Color Measurement on Metalized Substrates
Fig. 1: 45°/0° spectrophotometers use directed geometry. (Double click illustrations to enlarge.)
Fig. 2: Spherical instruments use diffuse geometry.
Fig. 3: A 45° instrument may yield values that indicate that the shiny sample is darker and more saturated.
Fig. 4: Using the specular-included mode, a sphere spectrophotometer will yield similar measurement results, regardless of the reflectance of the surface.
Fig. 5: Using the specular-excluded mode, a sphere spectrophotometer will yield measurements that take into account the reflectance of a surface and produce results that are pretty similar to a 45° instrument, which is usually the condition closest to the visual assessment.
Fig. 6: With mirror-like surfaces, a 45° instrument will essentially be “blind” since almost none of the light is available to be measured by the sensor.
Printing on metalized or “mirror-like” substrates is becoming increasingly popular, as brand owners and package designers continuously strive to look for new ways to add vitality and interest to their products. Producing striking metallic images using translucent inks on foil brings a new set of color measurement challenges. This article looks at instruments that are best suited to meet the unique requirements of measuring color on metalized substrates.
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).