Kenneth Martin from Zumtobel about Comfortable Lighting — LED professional

An important question driving us forward these days is: What effects does light have on us, besides imaging information? Initial scientific indications show impacts via human skin with its melanopsin producing fat cells and the far red light activated mitochondria, but effects on the eyes’ cells are still the main focus of research.  Unlike skin, the eyes’ reception of light at the single cells of the retinal tissue is regulated by the iris and filtered by its directionality via the lens.  

Currently, five types of photosensitive retinal cells have been classified: Next to the cones in its three variants that are sensitive to short, middle and long wavelengths, there are also the rods which are most sensitive within the cyan (appearing) wavelength at low intensities. For the past few years we have gotten to know more about the fifth photoreceptors, the intrinsically photosensitive retinal ganglion cells (IPRGCs), containing melanopsin, which is most sensitive to the azure wavelength. These cells also get signals from other cell types, but how all the signals of the cells and from both eyes are combined for the physiological mechanisms is still being researched.

We can describe some average “stimulus” values by spectral irradiation measurements at eye level. But this is only directly comparable to another situation if it has the same relative radiance distribution within the whole visible field. The distribution of the different cell types on the retina and their relative degree of saturation may play an important role on the reactions. That’s why absolute readings of lux, W/m², cd or cd/m² are usually non-transferable without further ado. The spatial and spectral distribution of the signals from the retinal cells – a kind of histogram – is the minimum required information from a hyperspectral imaging measurement, to seriously describe the effects in detail.

Like a video camera, the eyes don’t have the possibility to greatly alter the signal integration time along the intensity levels. Instead, there is an impulsive pupillary reflex and a slower steady state pupil size regulation, optimizing the irradiation level at the retina and protecting it from excessive intensities. Other physiological effects are the main adaptation level (chromatic and brightness perception) and melatonin suppression. Those effects react to the “histograms” of the relative stimuli magnitudes of the five different photoreceptor types, and therefore on the spectral and spatial distributions of the reflected light.

When we focus our vision e.g. at work, the light that we receive, should be optimized so that we have the widest color and lightness contrast sensitivity range. And it should also induce the right non-visual effectiveness and the lowest strain level on the retinal cells, supported by correctly functional physiological mechanisms.

Based on the numerous new research results, we need to question whether illumination based regulations and quality standards should be replaced. To include long-term effects of light on health and stress to the eyes, we need new quality criteria based on the active photoreceptor stimulus distributions. Contrasts from bright spots should not be excessive nor should there be reduced object color contrasts and brightness gradients. Nowadays, most people have a smartphone that can roughly calculate lighting quality by the camera pictures’ histogram data. At good values of new quality criteria, it should be possible to easily make acceptable snapshots without effects from glaring lights or flicker – even at night. It helps if lighting is not just vertical or diffuse but also has one visible inclination.