optical axis—A prolongation of a line joining the nodal points of a lens. The line is normal to both surfaces of a simple lens or to all the surfaces of a perfectly centered lens system.

optical center—An intersection of lines representing rays whose emergent directions are parallel to their respective incident directions. The point of intersection lies on the optical axis. An oblique ray, even if it passes through this point, undergoes a longitudinal displacement which increases with the thickness of the lens.

optical flat—A piece of optical glass (usually a disk with parallel surfaces), whose surfaces have been ground and polished to within a fraction of a wavelength of light. Such a flat is used for testing the planeness of prism fairs. mirrors, etc. Also called “optical plane.”

optical parallax—See parallax, instrumental.

optical plane—See optical flat.

optical plummet—See collimator, auto; collimator, vertical.

optical square—A small hand instrument used in setting off a right angle. Some optical squares use two plane mirrors placed at an angle of 45° to each other. In use, one object is sighted directly and another is so placed that its twice-reflected image appears directly in line with the first object. If the lines to the point of observation from the two observed objects meet at a right angle. In other optical squares, a single plane mirror is so placed that it makes an angle of 45° with a sighting line; one object is sighted direct, and the other is so placed that its reflected image is seen so in the sighting line. Optical upturn are also found in pentagonal prisms, both single and double. Using the double pentagonal prism one can place oneself on line between two visible points and project a 90° angle ahead from the line.

optical tooling—A means of utilizing powerful telescopic sights to obtain precise reference lines and reference planes from which accurate measurements are made with optical micrometers, optical tooling tapes, optical tooling scales, and micrometer measuring rods. Applicable in manufacturing processes where larger measurements to small tolerances are impossible to attain with ordinary tooling procedures. Sometimes referred to as “optical alignment.”

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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