Color Measurement on Metalized Substrates
Mirror-like surfaces pose unique challenges for accurate color measurement.August 2011 By Felix Schmollgruber, X-Rite
Types of instruments
There are two primary types of spectrophotometers used in the printing and packaging industries today: traditional 0°/45° (or 45°/0°) spectrophotometers, and spherical (or diffuse/8°) spectrophotometers. While a third type of instrument, called a multi-angle spectrophotometer (MA) is also available, an MA is better suited to larger scale, industrial production applications where it would be typically used for measuring metallic inks or paint. For the purpose of this article, we will focus on traditional 0°/45° or 45°/0° instruments (45° instruments) and spherical (or diffuse/8°) instruments.
In measurement geometry nomenclature, the first number refers to the angle of illumination and the second number refers to the viewing angle. This means that in the case of 45°/0° geometry, the light source shines at a 45° angle from the sample’s surface, and the detector receives the reflected light at a 0° degree angle from the perpendicular surface of the object (Fig. 1).
In a spherical (or diffuse/8°) instrument, the object is illuminated from all directions and the detector receives the reflected light at an 8° angle from the surface of the measured object (Fig. 2). It is known as “sphere” geometry because these instruments are lined with a highly reflective white substance used to project and diffuse the light.
45° spectrophotometers are designed to optimally measure the appearance of samples, which means that they take into consideration color as well as gloss and texture. However, if a glossy specimen is viewed using a 45° spectrophotometer, it will yield values that indicate it is darker and more saturated in color than a matte sample, even if the color in the two samples is equally pigmented (Fig. 3).
Because 45° instruments “see” color in the same way as the human eye, they are generally preferred for applications such as measuring color on matte or textured surfaces.
Sphere (or diffuse/8°) instruments can provide reflectance measurements in two ways: specular-included and specular-excluded. A specular-included reading includes the gloss component (Fig. 4), whereas a specular-excluded reading excludes the gloss component (Fig. 5). As we have seen before, 45° spectrophotometers are only capable of specular-excluded readings.