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Originally Posted by 75 tinkerer
My understanding is that QH bulbs generate excessive UV which is usually filtered out by the glass housing.
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The 'housing' is the envelope containing the filament. They use a 'hard' gass to minimise UV emission.
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These days the housings are not glass and I doubt whether the 'plastic' (may be the wrong term for polycarbonate) filters it out. Hence the UV protection.
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The headlamp front cover (referred to as the 'lens') is made of polycarbonate. As noted earlier, this material is degraded by UV. However, it's UV from sunlight that does this. The UV emitted by QH bulbs is comparatively weak. The UV protectant in headlamp lenses degrades on the outside rather than the inside.
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Have a look at this link, particularly the the paragraph titled Spectrum.
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According to this, the UV emitted is relatively high wavelength 'near UV' which isn't particularly active. The greatest proportion of the emission spectrum is the Infra Red (heat).
TC