The objective evaluation of metallic and effect colours
von Björn Kammertöns,
In addition to the visual evaluation of print results, the use of appropriate measuring technology is by now widely established. Densitometers are used that reflect the layer thickness of the process inks and indicate the ratio of absorbed to remitted light. Despite the occasional impression to the contrary, this method is actually only suitable for use with process colours (CMYK) due to the physical filters deployed.
Over the past 15 years, mobile spectrophotometers have been increasingly used in the printing industry because they allow quantitative and thus objective colour evaluation. In addition, the use of portable devices enables quality control directly at the press.
A brief excursion into measurement technology
Spectrophotometers were developed in the USA at the beginning of the 1940s and the first devices made their way into Europe around 1950. However, due to the extraordinarily high costs, only very few devices were used – and almost exclusively for research purposes. The Eppendorf company from Hamburg then developed a much more cost-effective variant of this device, which quickly became the standard in laboratory medicine.
In the early 1980s, the dead centre of development was reached when a completely new photometer principle emerged from Japan. The idea originally came from a local agricultural university, where a research project was to investigate at which wavelengths of light certain plants thrive best. Instead of hundreds of monochromatic lamps, only one xenon high-pressure lamp was used and split into the different wavelength ranges by an oversized grating monochromator. The plants were then positioned in this light fan to find out in which wavelength range the plants grew best. This system was developed by the Hitachi company. In the course of the following decades, this technology and the corresponding devices were further developed for various other areas of application. As far as the graphic arts industry is concerned, the 45°/0° geometry became established. In extremely simplified terms, this means that a light source illuminates the substrate to be measured at an angle of 45° and the remitted light hits the sensor at an angle of 0° to the test sample.
The problem with metallic and special-effect inks
In addition to process and special inks, metallic and special-effect inks are also used in package printing. This is particularly the case with luxury products, for example in the cosmetics sector. Such products require a correspondingly elaborate and conspicuously designed packaging. Either monochromatic or polychromatic inks are printed (and sometimes overprinted with a transparent ink to achieve the desired colour impression), or a film printed with transparent inks is laminated against a metallised substrate. However, the metallic or special effect pigments used, such as pearlescent effects, have the property of a visually different colour impression at different viewing angles. Put simply, this is due to reflections on the metallic pigments and light refraction effects in the material mixture (pigments, binding agent, solvent and additives).
However, these effects can neither be measured nor described with the 45°/0° measuring geometry. In order to achieve a more accurate result, measuring devices with spherical geometry were used. Without describing the exact construction of these devices, it should be mentioned, that due to the spherical geometry (principle of the Ulbricht integrating sphere), the light reflected in all directions is collected by the sensor and is included in the determination of the colour value. But even with this, metallic and effect colours still cannot be fully described. To achieve this, multi-angle spectrophotometers must be used. The sample is measured at different viewing angles in order to capture the effect of the visually different colour impression. This technique is by no means new, but it has never really found its way into the printing industry. It has been used primarily in sectors such as the automotive industry.
This was mainly due to the user-friendliness of the measurement data. If the measurement with a 45°/0° spectrophotometer resulted in only one trio of L*a*b* values, this increased to 3, 5, 6, 10 or 12 value trios with a multi-angle device, depending on its version. From this it is only possible to deduce at which angles there are deviations from the original artwork. But this does hardly provide any information what must be changed in the print or in the ink to achieve the original artwork. However, the development did not stop at this point, but more and more work is being done to combine the accuracy of the multi-angle devices with user-friendly and relevant measurement values. In October 2022, the Paris-based company ColorGrail launched a device that precisely meets these requirements and includes many additional features to simplify colour communication between brand owners, agencies, pre-press companies and print shops.
The Spectrophone 2 Pro
The predecessor model of the Spectrophone 2 Pro, the Spectrophone 1 in particular enables the export of colour data in many different formats in order to simplify colour communication. In addition, the reports integrated in the software are easy to create and distribute. Including all these functions, the new Spectrophone 2 Pro is the first ultra-mobile measuring device that offers quantitatively measuring metallic and special-effect colours by means of multi-angle measurements and transferring these information in different formats to other systems, such as ink formulation systems.
The device measures metallic colours in three different angles of 20°/0°, 45°/0° and 75°/0°. For this purpose, a total of three LEDs are arranged on the back of the device. Spectraphone 2 Pro displays and compares the respective L*a*b* values of the individual angles. In addition to these values, other important properties are measured that allow the metallic or effect colours to be controlled. The gloss is measured and displayed as usual in gloss units (GU) and in addition, also the haze effect is determined.
This phenomenon is given when a surface shows a “milky” or “foggy” cloudiness or a dull, milky halo of light, which is also called a glossy haze. Although this effect is more common in coating applications such as in the automotive industry, it can also be
detected by Spectrophone 2 Pro. Haze usually changes according to the change of smoothness of the corresponding metallic or effect ink changes. In order to quantify the degree of change in the colour impression at different angles, the so-called flop index is determined. It describes the difference between two viewing or measuring angles. If this flop index is 0, the measured colour shows no differences even at different viewing angles. However, some metallic or effect colours can also have flop indices of 15 and more.
Besides the ability to measure metallic and effect colours, there is also the possibility to measure translucent colours and materials. For this purpose, the device is positioned on a small measuring table and the sample is fixed in. Measurements are taken on a defined white and black ceramic tile, which also allows the opacity of the respective sample to be determined. For example, this facilitates to measure coloured films and to record and check the deviations. The typical greying of recycled substrates can also be monitored. With a small, manual device, a small chip can be moulded from the delivered granulate or flakes and then tested for its colour. This can be done before the actual film production and thus provides process reliability.
ColorGrail also offers MiniChart, a new test chart feature for process control provided with 25 fields currently, which will soon be expanded to 42 fields. Solid and halftone areas can be measured with just one measurement and therefore takes very little time. In addition, it is possible to compare these measured values against an ICC profile. Either one of the pre-installed generic profiles, such as ISO Coated 39L, is selected or a self-created ICC profile is imported.
With just a few clicks, a complete report can be created that shows at a glance whether the selected colour space has been reached in printing. The first page of this PDF report provides a quick overview – shown with red and green fields – of which colour patches in the chart are within (green) or outside (red) the tolerance. On the following pages you will find the detailed values – also for the substrate or the printed white. In addition, the tone value increases and the correspondingly required compensations are calculated in the report. Before creating this report, one can select between the Murray Davis or SCTV method to calculate the respective tonal values.
ColorGrail’s Spectrophone 2 Pro combines the accuracy of today’s measurement technology with a high degree of colour communication options. Due to the MiniChart functionality and the possibility to create reports, it is also very well suited for process control from the agency to the print shop. Furthermore, it must be pointed out that Spectrophone 2 Pro represents a paradigm shift with the option to measure metallic colours with very high accuracy and repeatability.
About the author
Projects, Support and Consulting
Björn Kammertöns is a proven expert in package printing and colour management – particularly in gravure printing. He advises and supports brand owners, agencies, pre-press companies and print shops in realising packaging projects and optimising processes. He is also the ideal person to contact if you would like to delve deeper into the topic of “colour management”.
Find more information about Björn Kammertöns and his services at: www.bpacked.de