by teK-- » Mon Aug 17, 2009 4:50 pm
HIDs visually and cosmetically look OK when retrofitted to a halogen headlight, but in actuality they will fail performance tests. Daniel Stern Lighting has done many a writeup on the topic and also the US DOT has done tests in a lab which confirm these findings.
In most cases the HIDs will give vastly increased light in the immediate foreground (a negative trait as it causes glare to the driver and reduced ability to focus on objects in the distance), and causes glare to oncoming traffic. Despite the cutoff still being sharp for the vertical beam pattern, it is the heatspots and incorrect distribution of light below the cutoff point which deem HID retrofits unsuitable (unless you install a factory HID headlight and/or lens assembly from a more upspec model).
The reason is because in halogen globes the filament runs across the width of the tube, whereas in HID the arc runs along the length of the tube. Each of these two setups require different optics.
The most effective upgrade for H7 low beam bulbs is the Osram Hyper Rallye 65W globes which provide 2100lumens of correctly focused light, are extremely white in colour, and are a straight plug-in replacement with minor trimming of the globe base. (the stock H7 globes are around 1400lumens and HIDs are up to 3000.
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