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I`ve always fitted the OE bulbs. I saw an article in a magazine suggesting LED upgrade and have now seen articles suggesting these are likely to fail MOT as you have to fit Halogens. There seem to be conflicting views on this. I also see "uprated" Halogens are available but suggestions these may not last very long. Can anyone "shine a light" on all this (to coin a phrase !)
 

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Oversimplified answer:

Primarily headlight quality is a function of the reflector and the lens. Different and/or brighter bulbs make very little impact. Often times people confuse light temperature with lux - whiter, bluer lights trick the eye into thinking it can see more than it really can as these lights are perceived as "brighter."

"LED upgrades" are typically just a bulb that slots in where a halogen bulb once did. The light source is differently shaped, so the final light output is necessarily different. LEDs generally have inferior throw to halogen equivalents, but they do typically offer whiter light. The net effect is a lot of bright, scattered light near the front of the vehicle and no improvement (or detriment) at a distance. Since the light is very pure, people mistake this for "better headlights" but it's actually worse for you, and worse for oncoming traffic. Empirical tests show this, and that's generally why LED replacement bulbs fail inspections.

Modern headlights are fundamentally different than old-style reflector lights - different set of rules. So the things you can do with a modern projector headlight don't apply to a legacy reflector light. Probably magazines that endorse LED upgrades are assuming a car made in the last 10-20 years with projector lights.

Nothing here is hard and fast, but generally true.
 

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Headlight effectiveness depends on the bright bit (bulb filament) being in the right place to work with the reflector and lens. A few millimetres off and the results can be bad. (If you ever have not quite seated the bulb properly, you know what I mean--suddenly the headlight/foglight isn't working as it should.)

LED bulbs can't really replicate the filament placement that the headlight was designed for. The C900 uses H4 bulbs. which are a dual filament with a shield over the low beam (to get the cutoff). I doubt that any LED "replacements" could mimic in any way the optical properties of the H4 bulb.

The first step is to make sure that your lens and the reflector in the headlamps are good. If they're good, make sure the wiring to the bulb isn't damaged. If possible, check the voltage at the connector (unfortunately that has to be done with the bulb installed). Finally, you can get "+30%" or "+50%" or greater bulbs. They do tend to burn out more quickly--check the hour life ratings if they're on the package. And they won't do anything if the lens, reflector, or wiring are not up to snuff.

Here's a UK source:

I would stay away from any bulbs that are "styling", unless you believe it's better to look good than to see good.
 

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I did put some of the nightbreakers in my c900 5 dr and they did make a bit of difference, but put some in the wife's vert, and they had gone with in a few days as she had to follow another car home as no headlights(low beam), just thought I had a bad set, so replaced them and they went again. Potential reason they are thinner filaments to give the brighter light so are more fragile,. as someone else commented if you live down a country lane(bumpy) don't bother as they just break with the vibration, and of course FPT Vert's are usually stiffer, so went back to the originals all OK,
 
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