Metallic grating pattern on glass squares with opaque lines approximately 50% of the pitch. Stock items are on 50 mm x 50 mm squares of glass 1.5 mm thick. The glass is B270 with refractive index of 1.52. Mutually perpendicular pairs produce a screen of square apertures. Vacuum deposited chrome gratings on glass substrate. Typically used for testing of resolution, field distortion and parfocal stability, but may be paired to provide interference patterns. 50mm x 50mm B270 glass. 100 lines per mm ruled over 30mm