The geometry of an instrument is the relative position of the light source, sample plane and detector, and is one of the 6 key parameters that define a color measurement. There are two general categories of instrument geometries – directional 45°/0°(or 45°/0°) and diffuse d/8° sphere.
To tell the difference between directional and diffuse instrument geometries, look in the port. If the inside is black, when the lights are on, the instrument has a CIE directional 45°/0°(or 45°/0°) geometry. If the inside is white, then it is a CIE diffuse d/8° sphere geometry instrument.
While both geometries can be used for color measurement, it is best to measure some samples with a directional 45°/0° geometry instrument and others with diffuse d/8° sphere. More information in our Application Notes at AN 1033.00 Color versus Appearance.