- Paint it. Silver with a couple of coats of clear gloss. With care and practice, this actually works quite well. Getting a sharp, clean demarcation line around the “glass” and the housing is the key to a convincing result.
Larger lenses can be painted with blues and bright white “catch lights,” which can be attractive (if somewhat stylized).
- Drill it out and add an aftermarket lens. MV lenses are the standard but hard to find now. They already have the silver parabolic reflector surface on the rear.
There are clear styrene “generic” headlight lenses available for model railroad applications (treat as per 3, below). These usually come in sets with several different diameters. Sometimes you have to convert the model RR size, often given in “inches of prototype diameter” for the scale - usually HO, they’re sold for in order to know what you’re actually buying.
Several PE companies sell lenses and lights that are made from steel covered with colored or clear resins. Most of these are “kit specific,” but there are a few generic sets, and if you collect up enough leftover bits, you can sometimes get lucky and find a size-match in your PE stash. SKIP is one such PE lens maker. I think Eduard also has a few.
- Drill it out and add a scratch-built lens (punched disk from clear styrene coated with a drop of gloss clear). Paint the inside of the housing silver. An alternative is to cover the back side of the disk with foil vice painting the inside of the housing.
(This is my go-to if the lens diameter is a close match to one of my punch and die sets.)
An alternative is to make the lens out of a piece of clear sprue. Heat one end to make it melt and round over. Saw this domed bit off the rest of the sprue. This will look best if the backside is smoothed with fine grit sandpaper and the whole thing give and coat of clear gloss. Finishing the backside / inside the housing as above. The difficult is fining a piece of sprue that’s at least as large in diameter as the desired finished lens. (Smaller diameters can be simply be cut as slices of the face of the larger dome.)
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Drill it out. Paint the inside silver and fill with a clear material (epoxy glue, acrylic gloss gel medium, clear gloss acrylic). The downside here is dealing with shrinkage. The epoxy will shrink the least, but it’s hard to mix without any small bubbles, and you need to shop around to find an actual CLEAR epoxy.
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Most newer releases come with the headlight lenses as separate clear parts. Treat these as per 3), above.