Spectrophotometers illuminate the measurement subject, receive the light reflected by the subject, separate the light into wavelength components, and calculate numerical values to express the color. The direction at which the subject is illuminated and the direction at which the receiver receives the light are termed the "geometric conditions" of the instrument. The CIE (Commission Internationale de l'Eclairage) defines several sets of geometric conditions, such as those shown in Figure 1 below.
When light from a single direction illuminates objects such as a mirror or metal objects, almost all the light is reflected specularly (as if from a mirror), but for most objects such as solids, when the surface is illuminated from a single direction, the light is reflected as shown in Figure 2, with the surface (gloss surface) reflecting some light specularly like a mirror and reflecting the rest of the light diffusely in all directions.
Humans determine the color of an object by seeing this diffuse light. When specularly reflected light is viewed, the color of the light source is reflected almost unchanged, and it's difficult to see the color of the object.
Metallic paints contain lustrous particles like flakes or wafers made of mica or other material that act as tiny mirrors, so that when light illuminates a metallic coating from a single direction, in addition to specularly reflected light and diffusely reflected light, there is also light reflected from the surfaces of the lustrous particles (Fig.4). Metallic and pearlescent paints use the variations in reflected light from the lustrous particles to provide an effect in which the paint appearance varies according to the angle from which it is viewed.
When measuring coatings having this effect with a color-measuring instrument, if the light received in only one direction is measured, it is not possible to evaluate the effect of the lustrous particles that cause the paint appearance to vary according to the angle from which the paint is viewed. Because of this, for measuring colors which contain lustrous particles, a multiangle color- measuring instrument that measures the light from several angles (Fig.5) is necessary.
L*C*h is a color system that expresses color in 3 dimensions, with the vertical scale being lightness L*, the distance from the center being chromaticity C*, and the rotation angle in the horizontal plane being the hue angle h.
Since pigment color differences are most noticeable as hue differences, evaluation of color differences between a target and sample can be evaluated using ΔH (hue difference) in the L*C*h color system.
In the highlight direction (25°), the mirror-surface reflection component from the lustrous particles is too strong, and in the shade direction (75°), the diffuse reflection component is weak, making these two directions unsuitable for pigment color evaluation. 45° is a geometric condition which is also used in general color evaluation, and is ideal for judging the color of pigments.
A variety of geometric conditions for multiangle spectrophotometers to measure metallic paints have been proposed by various industrial or national standards. Konica Minolta initially developed instruments with two different 3-angle illumination/1-angle receiving geometric conditions (Fig.5)--the 25°/45°/72.5 illumination, 45° receiving geometry defined by DIN, and the 15°/45°/110° illumination, 45° receiving geometry advocated by Dupont (E. I. du Pont de Nemours & Co.). However, when actual automobile bodies were measured, the results of measurements were unstable with variations. In order to improve stability, Konica Minolta developed the 3-angle circumferential illumination, 1-angle viewing geometry (Fig. 7).
3-angle circumferential illumination with highlight/45°/shade is able to measure pigment color at 45° measurement, and is able to evaluate the level of variation of lustrous particles by highlight/shade measurement ratio, so that it is suitable for judging acceptance in quality control and inspection processes.
One particularly remarkable cause of variations in measurement values when measuring actual automobile bodies is tilting of the instrument when measuring curved surfaces (Fig.8). Circumferential illumination reduces measurement variations due to instrument tilt, providing more stable measurement values.