Even at the lowest brightness level, stray light from the thermal imaging camera display illuminates clothing, equipment and ultimately the bright face out of the eyepiece in the dark.
In addition, the used eye loses its ability to see in the dark. In theory, the latter would be less of a problem because I only use my left eye for my handheld. The visual center in the brain then practically lacks the left image for a high-contrast 3D image. This also applies to the use of thermal imaging attachments.