The degree of whiteness is a numerical measure for the reflectivity of a surface.
White is that color stimulus of a body color which , with the greatest possible brightness, triggers the same color valence in all three types of cones . The human visual system has an automatic mechanism for " color constancy ", which means that less bright fields of vision are perceived as white . However, the difference between unequally bright areas can definitely be determined if both areas are directly next to each other. In order to create an objective measure for this, the degree of whiteness was defined. By definition, a higher numerical value stands for a “whiter” white.
Since the degree of whiteness depends on various factors such as the amount of light or the color of the lighting, the measurement conditions and the calculation methods must be precisely described. There is no such thing as the “degree of whiteness per se”, but only the “degree of whiteness according to ...”. A method widely used in the paper industry is the Berger whiteness. This approach becomes understandable when one realizes that the variety of colors in the visual space is reduced to a measure. The degree of whiteness loses value as soon as the color cast becomes too great. The whiteness of a red can no longer make sense.
Using various formulas, the degree of whiteness is obtained from device data or by color measurement, controversially noted as degree of yellow.
- Whiteness according to Berger
- Hunter whiteness
- Whiteness to Ganz, Whiteness to Ganz / Griesser
- Whiteness according to Stensby
- Whiteness according to Stephanson
- Whiteness according to CIE
- Yellowness index according to DIN 6167
- Whiteness according to ASTM E313, according to ASTM D1925
- Whiteness according to ISO 2470, here referred to as brightness
- Whiteness according to Tappi 525 and R 457
- Degree of whiteness according to Taube
- Yellowness according to CIE
- Yellowness according to ASTM (yellowness)
- The L * value of the Lab color space is also suitable as a measure of whiteness.
The ISO whiteness should preferably be used, which is used as an indication of quality, especially for paper. A good, white paper that has not been lightened (copy paper) has a degree of whiteness of around 160 according to Berger.
A conversion between the degrees of whiteness without knowledge of the spectrophotometric values is not always possible. The reason for this is that a color cast in some formulas goes unnoticed. The metamerism, however, prevents the numerical value whiteness from being clearly assigned backwards to the spectral arrangement. It is not possible to “calculate out” the weighting of a color nuance when converting degrees of whiteness for a specific formula.
The degree of whiteness serves as a quality characteristic for assessing uncolored ( white ) products in the paper industry . The degree of whiteness is particularly important for coated papers. It is a common quality measure for white textiles. The use of this measure is of lesser degree in the plastics industry, here more in the selection of fillers or binders.
Objectives of the application
- The degree of whiteness is a standard test in paper production.
- Sufficient whiteness for papers that are used as printing material to achieve sufficient contrast.
- Recycled paper with a gray tone, for example due to ineffective deinking , is more difficult to read when printed than bright white paper after chlorine bleaching .
- A high degree of whiteness is particularly necessary for conventional photo papers so that the contrast between the photo silver and the background can be improved.
- Coatings for art paper must be bright white, since the paper white is the eighth coloring component alongside six chromatic colors and black.
- The degree of whiteness is used to measure the success of bleaching processes .
- It is a means of characterizing the effectiveness of detergents on textiles.
- The degree of whiteness is used to indicate the quality of white interior paints, exterior paints, plasters and other coloring agents.
- The whiteness is used to control the quality of fillers for the paper industry.
The degree of whiteness is usually determined under standard light , which differs from natural daylight due to the lower proportion of short-wave UV radiation.
However, optically brightened textiles or paper absorb UV light due to their fluorescence and emit visible light. Visually, this has the effect of (possibly) improving the whiteness, ultimately the degree of whiteness, but this additional light would not be measurable with UV-free light. The degree of whiteness only gives the achromatic portion of a measured area in relation to an ideally white ( density 0) or ideally black area (density 2). For optically brightened substrates, two pieces of information are required, which are to be measured with and without UV excitation. This corresponds to the “perceived white” in sunny daylight and in UV-free artificial light.
An increase in the degree of whiteness when washing textiles can also be achieved by blueing .
A shade of red or yellow is perceived as more annoying with white than a "metrologically" identical blue cast , to a lesser extent also as a green cast. Therefore, when it comes to quality indicators, not only the degree of whiteness and the method must be agreed, but also a color deviation. This permissible deviation can be determined as color temperature , verbally or colorimetrically.
- Konica Minolta Sensing: The Color of White