To understand and define circadian efficacy and pro-cognitive action, we define two regions in the visible spectrum (from violet to red) wavelengths:
- visual sensitivity curve that includes the region of the visible spectrum to which human vision is most sensitive – this curve therefore represents the photopic visual system of the eye mediated by the retinal cones and has a maximum at 555 nm,
- and the melanopic curve, which includes the region of the spectrum to which the human circadian system is most sensitive to (i.e. circadian entrainment, alertness, cognitive performance) – this curve therefore represents the non-visual system of the eye, which is primarily mediated by the so-called internally sensitive melanopsin ganglion cells of the retina (ipRGC cells, from English intrinsically photosensitive retinal ganglion cells) containing the light-sensitive photopigment melanopsin [22-24].
Melanopsin has an absorption maximum ranging from 464 to 484 nm, sometimes reported as low as 447 nm . This maximum is not precisely defined yet – this is not only due to the different methodology of the studies, but also due to the fact that melanopsin exhibits bi-stability, which may affect its spectral sensitivity [24,25]. In addition, there are at least five types of ipRGC cells that differ in their morphology and function. Other photoreceptors may also play a role in the non-visual system – an effect not fully described yet . However, despite these complex interactions and effects, most scientific data show that the maximum lies in the region of around 480 nm [23,24]. Since the melanopsin absorbance is defined by a function and not a specific point, the melanopic pro-cognitive region can generally be defined as the range 450-500 nm and especially 460-490 nm [22-24]. Stimulation of melanopsin during the day is essential for circadian entrainment, alertness and cognitive performance [26-30]. Exposure to a balanced full-spectrum light without any dips in the melanopic region is therefore extremely important during the day for circadian entrainment, mood, alertness and cognitive performance [22-24,27,31,32]. This is also why the International Commission on Illumination (CIE) recommends maximising exposure to full-spectrum daylight during the day not only outdoors, but also indoors .
An unbalanced discontinuous spectrum usually has a low insufficient CRI value with LED sources typically lacking in red spectral emission. CIE confirms that light that is insufficient for the visual (sight) or non-visual system of the eye, or light that has an imbalance between the effects on both systems or is inadequate for one or both systems, can harm human health .
Summary and comparison of the pro-cognitive effect of light sources
The pro-cognitive light source effect describes the effect on the non-visual system of the eye, i.e. the quality of radiation in the circadian cyan and neighbouring blue and green regions, also known as the melanopic region. It is a region defined by 460-500 nm (with a maximum at 480 nm) that signals the human brain that it is in fact daytime. As a result, it entrains the circadian system and supports wakefulness and cognitive performance of the brain.
- The fluorescent lamp has minimal to zero radiation in this region.
- A conventional LED light source has a typical drop in radiation right in this melanopic region, which is preceded by concentrated energy in blue wavelengths; this drop is caused by fundamental limitation of luminophore in LED technology.
- The Spectrasol LED light source emits continuous and balanced energy across the full visible spectrum, i.e. the spectrum is continuous also in the melanopic pro-cognitive region and analogous to the Sun.
- The sun at noon has a fully continuous spectrum, i.e., it also emits continuous and balanced energy in the melanopic pro-cognitive region of the visible spectrum.