How do water droplets on the camera lens affect the image? Do they affect the image as much as on a visible light range camera?

Water droplets are entirely opaque in the thermal IR. How water affects the image will depend on several factors. Lens focal length and focus position are two of the important considerations, after total percent of coverage. If the lens is focused at infinity, rain or water drops will be more out of focus than if the lens is focused up close. A longer focal length lens will also generally cause the droplets to be more out of focus. If the droplets are sufficiently out of focus (and they should be if the lens is focused at infinity), then water drops on the lens will reduce the image contrast in proportion to the ratio of obscured area to total lens front surface area.

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