Colour vs. Appearance

With the number of colour measuring instruments on the market, how do you choose the most appropriate one? It helps to understand the differences in colour and appearance.

When white light strikes a surface, two different light interactions occur. Gloss or shininess is a geometric attribute related to the scattering of light off the first surface of the material. Colour is a chromatic attribute related to the absorption of light by the colorant in the material. When we describe the “colour” of a material, we speak in chromatic terms such as lightness, saturation and hue. When we speak of “colour and appearance,” additional geometric terms related to gloss or shininess are added to our description.

To accurately communicate about a material, you need to understand the difference between colour and appearance and their relationship. Let's take a closer look at these attributes and methods for measuring them.

What Is Colour?

To perceive colour, light, an object and an observer must be present. Although all objects appear to have a colour when observed, in reality, colour is not an essential attribute that objects possess. Instead, the chemical composition of an object causes it to reflect or transmit light at specific wavelengths. The cones in the human eye receive these wavelengths, and the brain interprets them as colour.

The visible light spectrum ranges from violet light with wavelengths around 400 nanometers to red light with wavelengths around 700 nanometers. When light strikes an opaque object, the light interaction is characterized at the surface as a specular reflection for gloss and shininess and diffuse reflection for colour. People perceive colour when diffuse reflection occurs at the object's surface, causing light to scatter — or diffuse — in many different directions.

In contrast, people struggle to view colour when specular reflection occurs. Specular reflection is light that is directed at an angle equal but opposite to the incident light and is perceived by a human observer as glare caused by the shininess or glossiness of the sample. To see the apparent colour of the sample, observers must move their eyes away from the glare specular reflection and concentrate on examining the diffuse (scattered) reflectance from the sample.

These viewing conditions have led to standardized industrial methods for visual evaluation such as ASTM D1729 “Standard Practice for Evaluation of Colour Differences of Opaque Materials.”

What Is Appearance?

Colour is only one aspect of an object's appearance. In fact, the way you perceive an object's colour may change depending on other aspects of appearance, including characteristics such as texture, gloss and transparency.

A smooth or high-gloss surface facilitates specular reflection. This is because the lack of surface imperfections allows light to bounce back at the viewer at an angle equal to the light's incoming angle, producing a reflection of the source illumination. A more textured surface causes light to scatter in many different directions, revealing the object's colour instead of reproducing the source's image.

When the smoothness of a surface changes resulting in higher micro-roughness and lower gloss, the light-reflection characteristics from that surface become more complex. For a medium gloss sample, the specular reflectance is spread by the micro-roughness and becomes mixed with the diffuse reflected light. A matte surface having higher micro-roughness scatters almost all the white specular light, which mixes with the normally scattered diffuse reflectance, causing the perceived colour to appear lighter and less saturated. This means that objects with a glossy appearance look darker than objects with a rough surface, even if they contain the same pigmentation.

How Does Appearance Affect an Object's Colour?

Colour appearance is affected by the roughness of the surface independent of the colour. To the average observer, a high-gloss specimen would appear to be more chromatic or colorful and darker than an identically pigmented specimen with lower gloss or increased surface microroughness. The close relationship between colour and appearance makes it difficult to measure colour accurately and communicate about it in isolation — to understand a material's colour, you also need to recognize its texture and appearance.

Because specular and diffuse reflection scatter light in different directions, the viewing angle also impacts an object's perceived colour. By shifting your viewing angle, you can avoid the glare created by specular reflection and see the colour differently. This means that viewing conditions must match for two identical samples to appear identical in colour.

Measuring Colour and Appearance Accurately

To measure colour accurately, you need to consider the sample's total appearance. Spectrophotometers make it possible to compare colour between samples by changing viewing angles and accommodating differences in surface texture.

The best spectrophotometer for your application will depend on the material you want to evaluate and what you hope to accomplish. If you want to ensure the colour of a product meets specifications, directional geometry works well. On the other hand, if you want to see the effects of surface differences on a product, diffuse geometry may provide the best option.

At HunterLab, we offer a wide range of solutions for colour and appearance measurement. To learn more about the relationship between colour and appearance in spectrophotometry, contact us.

Colour vs. Appearance