An instrument that measures a surface's reflectance at many wavelengths, from which its colour under any light can be calculated.
It illuminates the sample with a known source in a defined geometry (commonly 45°/0° or diffuse/8°) and measures reflected light in narrow bands, typically every 10 nm across the visible range.
A colorimeter reports colour under its own fixed conditions; a spectrophotometer records the spectrum, so it can also predict metamerism.
Paint shops scan a sample with a spectrophotometer to formulate a match.
It is the reference instrument for colour matching and quality control in almost every industry that sells colour.
The instrument is specified by its geometry, its wavelength range and interval, its aperture and its measurement condition. Geometry is either directional, lighting at 45° and viewing at 0°, or diffuse, using an integrating sphere with the gloss reflection included or excluded. Graphic-arts instruments add the ISO 13655 conditions M0 to M3, which fix how much ultraviolet the lamp contains. Performance is stated as repeatability, the scatter when one instrument measures one sample repeatedly, and inter-instrument agreement, which is normally the larger figure.
Two sites measure the same production part and argue over half a ΔE unit.
Different instrument models can disagree by more than that, and a sphere instrument and a 45°/0° instrument are not comparable at all on a glossy or textured part. Agreeing one model, one geometry and one physical standard settles more disputes than tightening the tolerance.
Paper or textiles containing optical brighteners are measured on an older instrument.
A lamp with little ultraviolet does not excite the brightener, so the sample measures yellower than it looks in daylight. The M1 condition exists to make the instrument's light match D50 including its ultraviolet.