Despite all technical measuring
methods, checking with the human
eye is indispensable.

When measuring the visual paper characteristics, the result largely depends on the light source and the observer. The light source itself is standardised at a particular "light temperature" specified in K (Kelvin). Because natural daylight has a different spectrum depending on the time, place, weather and other influential factors, standard light types were defined for daylight on a representative basis, ensuring standardised, reproducible conditions. The visual characteristics of the paper surface can only be assessed under these conditions.

The eye's perception of colour changes with the angle of vision (object size). The normal observer first defined in 1931 by the CIE is based on a field of vision of 2°. In 1964 a normal observer was also defined with a wider field of vision of 10°.

The main visual paper characteristics are:

• Standard light type D65
• Standard light type D50
• Brightness and whiteness
• L*, a*, b* (CIE colour spectrum)
• Visual brighteners
• Opacity

Standard light type D65

The CIE standard light type D65 corresponds to a medium daylight (overcast sky) with a colour temperature of 65 00 Kelvin. This light has a high proportion of blue, which is essential for producing and assessing brilliant white, brightened papers. D65 standard light is used for reflective originals.

Most printers use standard light type D50 because standard light D50 is the binding basis according to ISO 366 4 for comparing originals and reproductions, checking samples, digital proofs, press proofs and production printing.

Standard light type D50

The CIE standard light type D50 simulates midday light with direct incident sunlight with a colour temperature of 5000 Kelvin. Standard light D50 is used to assess the printing results.

The visible light range is between 400 and 700 nm. Ultraviolet is less than 400 and infrared over 700 nm. Because of the lower proportion of blue and UV, standard light D50 is not sufficient for differentiating between paper surfaces with high CIE-Whiteness. Because of the smaller proportion of red, standard light D65 only permits a limited evaluation of the print result.

Different light types and angles of view thus lead to different colour measurement values.

Brightness and whiteness

Although the terms brightness and whiteness are sometimes used interchangeably, between them there are some differences in terms of how they describe paper characteristics.

Brightness (L* value) is the ability of the paper to reflect all the colours of the light evenly. It indicates the extent to which paper reflects visible light in all wave lengths. A really white paper will have a high degree of reflection and will not reflect the light of one wave length with greater strength than another. Most white paper types have an overall degree of reflection of 80 - 100 %. Brightness is a very important aspect for colour sensitivity and thus forms an important basis for colour management.

CIE-Whiteness (W10) corresponds to a visual appearance of white paper with or without white toner/ brightener under lighting conditions with a light source similar to daylight (D65 /10). Unlike ISO brightness, which is limited to the blue area of the visible spectrum, CIE-Whiteness is based on the reflections measured over the entire spectrum. Paper with a bluish tone will seem whiter to most observers. CIE-Whiteness corresponds to the perception of the human eye. It corresponds to an average value determined from examination involving a number of different test objects.

L*, a*, b* (CIE colour spectrum)

This reference colour system is based on the perception of opposite colours by the human eye, which indicates that colours cannot be green and red or yellow and blue at the same time. This enables the position of a colour on the red/green axis or yellow/blue axis to be described with a single value.

The measurements for the colour impression of the surface is specified with the help of three coordinate axes at right angles to one another (L*, a*, b*).

L* stands for brightness.
The brightness axis ranges from 0 = black to 100 = perfect white

a*: Positive a* values are red, negative values are green, 0 = grey

b*: Positive b* values are yellow, negative v alues are blue, 0 = grey

Visual brighteners

Optical brighteners are used as additives in paper production to produce bright white paper. To be able to evaluate the effectiveness of the brightener, the reflection is measured at 457 nm with and without a UV portion.

Optical brighteners (OBA or FWA) are used to transform invisible ultraviolet light into visible blue light. This increases the brightness L* and the chromaticity coordinate moves towards blue, i.e. the b* value becomes more negative.

Opacity

Opacity is the technical term for the light resistance of paper. Expressed in physical terms, opacity is the ability of the paper to absorb and reflect light. The greater the light resistance, the higher the opacity. Paper should be as resistant to light, i.e. as opaque, as possible so that text and images on one side do not show through on the other.

Some of the light that hits the surface of the paper passes through and some is absorbed, but most of it is diffused and reflected. The greater the reflectivity of the paper, the better the opacity. The light refraction depends on the number of fibre/ air and pigment/ air transitions in the paper.