The Physics of Visible Light

Light is a form of electromagnetic radiation that behaves as both a wave and a particle (photon). The visible spectrum for humans occupies a narrow band approximately between 380 and 750 nanometers (nm).

The Retinal Decoding Process

Our eyes contain specialized photoreceptor cells called cones, which are sensitive to different wavelengths:

• S-Cones (Short): Peak sensitivity at ~420nm (Blue)
• M-Cones (Medium): Peak sensitivity at ~530nm (Green)
• L-Cones (Long): Peak sensitivity at ~560nm (Red)

When photons hit these receptors, they trigger electrical signals sent to the primary visual cortex. The combination of these signals allows us to perceive millions of distinct chromatic variations.

Spectral Power Distribution (SPD)

In industrial design, we look at the Spectral Power Distribution. This is a plot of the power of each wavelength in a light source. Natural sunlight has a relatively continuous SPD, while LED or fluorescent lighting has sharp peaks.

Understanding SPD is critical for achieving metameric stability—ensuring that colors look consistent under different light sources (e.g., in a showroom vs. natural daylight).