to stand out. Using no (or the incorrect) filter would result in a great deal of back scatter being seen, which would make it very difficult to see the evidence.

http://foxfury.com/
http://foxfury.com/

Wavelengths that FoxFury uses

FoxFury's ALS utilize LEDs and are available in specific wavelengths. In some cases, we may offer a model in two different wavelengths for one light color. We selected these wavelengths after doing extensive testing with different evidence in various dilutions. We also offer white lights as well.

UV - 380 nm, 395 nm

Blue - 450 nm, 470 nm

Cyan - 495 nm

Green - 525 nm, 532 nm

IR - 850 nm

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      foxfury 4 years ago

      @bonnie-mackenzie2: Wow. That's a great question. Not really sure about FDA approvals for Wood's lamps. Wood's lamps are UV lights and are used by sexual assault nurse examiners (SANE) to look for evidence. Like UV light, blue light can be used to find serological evidence. If anything, blue light is safer to use than UV as it is a higher wavelength. As for subcutaneous bruising, typically UV or IR light is used for that purpose....not sure exactly about the FDA status / approval. Hope that helps.

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      bonnie-mackenzie2 4 years ago

      Do you know the FDA status of blue lights as they pertain to collecting evidence on patients who present with alleged sexual assault? Is there any FDA approval for the use of wood's lamp to detect subclinical bruising? (this application has been suggested in a pediatric emergency medicine textbook based on findings from this paper: Arch Pediatr Adolesc Med. 2002;156(3):265-268. doi:10.1001/archpedi.156.3.265.) The only FDA approval for woods lamp that I can find relates to detecting fluorescence. Thanks!