Hot Spots
Aside from lens flare hot spots in normal photography are virtually non-existent but in infrared light they are quite common.
A hot spot can be described as any area in an image (most frequent in the center) that appears to be lighter or brighter than the rest of the image. Sometimes a color shift also occurs within the hotspot. Fortunately majority of lenses don’t have a hot spot issue.
There are a few reasons that hot spots can occur, the most common culprit is the mate black coating on the inside of the lens barrel. The coating is designed to absorb stray light from bouncing around within the lens and creating hot spots. Unfortunately this coating may behave the opposite in IR light, instead of absorbing light it may reflect it and end up causing a hot spot.
The other less common reasons for a IR hot spot would be the coatings on the glass lens elements themselves behaving in a way other than what was intended when used in visible light. Sometimes it can also be the interaction of the lens and sensor/micro lenses that causes the light to bounce in a certain way between the surfaces that cause a hot spot.
