HID Headlights

HID lamps are gas discharge lighting devices, much like sodium or mercury vapor lamps used for street lighting. Ordinary incandescent lamps use a heated filament which glows and gives off light. Gas discharge lamps use a high voltage (generated by the ballast) to make gas glow. Gas atoms are excited by the high voltage, causing their electrons to jump energy states. When the electrons jump back, they give up the extra energy as photons, which shine forth as light.

So much for theory, now on to the practical questions:

1) Discharge lamps require the gas to heat up and become excited to work. Each time you turn them on they have to go through this warm up, which takes a toll on their life. That's why you don't want to flip them on and off. Flashing is okay, but only because of how the dimming function works (see the next answer).

2) High beam/low beam. There is only one bulb per fixture, which runs at a constant brightness. Low beam is accomplished by a mechanical cutout which drops in place and blocks the upper part of the beam. Think of it as being like the baffles used in stage lighting. When you go to low beam, the cutout flips into place and blocks part of the light. When you switch back to high beam, the cutout flips out of the way. This is why flashing is okay; you are not cycling the discharge lamps, only moving a little mechanical piece back and forth.

3) The advantages, other than superior lighting, is reduced wattage and longer bulb life. Gas discharge lamps are very tough, since they don't have a fragile heated filament. However, superior lighting is the main advantage.

Tom

 

My 2 cents worth:

The HIDs are always at the same voltage, same wattage, same output.

When u "flash" them (or just turn on high beams) you are just flipping a reflector, changing what light "escapes' out front. Yes, real technical, but cool! Have someone do this while you're looking (not directly!) at the HIDs. Preferably look from above and slightly in front of the headlight assembly.

Advantages are the reduced power consumption, and I'll say that means they run cooler. MPG-wise, I, too, doubt they make a difference.

I've been very impressed with their forward lighting. It's not a chore anymore to drive at night.

Curt.

 

<div class='quotetop'>QUOTE(unruhly @ May 3 2006, 05:52 AM) [snapback]249020[/snapback]</div>

Your other questions were covered pretty well, but this one needs a bit more.

When you "strike" an HID, the ignitor sees 23,000+ volts for a moment to create the Xenon gas. The brutal thing to do to HIDs is to strike them, turn them off and restrike immediatey before the gas has had a chance to cool. Striking them and leaving them on is groovy - and they should last way longer than traditional bulbs. Yes, they use less power when on, no you won't notice this in mileage unless you count inches per tank.

 

And with most theatre / rock lighting fixtures [the ones that move
around and do all kinds of colors and patterns under computer
control], you *can't* restrike the HID lamps without waiting for up
to ten minutes for them to cool down. Newer ones claim "hot
restrike" but they'll wind up having the same lifetime issues as
car lamps if abused.
.
_H*

 

<div class='quotetop'>QUOTE(finman @ May 3 2006, 06:29 AM) [snapback]249034[/snapback]</div>

Same here. When I had gotten the car, I hadn't driven it at night until I had to go to work early in the morning. When they kicked in, wow.

 

<div class='quotetop'>QUOTE(qbee42 @ May 3 2006, 09:26 AM) [snapback]249033[/snapback]</div>

Wow. What a great (and well detailed) explanation. It's amazing to me how, even with all the knowledge I've acquired in my life, there are some areas wherein I know absolutely squat!

 

<div class='quotetop'>QUOTE(finman @ May 3 2006, 09:29 AM) [snapback]249034[/snapback]</div>

I like to use the momentary flash option on the headlight stalk in the daytime as a signal to other drivers that I am yielding right of way. However, this means firing the light from off to on. Will this affect lamp life?

 

You would be surprised to the durability of current generations HID systems. On my Honda VFR800, I have replace the standard halogen H4 bulbs with HID. In addition, I have added a set of PIAA driving light to be the high beams. With this combination, I am only drawing 20W more energy while projecting more light than can be imagined with halogen.

The comment about the restrike of HID comes into play with street lights. I bounce the high beams for oncoming cars (this bike at night is a rolling sun) and have yet to have a problem cycling them back on quickly. The restrike is instantaneous with the full intensity being produced without the gradual build up from first firing. The Prius does have the distinct advantage of using the movable reflector to change the beam pattern.

Once you drive with HID's, halogens pale in comparison.

 

as a theatre tech, the only thing i have to add is to dispel the myth that HID's are cooler (as in, actual amount of heat coming off the glass) than halogens. this is not the case. they are plenty hot.

the main difference in terms of heat between normal incandescent lamps, quartz halogen lamps and HID lamps is that with HID's, the production of heat is not a byproduct (and thus waste)... you need to heat the gas inside the lamp for the whole thing to work.

with quartz halogens, the heat is also necessary to produce an effect called the halogen cycle, where little particles of the filament which flake off are vaporized and deposited back on the filament. doesn't work unless it's hot.

conventional incadescents are the only bulbs where the heat is wasted energy, plain and simple.

 

if the HIDs are on, then flashing to high beams is just that mechanical reflector moving. If they are off and u turn the stalk switch, then you are igniting them and un-igniting them quickly. This on/off action is what HIDs don't like. To what degree, I'm not sure...

 

<div class='quotetop'>QUOTE(efusco @ May 6 2006, 05:20 PM) [snapback]250839[/snapback]</div>

HIDs work by exciting gas atoms which then directly give off photos as the electrons drop back from their excited state.

Florescents work very much like HIDs, but instead of giving off visible photos, the excited atoms in a florescent give off UV radiation. The UV strikes a phosphor coating on the inside of the glass tube, exciting the phosphor. The phosphor in turn gives off visible light, which is why they are called florescent lights. The color of the phosphor determines the color of the light: cool white, warm white, black light, etc.

Neon lights contain low pressure Neon or Argon. They work very much like HIDs, but run at a cooler temperature with lower intensity. Tubes filled with Neon glow orange, while Argon produces a blue color. All other Neon light colors are produced by lining the tube with phosphor, which is excited by the photons from the gas and re-emit in some other color, depending on the type of phosphor. In this case they work much like a florescent lamp, but don't require the hot cathode. These are sometimes called cold-cathode florescent lamps.

LEDs work by driving a current through a semiconductor band-gap (made by the junction of P and N doped semiconductor material). Due to quantum effects, the electrons in the semiconductor can only exist at certain energy levels. The quantum levels are different on the two sides of the band gap, so when electrons are pushed across the gap they have to change energy levels. The released energy is given off as photons. Depending on the drop in energy levels, the photons vary in color (but always the same color for any given drop). Lower energy levels produce infrared, then red, orange, yellow, and so on up to blue and even ultraviolet. Most white LEDs in flashlights are actually blue or UV LEDs with some white phosphor. The photons from the LED excite the phosphor, which then emits white light. White can also be produced by mixing red, green, and blue LEDs inside the same package. This is used in color signs and displays, but is too expensive for flashlights.

Tom

 

<div class='quotetop'>QUOTE(efusco @ May 6 2006, 08:31 PM) [snapback]250903[/snapback]</div>

Duh! Yes, I meant photons. Funny how spell check doesn't save you when you use the wrong word. :blink:

Tom