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After searching around on the web I came across a very knowlegeable bulb/lamp expert: Don Klipstein.

The questions along with detailed answers are below.

> 1. In an LCD or DLP video projector, will the metal halide bulb shatter

> under any situations, like e.g., using the projector after the bulb time

> (say 1000 hours) has expired?

> I read in a NEC LT150 manual that it might.

This is largely true. The quartz bulb material can crystallize

("devitrify") and thereby weaken. This is largely a combined function of

age and temperature. Since an aged bulb has higher electrode voltage drop

(more heat) and more discoloration/darkening (more heat from absorbed

light), an aged bulb runs hotter than a new one, so bad effects of high

temperature accelerate with age of the bulb (in terms of operating hours).

Do not operate the bulb beyond recommended age in terms of operating

hours unless you are capable and willing to tolerate explosion of the

bulb. Beware that your liability has a sharp increase by changing from

a recommended operation mode (replacing the bulb when the bulb

manufacturer or the equipment manufacturer says to do so) to a

non-recommended operation mode. Sometimes this may be a way to make the

bulb manufacturer demand you to feed them money, but defy that only at

your own risk. Reasons for replacing the bulbs when they have trod their

specified mileage are usually true.

> 2. Is there any way to extend the life of such a bulb, e.g., by putting

> a resistor in series with the current supply to the bulb, or perhaps a

> different ballast (you can tell, I know little about these type bulbs)?

Mostly NO! In fact, HID bulbs can be damaged by underpowering almost as

badly and easily as by overpowering. In an HID bulb, things work best

when the electrodes are at some ideal temperature.

Excessive electrode temperature is obviously enough bad.

Insufficient electrode temperature is also bad - the normal mechanism of

getting electrons from metal to gas in an HID lamp is "thermionic emission"

and an arc utilizing this process is referred to as a "thermionic arc".

In this case, the electrode material or some coating thereon has a loose

grip on some of its electrons, making it largely conducting to the

gas/vapor/vacuum/whatever. (Not a vacuum in an arc discharge lamp.)

Thermionic arc mode requires the electrodes to be at some temperature

sufficient for making the electrode material atoms or coating atoms

loosen their frip on their electrons.

If the electrode temperature is too low, then the electrode has too

tight a grip on the electrons in its atoms to effectively conduct to the

gas/vapor/whatever. If conduction is forced under these conditions, it is

by positive ions in the gas/vapor being accelerated through the "cathode

fall" (voltage drop in the region around the negative electrode) so as to

hit the electrode hard enough to dislodge electrons.

Such an "underpowered" discharge may be:

1. Cold cathode arc (arc discharge, with high current density in the

cathode process region). Cold cathode arc is most damaging when the

electrode temperature is barely short of sufficient for thermionic arc

operation. Cold cathode arc is most tolerated in lamps designed to handle

this process and with electrode temperature minimized as opposed to

elevated but short of thermionic operation. Cold cathode arc operation in

modern practical lamps is mostly in xenon flashtubes and the cathode is

normally plain tungsten and of size sufficient to remain cold enough to

adequately maximize the bonding between tungsten atoms when ion

bombardment is dislodging electrons from the tungsten atoms.

2. "Normal Glow" - a discharge with a different cathode region structure

of low current density and where a "natural" current density exists and

is not exceeded, with a double layer of glow around the cathode resulting

from this process.

Normal glow is reasonably nondamaging to lamps designed to operate

in this mode (life expectancy can be well into the 10,000's to possibly

over 100,000 hours). Elevated electrode temperature makes it easier

for bombarding ions to dislodge whole atoms of electrode material in

addition to dislodging electrons from atoms of the elecrode material.

The "normal" current density in the cathode process is often merely

milliamps per square centimeter.

3. Abnormal glow - usually resulting from current exceeding that of

"normal glow" current for utilizing the entire utilizable electrode

surface, although also possible by forcing the cathode glow process

into a thickness less than that of normal glow for the materials/gas/

vapor involved. The cathode fall (voltage drop) is greater than that

of normal glow, and so is dislodging of atoms of electrode material.

The really bad case is abnormal glow resulting in electrode

temperature just short of achieving arc discharge operation - this

often totally ruins a lamp in just a few hours and may do so possibly

in just several minutes!

A bit more detail in: http://www.misty.com/~don/dschtech.html

> The LT150 bulb costs almost $500, and only lasts 1000 hours, so you can

> see my motivation for longer life.

If you can tolerate the lamp exploding, run it until it croaks or

changes to some unusual color (more orange/pink usually). If you see a

major color shift that seems to be from age, then replace the lamp - it is

probably overheating seriously due to either a primary effect or a

secondary effect of a malfunctioning worn-out electrode. Beware that

it may be possible for the bulb to dangerously crystallize before such

symptoms occur - this is why you must be able to tolerate a bulb explosion

if you operate the bulb beyong expected life.

Note that the bulb probably contains a few to several milligrams to

possibly a few 10's of milligrams of mercury.

Hope this helps!

- Don Klipstein


Jim Story

Live long and Prosper
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