High-performance lighting evaluated by photobiological parameters

Light adaptation characteristics of melanopsin

Following photopigment bleaching, the rhodopsin and cone-opsins show a characteristic exponential regeneration in the dark with a photocycle dependent on the retinal pigment epithelium. Melanopsin pigment regeneration in animal models requires different pathways to rods and cones. To quantify melanopsin-mediated light adaptation in humans, we first estimated its photopigment regeneration kinetics through the photo-bleach recovery of the intrinsic melanopsin pupil light response (PLR). An intense broadband light (~120,000 Td) bleached 43% of melanopsin compared to 86% of the cone-opsins. Recovery from a 43% bleach was 3.4X slower for the melanopsin than cone-opsin. Post-bleach melanopsin regeneration followed an exponential growth with a 2.5 min time-constant (τ) that required 11.2 min for complete recovery; the half-bleaching level (I_p) was ~ 4.47 log melanopic Td (16.10 log melanopsin effective photons.cm^-2.s^-1; 8.25 log photoisomerisations.photoreceptor^-1.s^-1). The effect on the cone-directed PLR of the level of the melanopsin excitation during continuous light adaptation was then determined. We observed that cone-directed pupil constriction amplitudes increased by ~ 10% when adapting lights had a higher melanopic excitation but the same mean photometric luminance. Our findings suggest that melanopsin light adaptation enhances cone signalling along the non-visual retina-brain axis. Parameters τ and I_p will allow estimation of the level of melanopsin bleaching in any light units; the data have implications for quantifying the relative contributions of putative melanopsin pathways to regulate the post-bleach photopigment regeneration and adaptation.

Prediction accuracy of L- and M-cone based human pupil light models

Multi-channel LED luminaires offer a powerful tool to vary retinal receptor signals while keeping visual parameters such as color or brightness perception constant. This technology could provide new fields of application in indoor lighting since the spectrum can be enhanced individually to the users’ favor or task. One possible application would be to optimize a light spectrum by using the pupil diameter as a parameter to increase the visual acuity. A spectral- and time-dependent pupil model is the key requirement for this aim. We benchmarked in our work selected L- and M-cone based pupil models to find the estimation error in predicting the pupil diameter for chromatic and polychromatic spectra at 100 cd/m². We report an increased estimation error up to 1.21 mm for 450 nm at 60–300 s exposure time. At short exposure times, the pupil diameter was approximately independent of the used spectrum, allowing to use the luminance for a pupil model. Polychromatic spectra along the Planckian locus showed at 60–300 s exposure time, a prediction error within a tolerance range of ± 0.5 mm. The time dependency seems to be more essential than the spectral dependency when using polychromatic spectra.

Thermodynamics of ideal illumination: a novel figure of merit for characterizing illumination efficiency

The advancement of modern lighting technologies has led to many revolutions in lighting efficiency and presentation. The progression from filament bulbs, to CFL, and now LED technologies have produced a bounty of energy-efficient lighting options for design engineers and consumers. As the light-producing elements of a lighting fixture improve, the limiting factor in efficient illumination is no longer the light source, but the optical system itself. There are many characterization methods and standards for defining light for illumination in terms of color and human response. With concerns of how things like light pollution and energy requirements impact our society and the world around us, it is critical to understand how well a lighting fixture can illuminate a desired area while minimizing light lost to the environment and maximizing the total radiative intensity (radiance) of a space. This work presents two figures of merit, one for over-illumination and another for under-illumination, to characterize the optics of a lighting system based on a ray tracing methodology. Five common simplified optical design, with four varying beam angles, were simulated to present these new figures of merit. Results showed that common imaging optical systems such as parabolic and ellipse reflectors struggled to produce a well-lit area without over illumination, while nonimaging alternatives like the compound parabolic and compound elliptical reflectors were able to reach the thermodynamic ideal of a fully illuminated area without light lost to the environment. This work hopes to inform illumination engineers and lighting designers to help improve their optical design to maximize performance and minimize waste.

Effects of photopic and cirtopic illumination on steady state pupil sizes

The conventional view was that cones are responsible for pupil constriction in photopic lighting conditions. With the discovery of intrinsically photosensitive retinal ganglion cells (ipRGC), it was found that signals from ipRGCs along with cones mediated the pupil light reflex in photopic lighting conditions. Although both signals contributed, it was unclear how these signals were summed. In the work reported here, steady-state pupil size was measured with an infrared camera under LED lighting conditions with different color temperatures and luminance. A formula was then derived for pupil size according to the linear summation of cirtopic and photopic luminance. This formula allowed direct calculations to predict pupil size well when LED photopic luminance ranged from about 50cd/m² to 300cd/m², which is the general luminance level range for computer and smartphone screens.