Instruments with either a 45:0 or the inverse 0:45 geometry are called directional (light comes from one direction), with three further sub-categories.
When there are one or two lights (or single light split by two mirrors) at the directional illuminating or viewing angles, this is called uniplanar. Opaque or translucent samples measured with uniplanar directional geometry should have uniform surface characteristics for a repeatable measurement with a uniplanar 45:0 or 0:45 geometry.
A more robust directional geometry design is to use multiple lamps, illuminating fiber optics or mirrors in a circumferential ring of discrete points or a continuous annular ring to illuminate the sample plane at 45 degrees with 0 degree viewing by the detector. These circumferential or annular configurations more evenly illuminate or view the sample plane than uniplanar designs, minimizing the measurement variation associated with sample directionality and surface roughness.