Most of images have 8 bit per channel. That means colour values are in 0-255 range. 16 bit images have this in 0-65536 range. Problem is that lot of cameras produce images with only 0-4096 range or smaller range. So brightest pixels in image have value 4096. Most of image viewer doesn't stretch displayed image so you end up with 4096/65536=6% brightness which is almost black.

You can try load these 16bit images in GIMP and use levels tools to stretch it. Actually GIMP can load FITS images directly.

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