Daylight indoors consists of two physically distinct components: direct sunlight and diffuse sky radiation. The blue color of the sky is a result of Rayleigh scattering—the intensity of scattered light is inversely proportional to the fourth power of the wavelength, so short-wavelength blue light scatters much more in the atmosphere than red light. The result is a broad, omnidirectional source of illumination that lights a space softly and without harsh shadows.
This property has a practical effect on visual comfort. Point light sources—whether incandescent bulbs or LED chips—create high brightness contrast between the source and its surroundings, which can cause glare and increased eye fatigue. Large-area luminaires with diffusing materials reduce this contrast by spreading the light over a larger surface. Optically, such luminaires approximate a so-called Lambertian emitter—an ideal surface source whose apparent brightness is the same from all viewing angles. This is an established concept in lighting design, used for example in high-quality LED panels with microstructured diffusers or OLED luminaires.
The effect of spatial light distribution on circadian stimulation is less direct. Activation of melanopsin cells (ipRGCs) depends primarily on the spectral composition of the light (peak sensitivity around 480 nm), its intensity, and exposure time. Spatial uniformity of lighting can indirectly help ensure that light reaches a larger portion of the retina, but ipRGCs are distributed across the retina, with higher density in central rather than peripheral areas. Thus, the spatial distribution of photons is a relevant factor, but in the hierarchy of parameters for circadian stimulation, it ranks behind spectrum and intensity.
Large-area diffuse Spectrasol luminaires therefore have a demonstrable benefit for visual comfort and glare reduction. Comparing them to a “sky model” is an intuitive metaphor—the physical analogy is partial, because the actual sky differs from an indoor luminaire in orders of magnitude higher luminance and a dynamically changing spectrum throughout the day.
