Photomask blank
An unpatterned glass or fused-silica plate prepared with the surface, chrome film and optional photoresist needed for a later exposure or laser-writing step. It becomes a finished photomask only after the approved artwork is transferred and inspected.
Chrome-on-glass photomask
A patterned chromium layer on a glass substrate. Clear and opaque regions define the exposure artwork; the glass remains the mechanical and optical support.
Fused silica
A high-purity silica substrate selected when ultraviolet transmission, low thermal expansion or dimensional stability is important. The exact grade and spectral range should be stated in the drawing.
Critical dimension (CD)
The defined size of a feature, such as line width, opening width or gap. CD is different from the coordinate of the feature and from the accuracy of an assembled instrument.
Pitch
The distance between corresponding points of repeated features. A local pitch result does not by itself establish accumulated position error over a whole disc or scale.
Registration
The positional relationship between layers, marks or a pattern and its mechanical datum. It should be defined against named references and a measurement method.
Reticle / graticule
A patterned optical reference placed in an eyepiece or viewing path. A reticle can carry a crosshair, scale, grid, circle or other comparison pattern; its divisions require calibration for the complete optical setup.
Resolution target
A known pattern used to test whether an imaging system can reproduce detail. USAF 1951, NBS 1963A, Ronchi, Siemens star and slanted-edge targets answer different measurement questions.
Optical density and contrast
Optical density describes attenuation under a defined measurement condition. The contrast delivered by a complete mask, readhead or camera also depends on wavelength, illumination, focus and the rest of the optical system.
Drawing revision
The identifier that ties geometry, materials, inspection criteria and the delivered part together. A file name or preview image is not enough to control a production change.