aperture disk

aperture disk

[′ap·ə‚chər ‚disk]
(engineering)
A disk with a small round opening used in a densitometer to vary the amount of light or the area to be measured.
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References in periodicals archive ?
Given the very small hole in the aperture disk, slightly off the perpendicular means you can't see through the aperture.
Aftermarket accessories, such as the Gehmann adjustable aperture and various optical aids, will fit the same hole as the removable aperture disk.
The sight features a rotating aperture disk with two sizes of peep: a large hole for CQB and low light work and a small hole for more precise work.