The unit of measure used to define eye correction or the refractive power of a lens. Usually, adjustments to an optical eyepiece accomodate for differences in individual eyesight. Most ITT systems provide a +2 to -6 diopter range.
Distortion There are two types of distortion found in night vision systems. One type is caused by the design of the optics, or image intensifier tube, and is classical optical distortion. The other type is associated with manufacturing flaws in the fiber optics used in the image intensifier tube.
- Classical Optical Distortion: Classical optical distortion occurs when the design of the optics or image intensifier tube causes straight lines at the edge of the field of view to curve inward or outward. This curving of straight lines at teh edge will cause a square grid pattern to start to look like a pincushion or barrel. This distortion is the same for all systems with the same model number. Good optical design normally makes this distortion so low that the typical user will not see the curving of the lines.
- Fiber Optics Manufacturing Distortions: Two types of fiber optics distortions are most significant to night vision devices: S-distortion and shear distortion
- S-Distortion: Results from the twisting operation in manufacturing fiber-optic inverters. Usually S-distortion is very small and is difficult to detect with the unaided eye.
- Shear Distortion: Can occur in any image tube that use fiber-optic bundles for the phospor screen. It appears as a cleavage or dislocation in a straight line viewed in the image area, as though the line were “sheared”.