ALL THAT GLITTERS IS NOT GOLD

Modern finishing techniques are becoming increasingly common for package printing on a wide range of products to convey the appearance of high-product quality. In addition to varnishing and embossing, metallic effects are commonly used. However, inconsistencies in communicating metallic effects pose challenges in printing and matching samples. Completed in 2024, the Fogra research project investigated the characteristics responsible for the perceived metallic effect of gold. Correlating physical measurements with visual assessments helped to establish quantitative parameters for evaluating the perceived metallic character of gold prints.

The research included a wide range of golden metallic prints with varying metallic intensities. Samples featured were metallic offset inks (using effect pigments from bronze alloys [Cu-Zn] or aluminium [Al] in yellow-coloured solvents), transparent inks on metallised papers and hot-stamping foils (semi-matt, silk-gloss, highly glossy imitation foils and real gold foils). 

To broaden the range, some offset prints were finished with high-gloss varnish. Substrates included various folding cartons and wet-glue labels, with smooth and matte surfaces selected to achieve diverse effects. All samples could be categorised with the colour ‘rich gold’.

Metals exhibit unique optical properties due to the interaction of light with free electrons in the metal. This causes rapid attenuation within the material and re-emission as directional, specular light. It leads to high reflectance at specular angles – both steep and grazing. Metals, such as gold, also show coloured reflections due to selective absorption in the visible spectrum – enhancing their distinctive appearance. 

Inconsistencies in communicating metallic effects pose challenges in printing and matching samples

Gloss – closely linked to specular reflectance – is a key factor in metallic appearance. Parameters, such as specular gloss, contrast gloss, haze or distinctness of reflected images, capture qualities including surface brilliance and smoothness. To measure these parameters, the detection needs to be extended beyond the specular angle to include the surroundings of the glossy highlights.

When analysing the appearance of metallic surfaces, human visual perception should also be considered. The eye detects light stimuli based on colour and brightness. This raw information subsequently undergoes complex neural processing, making visual perception distinct from purely physical measurements. 

Quantifying the metallic effect requires combining the physical data, such as reflection and gloss parameters, with psychological data from visual assessment experiments. Correlating these datasets, through psychophysical scaling, bridges the gap between measured properties and human perception. As a result, a more comprehensive understanding – essential for quantising the metallic effect – can be achieved.

Several measurement devices were employed to describe the metallic character of print samples. Amongst them, a sphere spectrophotometer, gloss meter and gonio-spectrophotometer.

With a sphere spectrophotometer, the reflection of a sample can be determined using the specular included (SPIN) and specular excluded (SPEX) configurations. It enables the calculation of the Specular Percentage Index (SPI), which indicates the specular portion of the reflection (see equation).

It showed that glossy hot-stamping foils exhibited the highest SPI values, followed by silver papers and semi-matte foils. Offset samples had the lowest SPI values, with unvarnished versions slightly higher than varnished ones (see Figure 2).

The gloss meter was used to measure typical gloss parameters, such as Gloss20, Gloss60 and Rspec, directly at the specular angle. Further parameters were used, such as Haze, which quantifies the milky halo effect around specular highlights. Alongside the typical commercial measuring devices, a gonio-spectrophotometer was used to provide high-resolution data for individual detection/illumination angle combinations. It enabled the calculation of Flop parameters. These parameters quantify brightness changes at different viewing angles. Both gloss and Flop values followed a trend similar to the SPI values (see Figure 2).

Ten test persons conducted the assessments. For each group, they identified two reference pairs – one with the greatest difference in metallic character and another with an intermediate difference. In each group, the samples were compared pairwise and the difference in their metallic character was assigned on scale of 0–6. The largest metallic difference being given a rating of six. The ratings from individual groups were consequently scaled and adjusted to create an overall rating averaged across all test persons. The results for individual samples (see Figure 3) are as follows. Glossy, hot-stamping foils (most metallic, rating ~5.6) were rated highest, while offset samples with aluminium pigments (least metallic, rating ~1.25) were rated lowest. Furthermore, more consistent results were observed under parallel lighting conditions.

A correlation between physical parameters and psychological ratings was established to identify key factors influencing the perceived metallic character. The study aimed to determine the most relevant physical parameters and develop mathematical correlations, enabling visual ratings to be predicted from measurement data. Initial AI-based investigations revealed strong correlations between individual parameters and visual ratings, allowing fitting functions to include just one parameter each. Linear, exponential and logarithmic approaches were used, and correlation coefficients (R2, ranging from 0–1, with 1 representing a perfect fit) evaluated the fits’ quality. Parameters correlating strongly with perceived metallicity included brightness descriptors – such as Spec – changes in brightness close to the specular angle (e.g.,FlopY,max) and gloss properties (for example, SPI or Haze). 

For enhanced relevance in the field, it was determined how suitable the parameters of the various measuring devices were for quantifying the visually perceived metallic character. Here, only the most meaningful parameters were considered, indicated by a very good correlation with R2>_ 0.90(see Figure 4). The sphere spectrophotometer’s SPI achieved R2 = 0.93–0.97 across the different fit functions. The gloss meter and gonio-spectrophotometer each provided approximately 40% of the meaningful parameters, with Haze (R2 = 0.97) and FlopY,max (R2 = 0.96) being top-performing parameters. Consequently, these parameters – together with the corresponding fitting functions – provide good mathematical predictions for the perceived visual rating yvisof the different samples (see below).

The gloss meter and gonio-spectrophotometer each provided approximately 40% of the meaningful parameters

In summary, the primary outcomes of this research show that the perceived metallic character is best quantified by measuring brightness at the specular angle or changes nearby. Gloss parameters also prove useful. In addition, relevant physical parameters can be measured using devices such as a sphere spectrophotometer (best parameter – SPI), gloss meter (best parameter – Haze) and gonio-spectrophotometer (best parameter –FlopY,max). Finally, more consistent results are achieved when metallic samples are viewed under parallel light rather than diffuse lighting.