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Inorganic chemicals
A broad class of substances encompassing all those that do not include carbon and its derivatives as their principal elements, yet not excluding carbides, carbonates, cyanides, cyanates, and carbon disulfide.
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Lens blank
Eyewear; Optometry
Piece of material, usually preformed, for the making of a lens at any stage before completion of the surfacing processes.
Horizontal centre line
Eyewear; Optometry
Line located at equal distance from the two horizontal tangents of the boxing system.
Infrared radiation
Eyewear; Optometry
Optical radiation with a wavelength longer than that of visible radiation.
Focal length
Eyewear; Optometry
Distance in an optical system from the focal point to the corresponding principal point, measured along the optical axis (axis of symmetry).
Fixed-location stimulus perimeter
Eyewear; Optometry
Perimeter which utilises test stimuli that are at permanent locations on the background.
Fixation target
Eyewear; Optometry
Target used to locate the point where the patient should look during testing.
