Introduction to Alternate Light Sources

Fluorescence

Alternate light sources (ALS) enhance the visualization of evidence not readily apparent to the naked eye, facilitating collection, documentation, and processing of evidence. The technology uses light emitted at a controlled range of wavelengths to improve the contrast of evidence against a background. Alternate light sources that emit light in the visible range (400–700 nanometers [nm] within the electromagnetic spectrum) cause multiple types of evidence to be visualized through fluorescence.

Fluorescence is the emission of light of a longer wavelength by a substance that has absorbed light of a shorter wavelength. When a fluorophore, or molecule capable of fluorescence, is subjected to a light source at a specific wavelength (called the excitation wavelength), it absorbs energy and transitions into a higher-energy excited state. Shortly after light exposure, the molecule returns to its normal state, or ground state, and the excess energy is emitted as light. This wavelength of light (called the emission wavelength) is of a longer wavelength than the excitation wavelength, and is detected as fluorescence.

Some types of evidence important to a crime scene investigation—such as hairs, fibers, or biological fluids that may contain DNA—naturally fluoresce when excited by a certain wavelength or range of wavelengths. Developing agents applied to evidence such as fingerprints may also fluoresce with illumination. ALS devices in the visible spectrum emit light at different colors, which are controlled ranges of visible light wavelengths with specific peak wavelengths (most of the light emitted by the device is at this wavelength). Figure 1 shows the types of evidence that can be detected at specific wavelength ranges (colors of light). Table 1 gives the general wavelength ranges for each color of visible light.

Barrier Filters

Visualizing and documenting evidence by ALS illumination requires use of a barrier filter. During the fluorescence process, light emitted by the ALS (at the excitation wavelength) is reflected back to the eye, overpowering the emitted fluorescence (at the emission wavelength), often rendering it undetectable. Barrier filters enable visualization of fluorescence by preventing transmission of light at the same wavelength as the excitation light to the eye or detector (such as a digital camera). Light produced by the fluorescing compound passes through the barrier filter and is detected by the eye or camera. Figure 2 demonstrates the role of a barrier filter in visualizing evidence. Barrier filters are manufactured in the form of goggles, flat viewing panes, and filters for digital cameras — any combination of the aforementioned barrier filters and detectors can be used with single and multiwavelength units. The appropriate barrier filter depends on the wavelength of light used; for example, evidence illuminated by blue light can most often be visualized through an orange barrier filter. Without these filters, the illuminated evidence would most likely be undetectable to the naked eye.

In most circumstances, the barrier filters used for ALS that emit visible light are longpass filters. These optical filters block transmission of light with wavelengths below a certain range. Light with a longer wavelength (and lower energy) cannot be perceived by the viewer. When the appropriate color barrier filter is used, it can effectively block the excitation light emitted by the ALS.

Barrier filters must transmit precise wavelengths of light to effectively detect evidence. Filters that block light transmission at a defined wavelength range may more reliably block the transmission of ALS excitation light, enabling better visualization of evidence. This means that undesired wavelengths of light are less likely to "leak" through the barrier filter, obscuring the fluorescence. Figure 3 demonstrates the importance of barrier filters with a semen stain illuminated by blue light and visualized (a) with and (b) without an orange barrier filter.

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