Color cast WTI calculator

Unfortunately, the issue of color cast is often overlooked, but in practice it is actually even more important than the topic of color rendering (CRI).

The reason is that a color temperature of, for example, warm white at 3,000 Kelvin is only an imprecise, one-dimensional indication of a light source’s effect. White light can not only be warm (yellow-orange) or cool (bluish), but can also tend toward green or magenta. While the good old incandescent bulb, for technical reasons, actually always managed to produce light perfectly without any green or magenta cast, the situation is different with LEDs.

Everyone has stood under LED lighting at some point that seemed somehow too green or even too magenta or pink. This phenomenon was already familiar from fluorescent tubes. A color temperature specification of, for example, 3,000 Kelvin says nothing about this.

These 3,000 Kelvin can be neutral, but they can also have a distinct green or magenta cast.

A statement about this can be made using the so-called color distance to Planck, measured in delta uv, or D_uv for short. Unfortunately, D_uv is a very difficult value to communicate – with four decimal places – and is even harder to explain to the end customer than the color temperature in Kelvin or the color rendering index (CRI) of a light source.

I therefore propose a new metric called the WTI (White Tint Index), which simply multiplies the D_uv by 10,000. This results in WTI values ranging from 0 to over 100, which – like the CRI, for example – can be easily classified and communicated to the end customer.

WTI Rating

• WTI < 10: Not visible (museums, galleries, reference lighting)
• WTI < 30: Barely visible (high-quality architectural lighting)
• WTI < 60: Acceptable (general lighting)
• WTI > 60: Noticeable color cast

Note that the WTI can be positive or negative. It is positive if the light source lies above the Planckian curve, meaning it tends toward green. It is negative if the light source lies below the Planckian curve, meaning it tends toward magenta.

The following calculator determines the color temperature and corresponding WTI of a light source based on its x and y color coordinates.

Note:
People often find light sources that fall slightly below the Planck curve more pleasant. This is especially true for warmer color temperatures below 4,000 K. A negative WTI can therefore even be advantageous in this case and is preferable to a positive WTI. Ultimately, of course, it always comes down to personal perception and preference.