Selecting a Spectrophotometer for measuring and comparing colors is especially important to use the correct geometry for the application. For example, spherical instruments are optional for weatherability studies and tint strength. Optional 45c:0 when coordinating multi-component objects that may contain parts with different gloss levels. Multi-angle is optional for materials that change appearance depending on viewing angle, such as coatings with metallic or pearlescent pigments. A problem that sometimes arises with multi-angle Spectrophotometer s is that a 45° measurement angle is the same geometry as a 45/0 or 45c:0 instrument. The answer is yes, no.
The figure below shows the geometry of the 45c:0 Spectrophotometer :
The sample is illuminated in a 45° circle and the reflected light is measured normal to the surface, here referred to as 0°.
The figure below shows the illumination angle and measurement angle of a multi-angle Spectrophotometer :
The illumination angle of the surface is 45° from the normal, and the measured angles are given relative to the specular angle (the angle opposite to the light source). Thus, a measurement angle of 45° in a multi-angle spectrum corresponds to a measurement angle of 0° in a 45/0 spectrum. In both cases, the samples were illuminated at an angle of 45 degrees from normal, and the color was measured normal to the surface. So what's the difference? Peripheral lighting.
The 45c:0 Spectrophotometer illuminates the sample with a uniform 360° ring of light, all at an angle of 45° from normal. This removes the effect of directionality, meaning the spectrum can be oriented in any direction and the measurement will be the same. A multi-angle Spectrophotometer does not use peripheral illumination, but instead uses a single beam from one direction to illuminate the sample. This is by design; multi-angle Spectrophotometer s are primarily used to measure painted vehicles such as cars, boats, and airplanes, all of which are often viewed outdoors. Since typical viewing conditions involve light from a single direction (the sun), the light source in a multi-angle spectrometer is designed to simulate these viewing conditions. Thus, a 45° measurement angle in a multi-angle spectrum will yield similar results to a 45c:0 spectrum, but may be slightly different due to directionality effects. Note also that the notation "45c:0" is relatively new, and the "c" signifies circumferential lighting. Many spectrometers labeled 45/0 or 45:0 also use peripheral illumination.
