Alternate Light Source Technology in the Detection of Evidence and Injury
Written by Debra Holbrook   

A VICTIM CALLS 911 with a domestic complaint involving assault and strangulation. Law enforcement personnel arrive but see no visible injury to either party and no arrest is made. The victim transports herself to a local hospital where she is discharged in stable condition, having no neck CT as no injuries were noted.

This article appeared in the March-April 2021 issue of Evidence Technology Magazine.
You can view that full issue here.

Article Courtesy of the Academy of Forensic Nursing

This is a scenario that plays out across towns and cities every day involving unwitnessed histories of assault where no injury is seen. Variations to this complaint may include the victim being a person of color where detection of bruising is more challenging, or a victim whose emergent bruising is so faint that it is obliterated with a flashlight, or unable to be seen in a photograph. But all have one thing in common: a victim with injuries that are poorly visualized by the naked eye.

Bruising and Wounds

So, the question remains: Why are bruises sometimes so difficult to detect?

To answer this, we must understand the causation and properties of bruising. A bruise is caused by blunt- or squeezing-force trauma and may also be referred to as a contusion. It may present as firm (indurated), painful, and initially varies in color from red to purple. Bruises are caused by capillary blood vessel rupture and extravasation of the bleeding in the epidermal, dermal, and subdermal tissues without the skin being broken.

Visualization of bruising secondary to trauma is dependent on many factors including but not limited to: victim age, amount of force used, depth of the wound, location on the body, medications taken by the victim, as well as their physical condition and degree of pigmentation in the skin. Even bruising that appears at the epidermal level may be difficult to visualize on a victim with darkly pigmented skin as compared to one who is lightly pigmented.

Small, blunt-force trauma and ligature strangulation will generally leave visible wounds due to energy being focused to a smaller area and the rupture of capillaries; while trauma by a large object, including manual strangulation, will rarely leave bruising at the epidermal level since the energy dispersed to a larger area of the body decreases capillary rupture at the surface. This does not mean that there is not bruising or bleeding into deeper layers of the dermis, or subcutaneous layers of the skin—it only means that this bruising is too deep to be seen with an unaided eye. Deep bruises may take days or weeks to come to the surface, or they may never do so.

The cycle of the bruise is as follows: blood vessels break to release erythrocytes; erythrocytes break down to release hemoglobin; and hemoglobin breaks down to release bilirubin. This cycle demonstrates the body’s process of dissolving the bruise and absorbing it as waste. During the hemoglobin-to-bilirubin phase, the red to purple bruise may turn green to yellow in color.

But what if there was a method of assessing a victim/patient that could aid the eye in seeing wounds beneath the skin? What if it did not matter how darkly pigmented the victim was or how faint the wound appeared, but gave all victims the same advantage in regard to having their wounds documented? That method is alternate light source technology (ALS).

Alternate Light Sources 101

An ALS is nothing more than a powerful lamp that emits light in the forms of ultraviolet, visible, and infrared wavelengths. The ALS filters this light into color bands, or wavelengths, that allows one to see evidence by fluorescence (evidence glows), absorption (evidence darkens), and oblique lighting or reflection (where small particles of evidence are visible). When light strikes a surface or compound, it will either be absorbed, reflected, transmitted, or a combination of all three. The actual interaction is between the photon of light and electrons bound to the atoms of the surface.

Where white light is composed of a combination of wavelengths ranging from 190–700 nm, an ALS allows the selection of specific ranges of wavelengths by filtering the rest. For instance, a setting of 350 wavelength on most commercial units would give a range of approximately 330–370 nm, allowing one to visualize evidence at a surface level.

Image 1. Fluorescence at 350 nm wavelength.

Commercial ALS units come as flashlights with one wavelength per flashlight, or as a unit where different wavelengths may be selected (generally four to six pre-selected wavelengths). As the wavelength increases, the amount of energy decreases—so the higher the wavelength, the deeper the penetration of the skin. The deeper the penetration, the better one is able to visualize latent wounds beneath the epidermis and into the dermis and sub-cutaneous layers of the tissue.

Evidence tends to fluoresce on the skin at lower wavelengths (190–350 nm). You may be familiar with this set of wavelengths as a “blacklight” where substances and fibers tend to glow. Blood does not have properties that fluoresce, so blood—even on the surface of the skin—will appear darkened due to absorption of the wavelengths. As the wavelength is increased, the light begins to penetrate into the dermis (300–400 nm) and even into the subcutaneous layers under the skin (400–650 nm.) (Figure 1). Infrared wavelengths are not used in victim/patient evaluation due to the damaging effects of this light at a cellular level that is known to cause cancer (700–900 nm).

Figure 1. Penetration of skin layers at different wavelengths.

Due to the intensity of the light, it is essential that goggles are worn, not only for the protection of the eye, but to further filter light and determine the best visualization of the surface area. When using a light source and goggles to detect evidence, you are using different wavelengths to detect fluorescence, absorption, or reflection. Goggles are long-pass filters that block the excitation wavelength (also known as the light we are using to see the evidence) from hitting our eyes, allowing us to see the weak fluorescence or absorption created by the light source. Goggles are yellow, orange, and red in color. Some manufacturers use a mixture of these colors in their goggles, so orange may be a mixture of yellow and orange. We use multiple wavelengths to attempt to reduce the visualization of backgrounds so we can get a better contrast of the evidence that we are looking for. As we go from lower to higher wavelengths, the goggles lose their ability to block the light coming from the light source, so we need to change goggles as we change wavelengths to make sure no light is “leaking through” our goggles—which would cause us to either visualize less evidence or none at all.

Image 2. Absorption at 460 nm wavelength. Image courtesy Debra Holbrook.

Because all people have capillary rupture (bruising) at different thresholds due to injury, there is no constant ALS wavelength that can be used to detect latent injury on every victim. It is essential that all available wavelengths—most often 300–600 nm—be used in combination with all goggle colors for optimal evaluation of an injury. Tattoos, blood vessels, and areas of hyperpigmentation will also show absorption and must never be confused with wound absorption. An ALS can be a valuable tool in defining wounds such as bitemarks, and defining patterns associated with mechanism of injury and central clearing.

Simplified: an ALS is a tool to aid an investigator or practitioner in seeing that which is not apparent, much like corrective lenses aid one in reading or a microscope aids in seeing that which is too small to see without magnification. An ALS does not require calibration. Maintenance is limited to changing batteries and the light bulb, and modern units are lightweight and easy to use.

Image 3. ALS absorption on the neck. Image courtesy Jon Goldey.

Synopsis of Research Supporting ALS in Identification of Wounds

Since the early 1990s, research has been published linking ALS technology to the ability to identify latent wounds on the body:

  • West discussed the “Stokes shift,” as defined earlier, where light either fluoresces, is absorbed, or is transmitted.
  • An ALS has been suggested for use by the Department of Justice as a tool to assist in the identification of “subtle injury” during a forensic examination.
  • In 2006, Kaczor et al. published that small, blunt-force bruising generally causes bruising detected by the naked eye, but large surface trauma causes intradermal bruising not seen without the aid of ALS, and that absence of visible bruising does not mean an assault did not occur.
  • In 2010, Holbrook and Jackson pioneered the use of ALS in strangulation after linking latent wounds with patient symptoms in a retrospective chart review model of 172 victims. This study showed that 415–450 nm worked best to detect latent wounds with orange goggles, and that wounds could be visualized up to and including 30 days from the time of the assault.
  • The Holbrook study also specified that all wavelengths and all goggle colors should be used to conduct an ALS exam, and that 500 nm wavelengths were optimal for older wounds.
  • When challenged with whether the absorption could be topical agents on the skin, Politt et al. published findings demonstrating that of 14 commonly used topical products, only makeup containing a sunscreen showed absorption under ALS scrutiny.
  • In 2020, Scafide et al. conducted a randomized control trial to measure the effectiveness of an ALS within visible and ultraviolet long-wave spectrums at improving bruise detection compared to white light over time. Results demonstrated that 415–450 nm wavelengths were optimal with yellow-orange goggles on six skin tones of pigmentation.

Image 4. ALS defines wounds and mechanisms of injury. Image courtesy Jon Goldey.

ALS in Judicial Proceedings: Maryland Challenge

The relevant standard of the Frye standard adopted by the Maryland Court of Appeals in Reed v. State, 283, Md. 374 (1978) holds that before expert testimony based on the “new” scientific process can be admitted into evidence, it must first be established that the technique used is reliable.

The first argument is that ALS is not a process or a technique. There is nothing subjective about ALS. A light is turned on and an examiner either sees florescence or absorption… or they do not, making ALS more of a tool than a process.

As defined earlier, ALS is not a “new” technology and has been used in crime scene investigation and forensic medical examinations for many years. Since the early 1990s studies have shown that ALS can be used to identify latent injuries, including bruising—thus supporting its “validity and reliability” and acceptance in the scientific community. Forensic Nurses use these units on a daily basis to perform forensic medical evaluations of victims of crime. ALS rulings have been upheld in Baltimore Circuit Courts that use of an alternate light source has gained general acceptance and is standard in the field of forensic nursing.

Questions used to successfully challenge ALS motions may include:

  1. What is an alternate light source? Are there different kinds? How do they work?
  2. How long has an ALS been used at (center in question)?
  3. Is ALS evidence-based practice? When is ALS used? How many times have you used ALS?
  4. Do other facilities use ALS? Which facilities?
  5. Speak to your training in the use of ALS—specifically, when and where.
  6. Is (your facility) involved in training in ALS? Any national or international training forums? Other ways that ALS is used?
  7. Based on your knowledge, training, and expertise, is the use of ALS a standard in forensic examination?

ALS units may be brought into court for motion trials along with goggles or filter shields. Demonstration of the unit and how it is used on a victim/patient are particularly useful in educating the judge and/or jury in ALS use. Generally, it is more confusing to a jury to explain ALS than showing that it is a light that is relatively easy to use.

Image 5. ALS strangulation markings at 500 nm wavelength. Image courtesy Debra Holbrook.


An alternate light source is a powerful lamp that emits light in ultraviolet, visible, and infrared wavelengths and goggles must be worn to fully visualize florescence, absorption, or reflection to identify evidence.

It is essential that all wavelengths and goggle colors be worn to adequately identify evidence or latent injury. Documentation should include: 1) type of finding (fluorescence, absorption), 2) wavelength under which evidence was best visualized, 3) filter color used to visualize (goggle color), 4) topical products applied (if used on skin surface), and 5) a detailed history that may have contributed to the wound.

ALS units are easy to use and require no calibration. However, it is essential that staff be trained to use these units and have an understanding of what they are seeing with the light to assure effective documentation and potential expert court testimony.

About the Author
Debra S. Holbrook (MSN, RN, SANE-A, SANE-P, FNE A/P, DF-AFN, FAAN) completed both undergrad and graduate studies through Wilmington University. In 2002, she testified on Capitol Hill before a Senate Judicial Subcommittee on Crime and Drugs on behalf of the bill that was signed into law in 2005 as the DNA Justice Act. She is the recipient of numerous international awards, including the ANCC Magnet International Nurse of the Year, the Delaware Nurse of the year, the International Association of Forensic Nurses Pioneer Award, the 2014 Most Influential Marylanders in Healthcare, Distinguished Fellow—Academy of Forensic Nursing, and the prestigious Fellow American Academy of Nursing. She serves on the board of the Academy of Forensic Nursing and lectures nationally and internationally on forensics across the lifespan. Holbrook has integrated comprehensive forensic practice into the SANE model and her programs have cared for patients of interpersonal violence including domestic, elder, child, institutional, vulnerable populations, gunshots, stabbings, non-accidental poisonings, and burns. She has pioneered the use of the alternate light source in strangulation cases and set precedence in national court systems. She is currently Director of Forensic Nursing at Mercy Medical Center in Baltimore, where she coordinates care to victims of interpersonal violence for all hospitals in Baltimore City.


Holbrook, D., and M. C. Jackson. 2013. Use of alternative light source to assess strangulation victims. Journal of Forensic Nursing. 9(3):140–145.

Kaczor, K., M. C. Pierce, K. Makoroff, and T. S. Corey. 2006. Bruising and physical child abuse. Clinical Pediatric Emergency Medicine. 7:153–160.

Pollitt, E., J. Anderson, K. Scafide, D. Holbrook, G. D’Silva, D. Sheridan. 1980. Alternate light source: findings of common topical products. Journal of Forensic Nursing. 11(3):97–103.

Reed v State, 283 MD. 374. 1978.

Scafide, K., D. Sheridan, N. Downing, M. Hayat. 2020. Detection of inflicted bruises by alternate light: results of a randomized controlled trial. Journal of Forensic Sciences. 65(4):1191–1198.

Sheridan, D., and K. Nash. 2007. Acute injury patterns of intimate partner violence victims. Trauma Violence Abuse. 8(930):281–289.

West, M., R. Barsley, J. Hall, S. Hayne, and M. Cimrmancic. 1992. The detection and documentation of trace wound patterns by use of an alternate light source. Journal of Forensic Sciences. 37(6):1480–1488.

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