Rimantas Vaicekauskas

Optimization of white polychromatic semiconductor lamps

A stochastic method of optimization of a white-light source that relies on additive color mixing of the emissions from colored light-emitting diodes (LEDs) was developed. The method allows for finding the optimal wavelengths of LEDs in order to obtain the best possible trade off between luminous efficacy and the general color rendering index (CRI) of the white source for an arbitrary number of primary LEDs. Optimal solid-state lamps composed of two, three, four, and five different LEDs were analyzed. We show that a dichromatic LED lamp can only provide high efficacy with a general CRI close to zero, whereas trichromatic and quadrichromatic lamps are able to cover the entire range of reasonable general CRI values. The optimization of quintichromatic LED lamps and lamps with a higher number of primary color LEDs yields a negligible benefit in improving CRI but provides for quasicontinuous spectra that might be required for special lighting needs.

Spectral optimization of phosphor-conversion light-emitting diodes for ultimate color rendering

We apply an optimization scheme based on rendering of all colors of the enhanced Munsell palette to phosphor-conversion (PC) light-emitting diodes (LEDs). This approach yields combinations of peak wavelengths and bandwidths for white PC LEDs with partial and complete conversion that enable lighting with better quality than that obtained using designs based on the standard color-rendering assessment procedure.

Solid-state lamps with optimized color saturation ability

Spectral power distribution of trichromatic clusters of light-emitting diodes (LEDs) was optimized for rendering the highest number of colors with a perceptually noticeable gain in chroma (color saturation) out of 1269 Munsell samples. The basic tradeoffs of the number of colors rendered with increased saturation with the number of colors rendered with high fidelity and with luminous efficacy of radiation were established. High-saturation RGB clusters composed of commercially available AlGaInP and InGaN LEDs were modeled for a standard set of correlated color temperatures and the stability of the color saturation ability of the clusters against the drift of peak wavelengths was investigated.

Quadrichromatic white solid-state lamp with digital feedback

White light with high color rendering indices can be produced by additive color mixing of emissions from several light-emitting diodes (LEDs) having different primary colors. White Versatile Solid-State Lamps (VSSLs) with variable color temperature, constant-chromaticity dimming, and efficiency/color-rendering trade-off can be developed using pulse-width modulation (PWM) driving technique. However, such lamps exhibit chromaticity shifts caused by different temperature and aging coefficients of the optical output for primary LEDs of different colors. To overcome this drawback, we developed a polychromatic white solid-state lamp with an internal digital feedback. The lamp features a quadrichromatic (red-amber-green-blue) design based on commercially available high-power LEDs. The design is optimized to achieve high values of the general color rendering index (69 to 79 points) in the color-temperature range of 2856 to 6504 K. A computer-controlled driving circuit contains a pulse-width modulator and a photodiode-based meter. The software performs periodical measurement of the radiant flux from primary LEDs of each color and adjusts the widths of the driving pulses. These VSSLs with feedback found application in phototherapy of seasonal affective disorder (SAD).

Optimization of solid-state lamps for photobiologically friendly mesopic lighting

The circadian and visual-performance-based mesopic systems of photometry were applied for the optimization of the spectral power distributions (SPDs) of the solid-state sources of light for low-illuminance lighting applications. At mesopic adaptation luminances typical of outdoor lighting (0.1-2 cd/m(2)), the optimal SPDs were obtained through the minimization of the mesopic circadian action factor, which is the ratio of the circadian efficacy of radiation to mesopic luminous efficacy of radiation. For correlated color temperatures below ~3000 K, the optimized dichromatic light-emitting diodes (LEDs) are shown to pose a lower circadian hazard than high-pressure sodium lamps and common warm white LEDs; also they are potentially more efficacious and have acceptable color rendition properties under mesopic conditions.

Rendering a color palette by light-emitting diodes

We present a new approach to the optimization of sources of white light based on colored light-emitting diodes (LEDs) and show that a pentachromatic source composed of LEDs with the equidistant peak wavelengths at about 450, 500, 550, 600, and 650nm renders all 1269 colors of the Munsell palette almost indistinguishably from daylight.

Cultural Preferences to Color Quality of Illumination of Different Artwork Objects Revealed by a Color Rendition Engine

The preferences to color quality of illumination were investigated for American and Chinese subjects using a solid-state source of white light with the continuously tunable color saturation ability and correlated color temperature of quadrichromatic blends. Subjects were asked to identify both “most natural” and preferred blends. For very familiar objects, cultural differences did not affect the average of the selected blends. For less familiar objects (various paintings), cultural differences in the average selected blends depended on the level of the familiarity of the content. An unfamiliar painting also showed preferences to color temperature being dependent on the cultural background. In all cases, the American subjects exhibited noticeably wider distributions of selection rates.

Optimization of multichip white solid-state lighting source with four or more LEDs

Polychromatic solid-state lamps that produce white light by additive mixing of the emissions from primary colored light emitting diodes (LEDs) should have a higher luminous efficiency that those using phosphors. These lamps require emission spectra that feature an optimal trade-off between luminous efficacy and color rendering. We developed a mathematical technique that allows us to maximize the luminous efficacy and general color rendering index (CRI) for the white solid-state lamp composed of an arbitrary number of primary LEDs with given spectra. We use this method in order to compare the optimal efficacy and general CRI for 4 and 5 primary LEDs with that for 2 and 3 LEDs. For a particular color temperature, the required number of primary LEDs depends on the trade-off between efficacy and general CRI. The quadrichromatic lamp is shown to meet requirements for most practical applications. Quintichromatic lamps and lamps with a higher number of primary LEDs yield negligible benefit in improving color rendering. However, quintichromatic LED lamps are capable of producing quasi-continuous spectra that might meet special lighting needs.

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.

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.