Prolonged warmth could cause a thermal runaway where a slight darkening of the lens over time increases the absorption, causing the darkening process to accelerate quickly.

It could be that at IR wavelengths the transmission of the plastic is low and the absorption increases which would result in heating of the lens. But somehow sufficient energy is making it past this IR filter, through the dichroic color wheel and a light pipe, two glass lenses, and a first-surface mirror to cook the plastic lens. The module does contain an integral dichroic glass infrared (IR) filter to block most of the heat from reaching the more delicate optical elements downstream. The 230 watt lamp module cranks out a tremendous amount of light and with it a lot of heat. What causes this lens, a lens that is last of several elements, to go into meltdown is a bit of a mystery. Nevertheless a lump of black coal obscuring a quarter or half of the lens area is going to do some damage to the light uniformity across the screen. This one is part of the illumination path for the micro mirror chip, and as such it is tolerant of some defects. This lens is not part of the image path – the large projection lens performs that function. Light is collimated, guided horizontally under the main projection lens onto an angled mirror, and finally reflected upward through the offending plastic lens onto the face of the DMD (Digital Micromirror Device) chip. The cause of the dark region on the projected image is an oddly-shaped aspherical plastic lens that is the final element in an optical path that channels light from the reflector lamp on the left front of the unit. Almost Unbelievable Meltdown of Internal Lens

Black beamer