There is not a plastic seal over the lamp, it's the optical adhesive used to hold the lamp assembly and lens the lamp to diameter. The Siglite lamps are meprolight products, which use a white plastic sleeve, glass lens, and optical adhesive over the lens. Other models you might be familiar with, like the XS dots, are a steel sleeve with a glass lens, also installed with optical adhesive, then a painted white ring around the lamp, countersunk into the sight body. The meprolights are proven to be a brighter product, but often appear more dull because they have less of a contrast at the edges. The plastic sleeve takes on a dull glow, so it offers a larger illuminated circle than the Trijicon lamp design, but the edge isn't as sharply defined. None are painted white over the top of the lenses - but the mepro's will often look as though they are, since the tritium won't glow strongly enough during the day to be visible, and the bottom and sides of the sleeve are white. The trijicon's typically reveal themselves with a more grey center in the day time, and less uniform across the dot, since there is a clear demarcation between the painted white ring, the steel sleeve, and the gas lamp core.
