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Whiteness is illustrated by this icon for visual sensations that are greyish or achromatic.

Whiteness is a number that describes any vision, sight or ocular experience that could loosely be described as greyish or achromatic. Words like grey, black, white, dark, bright, silvery, taupe, leaden, ecru, ashen, beige, pale etc. are used to convey greyish impressions. The reference experience for achromatic sensation is seeing the Sun. So to make a binary description of a greyish visual sensation compare it to seeing the Sun. Report the result using one of the following algebraic statements. If the two experiences are not comparable, then say that the sensation is not an achromatic visual sensation and express this as \delta_{w}=0. If the sensation is like seeing the Sun, then say that it is white. Express this as \delta_{w}=+1. If the sensation is not like seeing the Sun, then say that it is black and that \delta_{w}=-1. The number \delta_{w} is called the whiteness. These definitions are expressed mathematically by

\delta_{w} \equiv \begin{cases} +1 &{\text{if a visual sensation is white }} \\ \; \; 0 &{\text{if a sensation is not achromatic }} \\ -1 &{\text{if a visual sensation is black }} \end{cases}

Reference Constant for Whiteness

The numerical constant associated with the visual sensation of seeing the Sun is called the mean internal energy of down quarks. It is symbolized by {\widetilde{U}}^{\sf{D}} and has a value of {\widetilde{U}}^{\sf{D}} = -27 \,   \left(  \mu \textsf{eV}      \right) For more about how this number is used, please see the discussion of fine-structure in the hydrogen spectrum.