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HID - High intensity Discharge
HID - High intensity DischargeLike the name implies, high intensity discharge lighting creates a very bright light that is ideal for night time driving. Though the color of the light is often perceived as having a bluish tint when viewed at night, most of the light that is produced by HID headlamps is actually very close in color to natural noontime sunlight -- though some of the light produced is also in the blue and ultraviolet spectrum. Halogen headlamps, by comparison, are more yellowish in appearance but are brighter and whiter than older incandescent style headlamps. The near-white light produced by HID headlamps improves visibility and makes it easier to see distant objects. The color of light can be measured in "degrees Kelvin," which refers to the "temperature" (shade) of light. Natural sunlight at noon is 4870 degrees K. Light produced by a HID xenon bulb is 4100 degrees K. Light from a standard halogen bulb is 3200 degrees K, and that from an ordinary incandescent bulb is 2800 degrees K. The lower the temperature rating, the more yellowish the light appears. Blue-white light is better for visual perception, but yellow light is actually somewhat better for reducing glare in fog, rain and snow (that's why fog lights are yellow). The xenon bulbs that are used in HID lighting systems also produce three times the light output of standard halogen headlamps (3000 lumens versus 1000 lumens), and require less energy (35 to 42 watts versus 55 watts). This is possible because HID lighting systems work like a vapor-filled street light or metal halide lamp. HID bulbs typically produce 71 lumens of light per watt compared to 18 lumens of light per watt for standard halogen bulbs.
How HID Headlamps OperateHID lighting systems use a special quartz bulb that contains no filament and is filled with xenon gas and a small amount of mercury and other metal salts. Inside the bulb are two electrodes separated by a small gap (about 4 mm or 3/16th inch). When high voltage current is applied to the electrodes, it excites the gases inside the bulb and forms an electrical arc between the electrodes. The hot ionized gas produces a "plasma discharge" that generates an extremely intense, bluish-white light. CAUTION: Once ignited, the pressure inside an HID bulb builds to over 30 atmospheres due to heat (up to 1500 degrees F inside the bulb!). This creates a potential explosion hazard so do not attempt to power a HID bulb outside of the headlamp assembly to "test" it. Also, the bulb must be in a horizontal position when it is on, otherwise it may overheat and fail. Like street lamps and fluorescent bulbs, HID headlamps require a high voltage ignition source to start. It typically takes up to 25,000 volts to start a xenon bulb, but only about 80 to 90 volts to keep it operating once the initial arc has formed. The normal 12 volts DC from the vehicle's electrical system is stepped up and controlled by an igniter module and inverter (ballast), which also converts the voltage to AC (alternating current) which is necessary to operate the HID headlamps. The ballast adjusts the voltage and current frequency to operating requirements. The AC ballast frequency is usually in the 250 to 450 Hz range. Power to the HID system is usually routed through a relay and fused at the power distribution center. When HID headlamps are first turned on, the light appears more bluish but quickly brightens as the bulbs warm up. On most applications, the HID headlamps are only used for low beams (conventional halogen high beams are used). But on some of the newest HID systems, the position of the shielding around the bulb changes position to provide both high and low beams. Because there is no brittle filament inside a xenon HID bulb to break or burn out, the headlamps typically last up to three times longer than halogen headlamps (3000 hours versus 1000 hours of continuous operation, which is equivalent to 5 to 10 years of normal driving).
What is color temperature?Many people believe that the higher the color temperature the brighter the lamp. This is totally wrong. The color temperature is purely a scale to measure the color of the light output. It is a reference purely for color and could equally be called White, Green or Blue. The reality is the higher up the scale the lamps are the less bright they become. 5200K lamps are approx 10% brighter (measured in Lumens, not degrees K) than the 7000K. If you want lighting performance the 6000K HID lamps are the best. In our opinion 7000K has the best and most attractive light output. Degrees K = ONLY COLOR "It should be noted that halogen technology is not comparable to the Xenon discharge technology, fitted as original equipment to more and more of the world's finest cars." - Philips
DefinitionsCandela (cd) Lumen (lm) Kelvin (K)
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