Measurement geometry (colorimetry)
The defined arrangement of illumination and viewing in a color-measuring instrument, such as diffuse/8° or 45/0, which changes the measured values.
Diffuse/8° geometry (d/8): Sample illuminated diffusely from an integrating sphere; viewed at 8° from the normal, with specular included or excluded. The most common sphere geometry, defined by whether the specular port is open or closed.
45/0 geometry: Illumination at 45° from the normal with detection perpendicular to the sample, mimicking human viewing. The directional alternative to sphere geometry; sensitive to surface texture.
CIE publication 15: The CIE technical report defining standard illuminants, observers, and recommended measurement geometries. The authoritative reference that names and standardizes the recognized geometries.
Orange peel effect (automotive coatings): Waviness in automotive paint finish that alters reflectance at different angles. Directional and sphere geometries give visibly different readings on textured coatings like this.
Reflectance factor: The ratio of light reflected by a sample to that reflected by an ideal diffuser under the same geometry. Geometry enters the definition itself: the reference diffuser is measured identically.
Specular component included (SPIN): A d/8 configuration with the specular reflection port closed, so mirror-like reflection stays in the measurement. One of the two d/8 operating modes; captures total reflectance regardless of surface gloss.
Sphere geometry: Any geometry using an integrating sphere to illuminate or collect light diffusely, such as d/8 or 8/d. The family that d/8 belongs to, contrasted with directional designs.
ASTM E1164: The ASTM standard describing instrumental requirements for measuring object color, including geometry specifications. The industry practice document manufacturers cite when specifying instrument geometry.
Gonioapparent color: Color that changes with viewing angle, caused by metallic or interference pigments. Fixed geometries are inadequate for these samples, driving specialized angle-resolved measurement.
Perfect reflecting diffuser: The ideal surface reflecting all light diffusely with no loss, used as the 100% reference. Calibration depends on the geometry because the ideal diffuser is itself geometry-defined.