Measurement geometry (spectrophotometry)

The defined arrangement of illumination and viewing angles between light source, sample and detector, written as notations like 45/0 or d/8, which fixes what a spectrophotometer measures.

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Bidirectional reflectance distribution function: A function giving reflected radiance in every outgoing direction for every incoming direction. The mathematical object that any fixed geometry samples one slice of.
45/0 geometry: Illumination at 45 degrees from the normal, viewing straight on; the arrangement that mimics human viewing and excludes gloss. The most common directional geometry, contrasted with sphere-based diffuse designs.
Datacolor instruments: Commercial d/8 sphere spectrophotometers with SPIN/SPEX capability, widely used in textiles and coatings. A canonical d/8 sphere instrument line used across industrial color control.
Kubelka–Munk theory: A two-flux model relating reflectance of turbid layers to absorption and scattering coefficients. The classic theory that assumes a diffuse geometry and predicts what sphere readings should be.
CIE 15 colorimetry: The CIE technical report defining standard illuminating and viewing geometries for color measurement. The document that formalized geometry notation and requirements internationally.
Specular component included: A d/8 measuring mode that keeps the mirror-like gloss reflection in the reading. One of the two d/8 modes the SPIN/SPEX switch selects between.
0/45 geometry: Illumination straight down the normal, viewing at 45 degrees; optically the reciprocal of 45/0. Its readings differ from 45/0 on anisotropic samples, a classic reciprocity pitfall.
X-Rite instruments: A leading maker of both d/8 sphere and 45/0 portable spectrophotometers. Its product range illustrates how vendors split between the two main geometries.
CIE color matching functions: Standard observer sensitivity curves used to convert spectra into tristimulus values. Geometry determines which spectrum enters the calculation these functions perform.
ASTM E1164: The ASTM standard practice for obtaining spectrophotometric data on object color. The US standard that prescribes geometry requirements for industrial practice.