Standard white LED production is based on a simple physical principle. A blue InGaN chip emits photons with a wavelength around 450 nm, which strike a phosphor layer—most commonly YAG:Ce (yttrium-aluminum garnet doped with cerium). The phosphor absorbs part of the energy and re-emits it as photons with longer wavelengths in the yellow-green region. The combination of the remaining blue light and the yellow emission from the phosphor creates light perceived as white. This process is accompanied by unavoidable energy loss, known as the Stokes shift.
The resulting spectral power distribution (SPD) of such a source has a characteristic shape: a pronounced peak around 450 nm, a dip in the 480–520 nm region (known as the cyan gap), and a broad hill in the yellow region. This spectral unevenness is a physical consequence of using a single phosphor, not a technological failure—it represents a compromise between cost, efficiency, and light quality.
The cyan gap has practical significance, especially in terms of the biological effects of light. Melanopsin, the photopigment of retinal ganglion cells (ipRGCs) involved in regulating the circadian rhythm, has an absorption maximum around 480 nm—exactly in the range where conventional LEDs show lower output. This is not the only relevant factor for circadian stimulation (overall intensity and exposure timing also matter), but the spectral composition of the luminaire plays a demonstrable role.
An alternative is luminaires with multiple types of phosphors or added LED chips of different wavelengths. Multi-chip or multi-phosphor systems allow a more balanced coverage of the spectrum, including the cyan region and, if needed, deeper red (around 660–680 nm, where cytochrome c oxidase has an absorption maximum relevant for photobiomodulation at high intensities). The technical challenges of these systems include reabsorption of light between phosphors, thermal management, and maintaining spectral stability over time—real engineering problems whose solutions vary among manufacturers.
Light with a more uniform spectrum and well-controlled color (high color rendering index, Ra > 90) is demonstrably more comfortable for visual performance and can contribute to better circadian stimulation at the appropriate time of day. However, the biological effects of typical indoor lighting must be assessed realistically—the intensities and exposures in clinical photobiomodulation studies differ significantly from conditions in offices or homes.
