Understanding HID Lighting
High Intensity Discharge (HID) lighting replaces the filament of a light bulb with a capsule of gas. The light emanates from an arc discharge between two closely spaced electrodes. This discharge is hermetically sealed inside a small quartz glass tubular capsule. HID lights require a ballast, which carefully regulates the voltage supplied to the capsule of gas. The amount of light produced is greater than a standard halogen bulb while consuming less power; this light more closely approximates the color temperature of natural daylight.
In all High Intensity Discharge lamps, light is produced by passing a current through a metal vapor. Free electrons colliding with an atom in the vapor momentarily knock an electron into a higher orbit of the atom. When the displaced electron falls back to its former level, a quantum of radiation is emitted. The wavelength of radiation depends on the energy zone of the disturbed electron and on the type of metal vapor used in the arc tube.
HID bulbs produce 5 percent of their output when first ignited, requiring a few seconds (usually 15-20) to reach full output. Also, if power to the lamp is lost or turned off, the arc tube must cool before the arc can be re-struck and light produced. Halcyon HID lights require approximately 5-10 seconds before they can be re-lit.
Understanding LED Lighting
Light-Emitting Diodes (LED) are light sources utilizing diodes that emit light when connected in a circuit. The effect is a form of electroluminescence where LEDs release a large number of photons outward; the LED is housed in a plastic bulb, which concentrates the light source.
The most important part of an LED is the semi-conductor chip located in the center of the light source. The chip has two regions separated by a junction. The p region is dominated by positive electric charges and the n region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and the n regions. When sufficient voltage is applied to the semi-conductor chip, the electrons are able to cross the junction into the p region.
When sufficient voltage is applied to the semi-conductor chip, electrons can move easily across the junction where they are immediately attracted to the positive forces in the p region. When an electron moves sufficiently close to a positive charge in the p region, the two charges “re-combine.“
Each time an electron recombines with a positive charge, electric potential energy is converted into electromagnetic energy. For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light. This photon has a frequency determined by the characteristics of the semiconductor material (usually a combination of the chemical elements gallium, arsenic, and phosphorus). LEDs that emit different colors are made of different semiconductor materials. Said simply, LEDs are tiny “bulbs” fit into an electrical circuit. However, unlike ordinary incandescent bulbs, they don’t have a filament. LEDs are illuminated solely by the movement of electrons in a semiconductor material, making them energy efficient and extremely resilient over long periods of time.
LED Advantages
LEDs don’t have a filament to burn out or break; therefore, they last much longer than conventional bulbs. Given that a very small semiconductor chip runs an LED, they are very durable and tend to last many thousands of hours. Moreover, LEDs are “instant on,” much like halogen lamps, and thus convenient for use in applications that are subject to frequent or potential on-off cycling. Conversely, HID lamps are more fragile and have to warm up (15 – 25 seconds) during ignition. However, HID lights are much brighter than current LED configurations. This is because the side-emitting light found with HIDs is much easier to focus; the front-emitting nature of LEDs does not lend itself to a tightly focused beam. HID lighting is still preferred by most divers given the superior intensity and tight focus. However, LED technology is changing rapidly and it is foreseeable that LEDs will eventually surpass the performance of HID lighting.
