ATN PS-7 Operator's Manual page 37

C. Black Spots.
these are cosmetic blemishes in the image intensifiers or dirt or
debris between the lenses. Black spots are acceptable as long as
they do not interfere with viewing the image. No action is required
if this condition is present unless the spots interfere with the
usability of the device.
D. Fixed-Pattern Noise.
this is usually a cosmetic blemish characterized by a faint hexago-
nal (honeycomb) pattern throughout the viewing area that most often
occurs at high light levels or when viewing very bright lights (Figure
3-4). this pattern can be seen in every image intensifier if the light
level is high enough. This condition is acceptable as long as the
pattern does not interfere with viewing the image and usability
of the device.
FIGURE 3-4. FIxED-PATTERN NOISE
3-5