The Physics of Lighting

Planck (1900) formulated the relationship between the temperature and the spectral energy distribution in the radiation of a blackbody (a hypothetical body that completely absorbs all radiant energy falling upon it, reaches some equilibrium temperature, and then re-emits that energy as quickly as it absorbs it)

$$R(\lambda)=\frac{8 \pi h c}{\lambda^5 [exp(hc/k T \lambda)-1]}$$

where $c$ is the speed of light, $\lambda$ is the wavelength, $T$ is the temperature, $h$ is the Planck constant and $k$ is the Boltzmann constant.

As examples, the energy spectral distribution of several light sources, the direct sun light, the overcast sky, and a tungsten filament lamp are plotted.

While an object is illuminated by a light source, what the eye perceives is the light reflected by the surface of the object:

$$L(\lambda)=R(\lambda) I(\lambda)$$

$L(\lambda)$ depends on both the illumination $I(\lambda)$ and the reflectance of the object $R(\lambda)$. Typically the illumination has little spatial variation, while the reflectance may have drastic spatial changes representing the details (texture, color, smoothness, etc.) of the object surfaces.