Whiteness and Yellowness

Whiteness (WI) is a measure of a visual rating of whiteness for white and near-white surfaces. Some common WI are:

• ASTM
• CIE
• Ganz-Griesser
• Hunter

WI is used in textile, plastics and paint industries for near white samples.

Color has three parameters but manufacturers of white colored products find it easier to use a single index scale when qualifying their products. There are many whiteness indices and it is important to note that for a WI to be valid, the correct WI must be used and the materials must be in similar gloss, texture, opacity and other physical properties.

Yellowness Index (YI) is an indication of the degree of deviation from white to yellow. The yellowness of ideal white is almost zero. The more it deviates from the ideal white, the larger the yellowness value.

YI is most commonly used to evaluate color changes in a material caused by wear and tear, degradation through prolonged use or exposure to environment. YI is widely used as an indication of deterioration.

Why do newspapers turn yellow over time?
Why is my white shirt glowing?

Whiter Than White - Fluorescence Effect

All color including white can be defined with a minimum of three numbers (learn more about the science of color - from Perception to Instrumentation, click here). What distinguishes white from other color is it’s high lightness and low saturation. It is more attractive with a bluish than with a yellowish cast.

Whiteness is caused by a combination of high lightness and lack of yellowness. Yellowness is usually undesirable in whites as it is associated with product deterioration. Whiteness with blueness is perceived as clean and therefore seems brighter.

High brightness is achieved with the aid of fluorescent whitening agents (FWA) or optical brightener agent (OBA). These are additives designed to enhance the appearance of brightness in fabrics, detergent and papers.
It may also enhance or modify the appearance of the white. FWAs increase the whiteness of a substrate by absorbing ultra violet light and re-emitting blue fluorescence. This overcomes the natural slight yellowness of the substrate to give an intense whiteness. FWAs are commonly used in washing detergents, paper and fabric. White paper and fabrics with FWA appear whiter than normal under UV illumination. Most commercial white fabrics are treated with FWAs. Similarly, washing detergents are formulated with FWAs to maintain the whiteness.

The paper samples below are illuminated by tungsten, TL84, D65 and UV lightings. Tungsten lighting has little or no UV energy and the FWA is not excited to make it flourescence. TL84 and D65 has some UV energy and the samples looked 'whiter'. Under UV lightings, the FWA are excited and produce a whiter than white look.

The spectrum graph shows an incremental amount of UV energy gives higher reflectance.

Color Matching Light Cabinets

Color matching light cabinet is a light booth designed to provide a standard set of lighting conditions for visual color inspection. The three most common types of light sources used in the world are incandescent, fluorescent and daylight. In theory, if the product color matches under these light sources in the cabinet, it will match in the real world.

Image: GTI Light Cabinet

Typically:

1. the daylight light source replicates the natural lighting - outdoor lighting
2. the incandescent for the tungsten light bulb and
3. fluorescent for the CFL light bulbs used for lighting up the office, mall and home.

Another light source commonly found in light cabinets is ultraviolet (UV) light. UV is used to observe the presence of fluorescence materials. The next time, when you are in a disco, observe how white fabrics glowed under UV lighting.

Light cabinets are used by manufacturers to ensure that their products matches in these lighting conditions and minimise color differences and metamerism.

The color difference between the zipper and the master color is larger under daylight than under tungsten light. Which means that it would look odd outdoor or in a place lit by fluorescent lighting.

Observe that the zipper has optical brightener agent (OBA) or fluorescence whitening agent (FWA) which causes the zipper to 'glow' under UV lighting whereas the master color has no OBA or FWA and under UV light (which has no color component), the sample looks 'black'.

This is not an optical illusion. It can be scientifically explained.

Welcome to the world of colors.