why top downs ?

[Snaz]

Originally Posted by fishytime

Its the way the light refracts through the glass.

I don't think it is refraction through the glass. Refraction just slightly shifts the angle of the light as it passes through the interface, there is no loss or wavelength change due to refraction.

What is more likely is some light never gets through the glass, compare to the glass shield covering a MH light blocking 80% UV light.