Chapter 2
Understanding Ultraviolet Radiation

- Electromagnetic spectrum
- UVA, UVB and UVC
- Wave theory
- Quantum theory

In order to truly understand the tanning process of the skin you need to have at least a basic understanding of the properties and function of light. Although light has played a central role in the histories of religion, art and science and is so common to our everyday existance, it can actually be quite elusive.

Understanding Ultraviolet Radiation
To understand ultraviolet radiation (UV) one needs to know UV�s placement in the electromagnetic spectrum. Ultraviolet light is located between X-radiation and visible light. UV has a higher frequency and shorter wavelength than visible light, and it has a lower frequency and longer wavelength than X-radiation. UV with its longer wavelength and less energy is less penetrating than X-ray and is sometimes absorbed by matter. Photobiology studies the interaction of nonionizing radiation between the electromagnetic spectrum and biologic systems. Nonionizing radiation represents the ultraviolet, visible and near infrared regions of the spectrum. Tanning occurs as a result of exposure to ultraviolet radiation. To fully understand this reaction, you must familiarize yourself with the electromagnetic spectrum.

Electromagnetic Spectrum
The electromagnetic spectrum is a way of visualizing the frequency and wavelength proportions of different forms of energy. Electromagnetic radiation has properties of both waves and particles. We divide the electromagnetic spectrum in the UV range for medical purposes.

UVA is found in the region between 320 and 400 nm (nm = Nanometer = 1 billionth of a meter) and is the least powerful wavelength band of UV radiation. UVA acts primarily to cause the melanin pigments in the skin to oxidize (darken) creating the cosmetic tan and has limited power to cause erythema (sunburn).

UVB is found in the region between 280 and 320 nm. It comprises the wavelengths primarily associated with erythema (sunburn), is also necessary for the production of vitamin D in the skin and is primarily responsible for stimulating increased melanin production. UVB wavelengths (at 305 nm) have 1,000 times more erythemal power than UVA wavelengths.

UVC is found in the region between approximately 200-280 nm and is called germicidal UV because of its proven effectiveness in killing single-cell organisms. Solar radiation in the UVC range is absorbed almost entirely by the atmosphere and that is fortunate considering that even a short overexposure to UVC is very harmful to the eyes and causes severe erythema (sunburn). UVC is emitted by High Intensity Discharge (HID) lamps; therefore these lamps require special filter glass, to contain the output of the UVC spectrum (this will be discussed later). Another place where radiation in the UVC range can be found is in the arc of a welding torch. For that reason, optical damage referred to as �welders eye� is caused by UVC light.

Wave Theory
Ultraviolet rays are similar to X-rays, white (visible) light, infrared and other similar types of radiant energy. They are all electromagnetic waves, wavelike disturbances associated with vibrating electric charges. Most waves are transmitted by some medium; for example, you have all seen waves on the surface of the water, in which case the water is the transmitting material. When a stringed instrument is plucked, waves are set up in the string, so the string becomes the transmitting material. Strangely enough, no one knows what transmits electromagnetic waves, however, we have proof that they are in fact transmitted.

Electromagnetic waves all travel at the same constant speed as light, 186,000 miles per second in a vacuum. All electromagnetic waves have the same form and travel at the same speed, but differ in wavelength. Wavelength is the distance between two successive crests in the wave. The number of crests or cycles per second is the frequency of the wave. The unit of frequency is hertz or 1 cycle per second. Therefore, if the wavelength is decreased, then the frequency is increased. Frequency and wavelength have an inverse relationship which is calculated with one of two equations where the velocity of radiation is 186,000 miles per second.

Where the velocity of radiation is 186,000 miles per second. Frequency is calculated using cycles per second and wavelength is calculated in meters. The wavelengths of electromagnetic radiation vary in size from a fraction of an angstrom unit (an angstrom is equal to ten billionths of a meter) to thousands of meters, commonly called the "electromagnetic spectrum." Some of the wavelengths of electromagnetic radiation from this spectrum are classified as follows:

Therefore, the useful unit of measure for our purposes is the nanometer. Radiations shorter than 10 nanometers (i.e. gamma rays or X-rays) generally ionize molecules (remove electrons) producing positively or negatively charged ions and are, therefore, known as ionizing radiation. Ultraviolet radiation is absorbed by molecules and is known as nonionizing radiation.

Quantum (Particle) Theory
Another theory used in reference to the electromagnetic spectrum is the quantum theory. In order to explain energy transfer, a bit of energy called a photon was theorized. Photons have no mass and when absorbed this energy is passed on to the absorbing molecule (such as skin cells) and the photon no longer exists in its same state. The amount of energy in a photon is directly proportional to the frequency of the radiation. The energy of a photon increases as the frequency increases. The more cycles per second (frequency) of any given photon, the more energy the photon has. The energy of any given photon decreases as the wavelength increases. The longer the wavelength, the less the frequency.

Light energy is expressed differently. We often express radiant energy in terms of watts per square meter or milliwatts per square centimeter. Skin exposure is usually expressed in joules per square centimeter. A Joule, is a unit of measurement and is equivalent to the electrical work done in one second by an electrical current of one ampere through the resistance of one ohm; named for its inventor, British Physicist, J.P. Joule (1818-1889).