The concept of color and photoluminescence emission stands for the observation of physical phenomena due to the interaction of light with material. Light consists of electromagnetic energy, including microwaves, radio waves, etc. The visible waves, which make up white light, form only a part of the electromagnetic spectrum. The capability of our eyes to see visible light is called color vision and is similar to a filter that transmits only the radiation between approximately 400–700 nm (Fig. 13.1). Radiation at 420 nm is observed as blue, at 450 nm as alternate light or yellow, at 550 nm as green, and at 650 as red. The mixture of electromagnetic rays between 400 and 700 nm is seen as white light.

Photographic techniques utilizing these four types (visible light, ALI light, UV light, and IR light) of the light spectrum give very different looks into pattern injuries on skin. Because the human eye is unable to see beyond the visible light spectrum, these special types of photographic techniques are applied to produce images in the nonvisible zones of electromagnetic radiation such that they can be seen with the human eye. When light hits human skin, different types of processes take place, including reflection, absorption, fluores­cence, and scattering of the light. Reflection occurs as the shorter wavelengths of light strike the surface of the skin. Depending on different parameters of radiation, more than 60% of short wavelengths do not penetrate the surface of the skin and are reflected back (Anderson and Parrish 1982; Rai and Kaur 2010), while the longer wavelengths of light (700–900 nm) can penetrate the skin up to 2–3 mm (Anderson and Parrish 1982; Rai and Kaur 2010). When light rays strike the skin, a molecular-level excitation within the skin occurs that increases the resting state energy of the molecules within the skin, known as fluorescence. This phenomenon remains for about 100 ns. A special technique known as alternative light imaging is needed for the human eye to see and the camera to record this type of fluorescence (Wright 1998). This technique is named for Prof. G. G. Stokes, who reported in 1853 that the remitted wavelength of a predominant color of light is of a different frequency than the illuminating source. A part of the energy of the light at a particular frequency is absorbed by the subject matter it strikes. Once that energy is absorbed in the form of electrons, it creates a molecular excitation that looks for a return to its rest position. The return of the electrons to their resting position releases that energy as fluorescence (Stokes 1853).

It is most important for forensic photographers to understand the physiological changes that take place during a skin injury. These changes from a normal state to an injured state to a healing state permit us to differentiate among the contusions illuminated by light sources of various wavelengths. A skin injury leads to inflammation and repair (Wright and Golden 1997, 2010). Inflam­mation leads to vascular changes at the site of the injury, as the body starts to transport blood in response to tissue damage and to mediate the framework for healing. The damaged tissues further lead healing through the process of repair or scarring. The tissues heal by repair via cell replication, while if damage is too large for repair, the tissue will heal by scarring. Inflammation is the strongest soon after the injury occurs, with repair becoming the prevalent event as the tissue begins to heal. The chemical composition of the injured skin is very different from the surrounding normal, healthy skin. Damaged tissues respond a different way to electromagnetic radiation than the adjacent normal, healthy skin because of differences in photoactive agents, including hemosiderin, melanin, hemoglobin, beta-carotene, etc. (Wright and Golden 1997).

When the pattern injury in the skin shows signs of injury on the epidermis, it is best to use the UV range to photographically illustrate the surface damage because it allows more absorption and has a shallower depth of penetration (Wright and Golden 1997, 2010). UV photography requires a special armamentarium for taking UV photographs. If a skin injury shows signs of an injury on the dermis, it would require using longer wavelengths of light that could go through the skin to the level of the bruising, namely, longer wavelengths of light in the IR range. It is very important to distinguish between the healthy adjacent skin and the injured skin. It would require using a special type of photography known as fluorescent photography techniques (ALI) (Wright and Golden 1997). All photography requires a totally dark room. The forensic photographer can employ any or all of the photographic techniques to take and preserve the evidence.

Note: Visible light photography creates images of the injuries as seen by the unaided eye. UV photography shows details of the damaged epidermis surface of the skin, while IR photography exhibits the tissue injury at the dermis and below. ALI photography will record the difference between the uninjured skin adjacent to the injured skin using fluorescence (Fig. 13.2). It is important to understand that all techniques will create useful images at all times (Rai and Kaur 2010).

The light can be controlled three ways:

Digital cameras are becoming entirely automatic. This does not make them foolproof; it just means they are more complicated to use appropriately. Be familiar with your camera—read your manual—have your manual with you—refer to it.