HIDs installed (pics)

Now, what about those "retrofit" jobs in which the beam cutoff still appears sharp? Don't be fooled; it's an error to judge a beam pattern solely by its cutoff. In many lamps, especially the projector types, the cutoff will remain the same regardless of what light source is behind it. Halogen bulb, HID capsule, cigarette lighter, firefly, hold it up to the sun—whatever. That's because of the way a projector lamp works. The cutoff is simply the projected image of a piece of metal running side-to-side behind the lens. Where the optics come in is in distributing the light under the cutoff. And, as with all other automotive lamps (and, in fact, all optical instruments), the optics are calculated based not just on where the light source is within the lamp (focal length) but also the specific photometric characteristics of the light source...which parts of it are brighter, which parts of it are darker, where the boundaries of the light source are, whether the boundaries are sharp or fuzzy, the shape of the light source, and so forth.

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