What is whiteness?

Whiteness can be defined physically as the degree of reflection of light. Light sources, including both natural sunlight and artificial light sources, emit light. Most objects, including cardboard and paper, do not emit light. This means that the whiteness of a sheet of paper only becomes visible when the paper is illuminated by a light source. It is the paper’s reflectance that makes its whiteness visible to the eye. To determine whiteness, measuring instruments are used which illuminate the paper with an artificial light source and measure the reflected light. The results obtained express the degree of whiteness as a numerical value.

ISO-brightness

The primary colours of visible light lie within a wavelength range of approximately 400 nm to 700 nm. The wavelength range of 400 nm corresponds to blue, the range around 550 nm to green, and 700 nm to red. If an object reflects all wavelength ranges, we perceive it as white. If there is a high degree of reflectance in the blue region, the paper appears whiter. By measuring reflectance in the blue wavelength range, it is therefore possible to quantify different degrees of whiteness. Ingredients are often added during the paper-making process to promote an increase in blue reflectance. These may be nuanced dyes or optical brighteners that absorb the UV component below 400 nm and reflect it in the visible blue range. Various filters that take the UV component of the light into account can be used to measure ISO brightness. The definition of the measurement standard requires white calibration and the specification of the type of illumination.

CIE whiteness

When measuring CIE whiteness, the entire spectrum of visible light, including all colour nuances, is taken into account. For the measurement, the parameters standard illuminant D 65 (illumination of the sample) and an observer angle of 10° were specified. Standard illuminant D 65 is daylight-like light containing a UV component. In addition to taking optical brighteners into account, measurement in the CIE-LAB colour space also allows small colour differences within the white range to be represented. The measured values are plotted on the three orthogonal Lab coordinates. Here, the vertical coordinate L represents the value for brightness, a stands for the red-green axis and b for the yellow-blue axis. Paper with high brightness will also exhibit a high L value. If the a value is positive, the paper’s colour location falls within the reddish range. The same principle applies to the b axis: the more positive the value, the more yellowish the colour; the more negative the value, the more bluish the colour. The determination of CIE whiteness has led to its widespread adoption in the graphics industry, in conjunction with colour management. CIE whiteness measurement offers the possibility of standardising the assessment of whiteness. Furthermore, whiteness is still assessed subjectively and is therefore subject to significant variations in perception in some cases.