Mainly because the headlight reflector (and lens to a degree) are designed to work with a filament type bulb and not an LED, or HID for that matter. The nature of LED bulb design means that they are a bulky affair and there has to be a considerable supporting structure for the diodes which is opaque and interrupts the dispersal of light. Also, in the example shown in your photo, the three diodes are spread over a relatively wide area; the light emitting zone is not in one place so the light source is impossible to position at the focal point of the reflector (or the point at which the reflector was designed for). A traditional halogen bulb glows most brightly at the exact centre of the filament in a tiny but dense area which can sit at the focal point and crucially, the light emitted from that spot travels in every direction (360 degrees in every plane) and can make full use of the whole reflecting surface, unlike an LED bulb which can only emit light in one direction and at best to about 160 degrees.

A lot of baloney is talked about Lumens, but it is not a very good measure of the effectiveness of a light source. Lumens is an over simplistic measure of total light produced, not the amount that is focussed and 'useful'; it looks good on paper, which is why it's often quoted in the sales pitch. Lux is a far better measure at it is 1 Lumen per square metre, in other words focussed, useful light. An LED bulb may produce 3000 Lumens on the test bench and a halogen bulb just 1600, but with the halogen bulb nearly all the 1600 Lumens are useful. Generally speaking an LED bulb in a non-standard installation will be a third of the efficiency (two thirds of the Lumens are wasted). Your LED bulb will 'look' brighter (mainly because of the higher colour temperature tricking your eyes), but who looks at their headlight? It will more than likely put two thirds less 'focussed' light on the road, plus high colour temperature (bluer) light is far more prone to scatter than light towards the red end of the spectrum so in fog, rain etc. it will be even less efficient.

Aftermarket HIDs are slightly better, but the arc which emits the light is most intense at each end (think of it as 'dog bone' shaped). The weakest part of the arc is at the focal point meaning less focussed light and loads of glare, scatter and wasted light caused by the bright ends of the arc sitting outside the focal point. This is mainly why aftermarket HIDs 'appear' brighter (colour temperature helps this illusion) and why they are an annoyance to other road users.

HIDs work most efficiently in purpose designed refractor, or projector headlight units. LEDs only work anything like properly in purpose designed complete units such as you see on top end Audis, BMWs etc. and you should take a look at the complexity/expense of the design including the intelligent cooling system and massive heat-sinks required to reduce junction temperature.

Fit them, see how you get on with them... but don't expect miracles!

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