ATN PS15 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
hexagonal (honeycomb) pattern throughout the viewing area
that most often occurs at high light levels or when viewing very
bright lights (Figure 5-4). 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 5-4. FIXED-PATTERN NOISE
5-5