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[Part 1 of this article discusses the major components of a typical remotely operated vehicle system along with everyday underwater tasks ROVs perform.]
3.2 PRIMARY SUBSYSTEMS
The ability to sense the environment, either visually or through other means, and perform work at the desired location, is the mission of the ROV. The subsystems necessary for this task are discussed in the following sections.
This explanation of lighting comes courtesy of Ronan Gray of Deep Sea Power & Light. The need for underwater lighting becomes apparent below a few feet from the surface. Ambient visible light is quickly attenuated by a combination of scattering and absorption, thus requiring artificial lighting to view items underwater with any degree of clarity. We see things in color because objects reflect wavelengths of light that represent the colors of the
visible spectrum. Artificial lighting is therefore necessary near the illuminated object to view it in true color with intensity. Underwater lamps provide this capability.
Lamps convert electrical energy into light. The main types or classes of artificial lamps/light sources used in underwater lighting are incandescent, fluorescent, high-intensity gas discharge, and light-emitting diode (LED) " each with its strengths and weaknesses. All types of light are meant to augment the natural light present in the environment. Table 3.1 shows the major types of artificial lighting systems, as well as their respective characteristics.
- Incandescent - The incandescent lamp was the first artificial light bulb invented. Electricity is passed through a thin metal element, heating it to a high enough temperature to glow (thus producing light). It is inefficient as a lighting source with approximately 90 percent of the energy wasted as heat. Halogen bulbs are an improved incandescent. Light energy output is about 15 percent of energy input, instead of 10 percent, allowing them to produce about 50 percent more light from the same amount of electrical power. However, the halogen bulb capsule is under high pressure instead of a vacuum or low-pressure noble gas (as with regular incandescent lamps) and, although much smaller, its hotter filament temperature causes the bulbs to have a very hot surface. This means that such glass bulbs can explode if broken, or if operated with residue (such as fingerprints) on them. The risk of burns or fire is also greater than other bulbs, leading to their prohibition in some underwater applications. Halogen capsules can be put inside regular bulbs or dichroic reflectors, either for aesthetics or for safety. Good halogen bulbs produce a sunshine-like white light, while regular incandescent bulbs produce a light between sunlight and candlelight.
- Fluorescent - A fluorescent lamp is a type of lamp that uses electricity to excite mercury vapor in argon or neon gas, producing short-wave ultraviolet light. This light then causes a phosphor coating on the light tube to fluoresce, producing visible light. Fluorescent bulbs are about 40 percent efficient, meaning that for the same amount of light they use one-fourth the power and produce one-sixth the heat of a regular incandescent. Fluorescents typically do not have the luminescent output capacity per unit volume of other types of lighting, making them (in many underwater applications) a poor choice for underwater artificial light sources.
- High-intensity discharge - High-intensity discharge (HID) lamps include the following types of electrical lights: Mercury vapor, metal halide, high-pressure sodium and, less common, xenon short-arc lamps. The light-producing element of these lamp types is a well-stabilized arc discharge contained within a refractory envelope (arc tube) with wall loading (power intensity per unit area of the arc tube) in excess of 3 W/cm2 (19.4 W/in2). Compared to fluorescent and incandescent lamps, HID lamps produce a large quantity of light in a small package, making them well suited for mounting on underwater vehicles. The most common HID lights used in underwater work are of the metal halide type.
- LED - A light-emitting diode (LED) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color of the emitted light depends on the chemical composition of the semiconducting material used, and can be near-ultraviolet, visible, or infrared. LED technology is useful for underwater lighting because of its low power consumption, low heat generation, instantaneous on/off control, continuity of color throughout the life of the diode, extremely long life, and relatively low cost of manufacture. LED lighting is a rapidly evolving technology " look for more usage of LEDs in the underwater lighting field soon.
Table 3.1 Light source characteristics (table and lighting description courtesy of Deep Sea Power and Light).
Most observation-class ROV systems use the smaller lighting systems, including halogen and metal halide HID lighting.
The efficiency metric for lamps is efficacy, which is defined as light output in lumens divided by energy input in watts, with units of lumens per watt (LPW). Lamp efficacy refers to the lamp's rated light output per nominal lamp watts. System efficacy refers to the lamp's rated light output per system watts, which include the ballast losses (if applicable). Efficacy may be expressed as 'initial efficacy', using rated initial lumens at the beginning of lamp life. Alternatively, efficacy may be expressed as 'mean efficacy', using rated mean lumens over the lamp's lifetime; Mean lumens are usually given at 40 percent of the lamp's rated life and indicate the degree of lumen depreciation as the lamp ages.
An efficient reflector will not only maximize the light output that falls on the target, but will also direct heat forward and away from the lamp. The shape of the reflector will be the main determinant in how the light output is directed. Most are parabolic, but ellipsoidal reflectors are often used in underwater applications to focus light through a small opening in a pressure housing. The surface condition of a reflector will determine how the light output will be dispersed and diffused. The majority of reflectors are made of pure, highly polished aluminum that will reflect light back at roughly the same angle to the normal at which it was incident. By adding dimples or peens to the surface, the reflected light is dispersed or spread out. When a plain white surface is used, the reflected light is diffused in all directions.