Color Measurement on Metalized Substrates

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).

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.

In practice, this means that sphere instruments are potentially more versatile, since they allow for the measurement of a color with or without the impact of its substrate’s associated surface effects.

Measuring ink on foil
Printing any inks on foil presents some unique challenges for spectrophotometers, as metallic substrates are highly reflective. When measuring any mirror-like surface, a 45° instrument will essentially be “blind” to the substrates, since nearly 100 percent of the light will be reflected in the opposite direction, leaving almost none of the light available to be measured by the sensor (Fig. 6). The instrument would “see” less light, interpret this as black and would likely yield values that may indicate that the sample is darker and more saturated than it really is.

On the other hand, a sphere (diffuse/8°) instrument can provide accurate and repeatable measurements of these substrates thanks to its diffuse illumination. The mirroring effect of a metallic substrate is reflected by the coated sphere of the instrument, which will yield an adequate amount of light for the sensor to measure. Because the sphere instrument is able to measure the reflected light, it will return measurements that are very close to what the eye would see, since printed foils and metals are typically observed in lighter surroundings to take advantage of its reflective nature.

One such sphere spectrophotometer offered by X-Rite is the SP64. It offers three different aperture sizes to provide the flexibility needed for printing applications, can save 1,024 reference colors and up to 2,000 samples, and can link to quality control software for data reporting.

While it is feasible to measure inks on foil or metal using a 45° spectrophotometer, users need to understand the technology limitations, since the measurement values could indicate that the sample is darker and more saturated than it really is. If you understand this and can work around these issues, it is possible to use a 45° spectrophotometer for this purpose with success. However, most users working with metalized substrates prefer to work with sphere instruments given their ability to provide specular-included readings. pP

About the author: Felix Schmollgruber is X-Rite’s product manager for new platforms.