Pranciškus Vitta

Measurement of the junction temperature in high-power light-emitting diodes from the high-energy wing of the electroluminescence band

By using pulsed driving currents with a small duty cycle, the high-energy wing of the electroluminescence band in AlGaInP and InGaN high-power light-emitting diodes (LEDs) was calibrated to measure the junction temperature in the range of 223–358K. In a red AlGaInP LED with a thick active layer, an accuracy of 2% was achieved for the junction temperature derived from the high-energy slope in the spectral range free from parasitic absorption by taking into account the three-dimensional density of band states. Meanwhile, the far high-energy region of the slope distorted by parasitic absorption can be used for the extraction of the junction temperature by using only an appropriate linear correction procedure (∼7% accuracy). In a blue InGaN LED with multiple-quantum-well active layers, the junction temperature can be determined with an accuracy of 2% from the inverse derivative of the spectra in a narrow spectral region ∼150meV above the peak energy by using a linear correction.

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.

Sol-gel synthesized far-red chromium-doped garnet phosphors for phosphor-conversion light-emitting diodes that meet the photomorphogenetic needs of plants

We report the sol-gel synthesis and characterization of far-red garnet phosphors Gd(3)Ga(5)O(12) (GGG:Cr), Y(3)Ga(5)O(12) (YGG:Cr), Lu(3)Ga(5)O(12) (LGG:Cr), and Gd(3)Sc(2)Ga(3)O(12) (GSGG:Cr) doped with different chromium (III) concentration (3, 5, and 8 mol. %). The morphological and luminescence properties of the phosphors annealed at different temperatures (1000°C, 1300°C, 1400°C, and 1500°C) were examined using x-ray diffraction, scanning electron microscopy, photoluminescence (PL), and PL excitation (PLE) spectroscopy, and by the measurements of diffuse reflection, PL internal quantum efficiency (QE), and PL decay time. The PLE spectra of the samples were found to peak at around 450 nm depending on the host, and luminescence was observed in the region of 700-760 nm. The QE was found to strongly depend on doping concentration and calcination temperature, and the PL decay exhibited biexponential behavior. The investigated far-red garnet phosphors, in particular GGG:Cr and YGG:Cr, show a potential for use in phosphor-converted light-emitting diodes that meet the photomorphogenetic needs of plants.

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.

Thermal characterization of light-emitting diodes in the frequency domain

We report on a method for the measurement of thermal relaxation time constants within light-emitting diodes (LEDs) in the frequency domain. The method is based on the phase shift of the forward voltage waveform with respect to that of the harmonically modulated forward current due to the sensitivity of the forward voltage to junction temperature. The phase shift was shown to exhibit dips at angular frequencies equal to inverse thermal time constants. Extraction of thermal time constants was demonstrated for common low-power and high-power LEDs. The measured thermal time constants (∼0.1–100 ms) were linked to heat flows between the LED components.

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.

White Complementary Solid-State Lamp

Abstract A complementary source of white light composed of a conventional, white, phosphor-conversion light-emitting diode (LED) complemented with red-orange (625 nm) and cyan (505 nm) colored LEDs was modeled and optimized. In comparison with the conventional white LED, the optimal complementary device features a reduction of color temperature from 6725 K to 5400 K and improvement of the general color rendering index (CRI) from 73 to 90 with the minimal special CRI increased from −18 to 77. We demonstrated that the complementary source can be employed as a versatile white lamp with a trade-off between luminous efficiency and color rendering implemented by variation of contributions from the primary LEDs.