maxf

Halogen is the type of bulb technology - changes the gas & glass so that "stuff" that boils off the filament re-condenses back onto it most of the time - hence can have hotter filament (brighter/whiter) and no "gone dim from silver inside" problems.

Easyish to tell if bulb is halogen:
- will usually be cylindrical (gas is under pressure)
- will be lots of warnings to not touch glass with fingers - (fingerprints can etch/distort the glass at high operating temps & cause explosions - if touched, use meths/alocohol to clean)

Electrically, they're the same - you don't need to do anything as long as it's the same base and fits the same plug. Physically, you might have to watch the plug and housing as halogens do run hotter, but running a DOT/ECE halogen should be OK as they'll be approved for filament layouts to "drop-in" replace a similar format bulb... that means the lens/reflector combo won't cause glare etc.

"Xenon" is a PITA - usually a misnomer. Halogen bulbs can use some Xenon as the halogen gas; but now it's mis-used as a reference to HID (high intensity discharge) lights. Simple to check - "xenon" bulbs with filaments are still halogen bulbs - HID lights have no filament, they're an arc light with 2 electrodes for the arc to jump across in a special pea-sized bulb (usually put inide another glass cylinder as it needs UV and explosion protection!)

But wait - there's more

Halogen bulbs have also started changing - you can now get the "blue cut", "4300k" etc variants. They're normal halogen treated with some colour filter to make the light even less "red" and more "blue" - the idea being that most eyes as they age they see the blue end of the spectrum better, so giving a bluer beam gives a better perception of brightness & beam distance/penetration down the road.

The most extreme of these variant halogens are trying to emulate the "pure white" of a HID light - but if they overdo it, the blue filter actually starts absorbing light across the whole spectrum, diminishing the high-end as well as the low end (and getting hotter to boot). Have a look at the degree of blue tint on the Philips BlueVision bulbs in the bubblepacks, the tint isn't much!

(PS - the "k" is the "colour temperature" in Kelvins... the lower, the redder, higher the bluer (and eventually going into purple/violet/UV)... see these kinds of charts, eg http://www.manyadeal.com/page.html?id=24 - although most over-exaggerate the "yellowness" of halogens.)

SO

- Xenon bulbs usually aren't a given guarantee of better performance - you're probably better going for a name-brand and getting reliability

- HID look-alike bulbs are probably not worth the premium

- Name-brand "blue" bulbs might help you, depending on your age

Other tricks for improving brightness:

- Clean the headlight, and everything in front of it (guards, etc). Common sense!

- Check the reflector is clean, and the inside of the headlight lens... dunno how you do it (have tried distilled water skiped with meths and sloshing it around then throughly drying, but it's your headlight so go careful. You might want to skip this step if you're not sure how to get into the headlight and/or you can't get int to fix anything you upset!)

- Even if you don't clean it yourself, check that the gasket around the bulb is OK (to keep the headlight insides clean)

- Have a think about using relays to power the bulb. Voltage drop at the bulb (due to wiring and switches) will dim the light - quite surprisingly so - see http://dsl.torque.net/images/Relays.pdf: 12.825v at the blub = 83% efficiency, 12.15v = 67%! Using relays and beefed-up wiring (direct to the battery and a good earth, and don't forget the fuse!) means less voltage drop - you still use your switches, the original wiring gets to control the relays, not directly power the bulb.

The HID question - worth it or not?

HID lights are the holy grail of vehicle headlights currently... approx 300% the light for 35w, yada yada yada. However, they are definitely not the best for bikes, IMHO (and after having tried one)...

- The potential for dangerous glare to oncoming traffic is enormous. You need to have a projector ("fish-eye") headlight to get the precision beam - the old external ribbed lens & reflector will spray light everywhere... remember, it's an arc, not a fixed filament, so the focal points can change (assuming the no-name manufacturer of your drop-in bulb got it right to start with!) THIS IS THE MAIN REASON THEY ARE ILLEGAL TO RETROFIT TO ANY HEADLIGHT NOT DESIGNED FOR THEM.


- Even worse, most bike lights are way higher off the road vs the average car - again, glare. Being physically smaller also means more intensity, more risk of blinding oncoming traffic.

- The arc is "struck" by an electronic "ballast" unit (jumps your 12v up to about 20000v to start the arc, then to about 50 to run it). Most ballasts are built for car use, and don't assume any interruption to power - and even if there was, you could still drive by the other light... and so they have overload protection (so as not to blow up the bulb!) and won't restart the arc for 10-20seconds. On a bike with only one headlight this is brown-pants time.

- They can damage the inside of your headlight - possible in a couple of ways:
(a) they can give off a lot of UV, aging and brittling plastics (including their own mounts!) If your headlight reflector isn't metal - just metal sprayed on plastic - then this might be worth considering
(b) they can explode - rare but possible. One I had lost chunks of mounting cement into the headlight, PITA to get back out!
(c) they can "leak". One I had, the bulb failed, but didn't explode - it did however leak it's halogen gas. Stinky acrid sulphur smell, AND a foggy stain on the inside of the lens that needed dismantling & meths to get off.

Anyway, you can try them if you want - for you they might be 150% great and toast bread too.