Why your eyes see color differently under different lights

Gardening · Published 18 October 2026 Swipe more like this →

In short: The color of a flower isn't just a property of the petals; it is a conversation between light and matter. Because different light sources—like a dim bulb or a bright sun—provide different spectral energy, scientists use theoretical "standard illuminants" to ensure we can measure and compare colors consistently.

Because colors shift depending on the light source, the International Commission on Illumination (CIE) established standard illuminants. These are theoretical models with published spectral power distributions (SPD) that provide a universal baseline for colorimetry. By using these standards, researchers and manufacturers can ensure that a specific shade of red remains consistent whether it is being viewed under a fluorescent lamp or in the middle of a field.

The history of these standards reveals a shift from simple approximations to complex mathematics. In 1931, the CIE introduced Illuminants A, B, and C to represent incandescent light, noon sunlight, and average daylight, respectively. However, B and C were eventually deprecated because they were poor approximations of natural light, especially in the ultraviolet and short-wave visible ranges. They were created using liquid filters—complex solutions of distilled water, copper sulfate, and other chemicals—designed to mimic daylight by absorbing red light, similar to how photographers use color gels.

Modern standards, particularly the D series, are more mathematically rigorous. The D series represents natural daylight and follows the "daylight locus," a quadratic relation discovered by researchers like Deane B. Judd. This locus accounts for variations in light caused by the proportion of direct to indirect sunlight (yellow–blue variation) and the presence of water vapor or haze (pink–green variation). While D65 is the standard for average daylight with a correlated color temperature of approximately 6500 K, it remains difficult to produce perfectly as an artificial source.

Interestingly, even the most basic standard, Illuminant A (representing tungsten-filament lighting), faced a technical correction. In 1968, a revision of the constants in Planck's law shifted its color temperature from 2848 K to 2856 K. To prevent future shifts, the CIE now specifies the spectral power distribution directly. These standards matter because they allow us to calculate the metamerism index, which tests how well different materials match under specific lighting conditions, ensuring visual consistency in a world of varied illumination.

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