Andrius Petrulis

Color rendition engineering of phosphor-converted light-emitting diodes

We present an approach to the optimization of the trichromatic spectral power distributions (SPDs) of phosphor-converted (p-c) light-emitting diodes (LEDs) in respect of each of four different color rendition properties (high color fidelity, color saturating, color dulling, and color preference). The approach is based on selecting a model family of Eu2+ phosphors and finding the optimal peak wavelengths of the phosphor bands as functions of the luminous efficacy of radiation. A blue component due to either phosphor photoluminescence or InGaN electroluminescence with the peak wavelength at about 460 nm was found to be an optimal one for the high-fidelity, color-dulling, and color-preference LEDs. The high-fidelity and color-preference LEDs need red phosphors with the peak wavelength of 610-615 nm. The high-fidelity LEDs were shown to require a true green (~530 nm) phosphor component, whereas a cyan (~510 nm) component is the prerequisite of the color-saturating and color-preference LEDs. Deep-blue (~445 nm) and deep-red (~625 nm) components are required for the color-saturating LEDs. A broad yellow band similar to that of Ce(3+) emission is to be used in the color-dulling LEDs. The SPDs of practical phosphor blends for the high-fidelity, color-saturating, and color-preference p-c LEDs are demonstrated.

Subjective evaluation of luminance distribution for intelligent outdoor lighting

A large number of physical dimensions and different criteria make choosing the optimal luminance distribution for outdoor lighting difficult. The optimisation becomes even more complex for light-emitting diode-based lighting installations that can be intelligently operated, providing different luminance distributions around a moving traffic participant. This study aims at establishing the main subjective factors for the assessment of the luminance distribution of an intelligent light-emitting diode-based outdoor lighting installation for pedestrians. Semantic-differential scales and Likert scales have been used to identify subjective impressions and to find the main factors for the optimisation of the control of the luminance distribution. Our results reveal two factors that need to be considered when assessing intelligent outdoor lighting installations: a major factor that is related to the subjective feeling of well-being and a minor factor that is related to the physical properties of the environment.

Firelight LED Source: Toward a Balanced Approach to the Performance of Solid-State Lighting for Outdoor Environments

We report on a blue-amber (“firelight”) cluster of light-emitting diodes (LEDs) with extra-low correlated color temperature (~1860 K) optimized for outdoor lighting under mesopic conditions. When compared with common white LEDs, the firelight LED cluster shows considerably reduced indexes of melatonin suppression and skyglow, increased retinal illuminance for elderly people, but a reduced performance of perceiving colors, which, however, can be tolerated at mesopic luminance. In comparison with an almost metameric high-pressure sodium lamp, the cluster exhibits a potentially higher luminous efficacy, similar reaction time and detection threshold of luminance contrasts for achromatic targets, and noticeably improved color discrimination characteristics.

Artwork visualization using a solid-state lighting engine with controlled photochemical safety

A concept of a solid-state lighting engine for artwork-specific illumination with controlled photochemical safety is introduced. The engine is based on a tetrachromatic cluster of colored light-emitting diodes wirelessly controlled via an external smart device. By using an instantaneous dimming functionality, the driving software allows for maintaining the damage irradiance relevant to a particular type of photosensitive artwork material at a constant value, while varying the chromaticity and color rendition properties of the generated light. The effect of the constant damage irradiance on the visual impression from artworks is demonstrated for the lighting engine operating in three modes, such as selecting color temperature, tuning color saturating ability, and shifting chromaticity outside white light locus, respectively.

Energy-saving approaches to solid state street lighting

We consider the energy-saving potential of solid-state street lighting due to improved visual performance, weather sensitive luminance control and tracking of pedestrians and vehicles. A psychophysical experiment on the measurement of reaction time with a decision making task was performed under mesopic levels of illumination provided by a highpressure sodium (HPS) lamp and different solid-state light sources, such as daylight and warm-white phosphor converted light-emitting diodes (LEDs) and red-green-blue LED clusters. The results of the experiment imply that photopic luminances of road surface provided by solid-state light sources with an optimized spectral power distribution might be up to twice as low as those provided by the HPS lamp. Dynamical correction of road luminance against road surface conditions typical of Lithuanian climate was estimated to save about 20% of energy in comparison with constant-level illumination. The estimated energy savings due to the tracking of pedestrians and vehicles amount at least 25% with the cumulative effect of intelligent control of at least 40%. A solid-state street lighting system with intelligent control was demonstrated using a 300 m long test ground consisting of 10 solid-state street luminaires, a meteorological station and microwave motion sensor network operated via power line communication.

Exploring Preferred Correlated Color Temperature in Outdoor Environments Using a Smart Solid-State Light Engine

ABSTRACT The existence of regions of correlated color temperature (CCT) and illuminance at which the illumination is considered “pleasing” (Kruithof hypothesis) has been tested in several outdoor environments using a smart tetrachromatic solid-state light engine with tunable CCT and high-fidelity color rendition. Subjects from a culturally homogeneous group were asked to find the most “pleasing” illumination conditions at the set illuminance levels of 5 lx and 50 lx by performing a CCT adjustment task within a wide range of 1850–10,000 K at a fixed initial condition of 1900 K. The selected intervals or preferred CCT were found to shift to higher values with increased illuminance almost independently of the content of the viewed scene with statistical significance, which qualitatively is in agreement with the Kruithof hypothesis. However, the intervals of CCT for pleasing illumination were found to be shifted to a broader and higher “warm white” range than those predicted by Kruithof. In addition, subjects were asked to find the most pleasing illumination conditions when the adjustment of CCT was accompanied by instantaneous dimming in order to maintain constant circadian irradiance. At low and high set circadian irradiance levels corresponding to 5 lx and 50 lx illuminance at the CCT of 1900 K, subjects preferred moderate dimming with lower CCTs and more noticeable dimming without the reduction of CCT, respectively.

Metameric Light Sources: A Recent Paradigm for Functional Lighting

Abstract Recent progress in tuneable solid-state light sources opens new opportunities in a niche and high added value lighting applications. The standard colorimetric technique of composing an identical colour coordinate metameric light spectra are facing the challenges due to the colour discrepancy noticed by the observers. A side-by-side colour matching experiment was performed with two tuneable tetrachromatic RAGB lamps (638 nm red, 598 nm pc amber, 518 nm green, 451 nm blue LEDs) to compare the colour of the metameres in a 10-deg viewing angle. The metameric light matching was investigated using a 2-deg (CIE 1931) and 10-deg (CIE 1964) XYZ colour matching function. Both colour matching functions of standard colorimetric observers were shown to be inaccurate for aligning of metameric spectral power distributions without a noticeable difference in a perceived colour. On the other hand, a wide scatter of individual results revealed that the standard colour matching functions are inherently limited and in some cases, especially in professional lighting applications, the individually adjustable metameric lighting approach is to be considered.

Solid-State Sources of Light for Mesopic Illumination with Minimized Circadian Action

The spectral power distributions of solid-state sources of light for low-illuminance applications were optimized using the circadian and mesopic photometry systems and practical lamps for photobiologically friendly outdoor lighting were proposed.