When selecting a spectrophotometer to use for measuring and comparing color, it is particularly important to use the right geometry for the application. Sphere instruments are preferred for weathering studies and tinting strength for example; 45c:0 is preferred when harmonizing multi-component objects that may contain parts with different gloss levels; and multi-angle is preferred for materials that change appearance based on observation angle such as coatings with metallic or pearlescent pigments. One question that sometimes comes up regarding multi-angle spectrophotometers is whether the 45° measurement angle is the same geometry as a 45/0 or 45c:0 instrument. The answer is yes, and no.
The diagram below shows the geometry of a 45c:0 spectrophotometer:
The sample is illuminated circumferentially at 45°, and the reflected light is measured normal to the surface, referred to here as 0°.
This diagram shows the illumination and measurement angles of a multi-angle spectrophotometer:
The surface is illuminated at 45° off the normal, and the measurement angles are given in relation to the specular angle (opposite angle to the light source). Thus the 45° measurement angle in a multi-angle spectro corresponds to the 0° measurement angle of a 45/0 spectro. In both cases the sample is illuminated at 45 degrees off the normal, and the color is measured normal to the surface. So what’s the difference? Circumferential illumination.
45c:0 instruments illuminate the sample using a homogeneous 360° ring of light, with all light hitting the sample at 45° off the normal. This eliminates the influence of directionality, meaning the spectro can be oriented in any direction and the results of the measurement will be the same. Multi-angle spectrophotometers do not use circumferential illumination, instead illuminating the sample with a single beam of light from one direction. This is done by design; Multi-angle spectrophotometers are primarily used for measuring painted vehicles such as cars, boats and airplanes, all of which are most often viewed outdoors. Since typical viewing conditions involve light coming from a single direction (the sun) the light source in multi-angle spectros is designed to mimic these viewing conditions. Thus the 45° measurement angle in a multi-angle spectro will produce similar results to a 45c:0 spectro but may differ slightly due to the influence of directionality. Note also that the notation “45c:0” is relatively new, with the “c” meant to indicate circumferential illumination. Many spectros that are labeled as 45/0 or 45:0 also use circumferential illumination.