The color rendition engine based on the statistical metric allows us to uniquely quantify the characteristics of color quality of illumination and assess the color rendition preferences. We now report on using the color rendition engine for revealing individual and cultural differences in color quality preferences of 205 American and Chinese subjects. Our study demonstrated that the majority of individuals preferred the color blend with the same statistical figures of merit on the average but with a much larger spread of blends for Americans. For both groups, the color rendition preferences depended on the object being illuminated. This was demonstrated by illuminating a set of common colored objects and three different paintings. We conclude that the color quality of lighting can be optimized and enhanced using the feedback to change the spectral power distribution of the illuminating source depending on the object being illuminated and on the preferences of an individual observer and a cultural group.
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.
Versatile spectral power distribution of solid-state light sources offers vast possibilities in color rendition engineering.
The optimization of such sources requires the development and psychophysical validation of an advanced metric for
assessing their color quality. Here we report on the application and validation of the recently introduced statistical
approach to color quality of illumination. This new metric uses the computational grouping of a large number of test
color samples depending on the magnitude and direction of color-shift vectors in respect of just perceived differences of
chromaticity and luminance. This approach introduces single-format statistical color rendition indices, such as Color
Fidelity Index, Color Saturation Index and Color Dulling Index, which are the percentages of test color samples with
particular behavior of the color-shift vectors. The new metric has been used for the classification of practical phosphor
conversion white light-emitting diodes (LEDs) and polychromatic LED clusters into several distinct categories, such as
high-fidelity, color saturating, and color dulling light sources. We also report on the development of the tetrachromatic
light source with dynamically tailored color rendition properties and using this source for the psychophysical validation
of the statistical metric and finding subjective preferences to the color quality of lighting.
We report on the effect of transient selfheating on the spectral modulation of electroluminescence (EL) in high-power
light-emitting diodes (LEDs). In AlGaInP LEDs, which emit due to the band-to-band recombination of free carriers, the
oscillation of junction temperature was found to result in that the modulation depth has a drop around the peak photon
energy, an increased magnitude at lower energies, and a linear increase with photon energy at higher energies. These
properties of the EL modulation spectrum can be explained by a model that takes into account the thermal modulation of
band gap energy and carrier distribution function. In InGaN LEDs, almost no thermal effect on EL modulation was
found around the peak photon energy and at lower energies, whereas at higher energies, the modulation depth also
increases with photon energy. Such a spectrum of EL modulation depth can be understood in terms of localized carrier
effect at peak photon energy and lower energies and of free carrier heating at higher energies. The frequency dependence
of modulation depth at particular photon energies was shown to sensitively replicate the thermal response function of the
LEDs.
On the basis of investigation of photoluminescence (PL) dynamics of highly excited structures with different In content
under strong excitation by short laser pulses and comparison with model calculations we analyze emission mechanisms
in these In-containing systems. The obtained PL spectra in various In-containing samples exhibit stimulated emission
line on the short-wavelength side of the spontaneous PL band. We suggest theoretical model for optical transitions in
InGaN compounds with strong compositional fluctuations leading to two distinct types of the regions, one of which is
In-rich islands. We assume that optical band-gap within those two regions is randomly fluctuating with Gaussian
distribution. Calculated PL spectra as a function of excitation and time are in fair agreement with experimental results
demonstrating all the observed peculiarities of luminescence dynamics obtained experimentally.
A set of UV light-emitting diodes (LEDs) with the peak wavelengths ranging from 255 nm to 375 nm was applied for
the investigation of spectral and decay-time fluorescence signatures in dry B. globigii spores and common airborne
interferants (albuminous, epithelium, and cellulosous materials as well as aromatic hydrocarbons). The fluorescence
decay signature was represented by a phase shift of the sinusoidal fluorescence waveform in respect of excitation
provided by high-frequency modulated LEDs. The obtained data matrix was used for the optimization a bioparticle
fluorescence sensor with a minimized number of excitation sources and detection channels and maximized
discrimination ability of bioparticles against common interferants. Based on the optimization, a new concept for a UV
LED based "detect-to-warn" bioparticle fluorescence sensor is proposed. The sensor contains a single deep-UV LED
emitting at 280 nm that is harmonically modulated at a high frequency (of about 70 MHz) and a dual-channel
fluorescence detector with the spectral windows peaked at 320 nm and 450 nm. The output parameters of the sensor are
the ratio of the fluorescence intensity in the two windows and the phase shift of the fluorescence waveform in the
320-nm detection channel in respect of the excitation one. Such a sensing scheme has a smaller number of optical
components and a potentially higher discrimination ability of bioparticles against common interferants in comparison
with the conventional approach based on just fluorescence intensity measurement under dual-wavelength excitation
(280 nm and 340 nm).
Recently developed deep-UV light-emitting diodes (LEDs) are already used in prototype fluorescence sensors for detection of hazardous biological agents. However, increasing of the sensor ability of discrimination against common interferents requires further development of measurement technique. In particular, LED-based fluorescence lifetime measurements are to be considered as a technique supplementary to fluorescence spectral and excitation measurements. Here we report on application of UVTOP® series deep-UV LEDs developed by Sensor Electronic Technology, Inc. for real-time measurements of fluorescence lifetime in the frequency domain. LEDs with the wavelengths of 280 nm (targeted to protein excitation) and 340 nm (for excitation of coenzymes NADH and flavins) were used. The output of the LEDs was harmonically modulated at frequencies up to 100 MHz and fluorescence lifetime on the nanosecond and subnanosecond scale was estimated by measuring the phase angle of the fluorescence signal in respect of the LED output. Dual-wavelength LED-based phase-resolved measurement technique was tested for discrimination of B. globigii against a variety of interferents such as diesel fuel, paper, cotton, dust, etc. We conclude that fluorescence phase measurements have potential to improve the discrimination ability of the "detect-to-warn" optical bioparticle sensors.
Recent progress in wide-bandgap semiconductor optoelectronics resulted in an appearance of deep-UV light-emitting diodes (LEDs), which can be used for fluorescence excitation in a variety of chemical and biological compounds. We used two generations of AlGaN-based UVTOP series deep ultraviolet LEDs developed by Sensor Electronic Technology, Inc. The peak wavelength of these fully packaged devices is 340 nm and 280 nm, line width at half maximum approximately 10 nm, wall-plug efficiency up to 0.9% and output power in the milliwatt range. The second-generation emitters are shown to have an extremely low level of unwanted long-wavelength emission what is important for fluorescence measurements. The UV LEDs were tested for fluorescence excitation in standard fluorophores (organic dyes), autofluorescent biological compounds (riboflavin, NADH, tryptophan, and tyrosine) and medical specimens (fluid secreted by prostate gland). Fluorescence lifetime measurements in the frequency domain were demonstrated using UVTOP-340 and -280 devices. The output of the LEDs was modulated at frequencies up to 200 MHz by high-frequency current drivers and the phase angle of the fluorescence signal was resolved using a radio-frequency lock-in amplifier. Nanosecond-scaled measurements of fluorescence lifetimes, which are the “fingerprints” of chemical and biological compounds, were demonstrated.
Rytis Stanikunas, Henrikas Vaitkevicius, Algimantas Svegzda, Vilius Viliunas, Zenius Bliznikas, Kestutis Breive, Rimantas Vaicekauskas, Algirdas Novickovas, Genadij Kurilcik, Arturas Zukauskas, Remis Gaska, Michael Shur
Lighting based on sources of light composed of colored light-emitting diodes (LEDs) offers versatile control of color and a possibility of trade-off between efficiency and color rendering. However, psychophysical issues related to such polychromatic solid-state sources have to be addressed. In this work, studies of the perception of standard colors under illumination with a quadrichromatic red-amber-green-blue (RAGB) solid-state source were carried out. An RAGB lamp containing primary LEDs with the emission peaks at 638 nm, 594 nm, 523 nm, and 441 nm and optimized for the highest value of the general color rendering index (86 points) was investigated and compared to a tungsten lamp. 40 standard Munsell samples of value 6, chroma /6, and hue incremented by 2.5 were used in the investigation. Changes in the saturation and hue of the Munsell samples illuminated by the RAGB lamp versus tungsten lamp (both with the correlated temperature of 2600 K) were obtained by colorimetric calculation comparisons and by psychophysical experiments on subjective matching of the samples. Subjective differences in hue and subjective color discrimination differences under the tungsten and RAGB lamps were found in the wavelength range of 440-500 nm and 560-580 nm. We attribute these differences to non-optimal peak wavelengths of the primary LEDs and to the narrow-band components of the RAGB spectrum.
We report on high-power solid-state lighting facility for cultivation of greenhouse vegetables and on the results of the study of control of photosynthetic activity and growth morphology of radish and lettuce imposed by variation of the spectral composition of illumination. Experimental lighting modules (useful area of 0.22 m2) were designed based on 4 types of high-power light-emitting diodes (LEDs) with emission peaked in red at the wavelengths of 660 nm and 640 nm (predominantly absorbed by chlorophyll a and b for photosynthesis, respectively), in blue at 455 nm (phototropic function), and in far-red at 735 nm (important for photomorphology). Morphological characteristics, chlorophyll and phytohormone concentrations in radish and lettuce grown in phytotron chambers under lighting with different spectral composition of the LED-based illuminator and under illumination by high pressure sodium lamps with an equivalent photosynthetic photon flux density were compared. A well-balanced solid-state lighting was found to enhance production of green mass and to ensure healthy morphogenesis of plants compared to those grown using conventional lighting. We observed that the plant morphology and concentrations of morphologically active phytohormones is strongly affected by the spectral composition of light in the red region. Commercial application of the LED-based illumination for large-scale plant cultivation is discussed. This technology is favorable from the point of view of energy consumption, controllable growth, and food safety but is hindered by high cost of the LEDs. Large scale manufacturing of high-power red AlInGaP-based LEDs emitting at 650 nm and a further decrease of the photon price for the LEDs emitting in the vicinity of the absorption peak of chlorophylls have to be achieved to promote horticulture applications.
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).
We review two complementary approaches to the development of white light solid-state sources. The first approach, which involves polychromatic LED modules, is targeted at advanced optimization of spectral power distribution in order to establish an optimal trade-off between luminous efficacy and color rendering. We apply a stochastic method of optimization of a white-light source that relies on additive color mixing of the emissions from colored primary LEDs. We present the results on optimized spectra for all-semiconductor lamps composed of four primary LEDs with the line widths typical of present AlGaInP and AlInGaN technologies. We point out the problem of the lack of efficient yellow-green (570 nm) emitters required for polychromatic lamps with four and more primary LEDs. The second approach is based on the development of AlInGaN-based UV emitters that can be tailored to directly excite different phosphors without sensitizers. AlInGaN materials system demonstrated potential for making UV LEDs with a high power and short wavelengths required for such applications. This has been achieved by using Strain Energy Band Engineering (SEBE) and Pulsed Atomic Epitaxy (PALE) techniques. SEBE relies on quaternary AlGaInN compounds for controlling strain and band offset and for producing UV emitters with improved device performance. PALE allows us to incorporate the required significant amount of indium (few percent) in AlGaN, since it can be performed at lower growth temperatures required for In incorporation. Further improvements in materials quality of AlInGaN layers with a high molar fraction of Al will be achieved by using bulk AlN substrates.
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.
Transient and quasi-steady-state photoluminescence of a dense electron-hole plasma was studied in GaN epilayers under high photoexcitation at room-temperature. High initial carrier heating up to 1100 K was observed. Decay of nonthermalized electron-hole plasma was analyzed both in homo- and heteroepitaxial GaN layers. The heating is shown to significantly influence the luminescence peak position and the rate of spontaneous and stimulated recombination. After the thermalization process is completed, the luminescence decay is exponential and the room-temperature carrier lifetime can be extracted. The lifetime in the heteroepitaxial layer grown on sapphire was found to be 190 ps, while the homoepitaxial layer exhibited an essentially higher value of 890 ps, which is one of the highest reported for free-carrier recombination in GaN. Additionally, optical gain spectra were studied using variable-stripe method. The threshold for stimulated emission was found to be considerably lower and the gain at a certain pump intensity was shown to be much higher in the homoepitaxial layer than in the heteroepitaxial one. Maximum net gain value of 300 cm-1 was observed.
Photoluminescence properties of short-period asymmetric GaAs/AlAs superlattices with the well and barrier thickness varied from 10 to 3 monolayers were studied at high optical excitation. It was shown that an asymmetric structure of the superlattice, in which the well layers are at least twice wider than the barrier ones, allows us to maintain the direct band gap and, hence, to improve emission properties for any well width. This is important for utilization of such structures in light-emitting devices. The stimulated emission at 80 K was observed for a GaAs/AsAs superlattice with the well and barrier thickness of 6 and 3 monolayers, respectively. At the same time, investigations of the dependences of the emission intensity on the pump intensity for different superlattices revealed an enhancement of nonradiative recombination with decreasing the well thickness due to an enhanced influence of interface roughness.
Strain Energy Band Engineering of Group III-N heterostructures should allow us to prevent defect formation at the heterointerfaces ad to reduce the built-in electric field in the quantum wells. The strain, caused by lattice mismatch, may be decreased by incorporation of In into AlGaN. To monitor structural perfection of the quaternary compound AlInGaN and to evaluate electronic potential profile, we employed optical methods: reflectivity, site- selectively excited photoluminescence, photoluminescence excitation and time-resolved luminescence. AlGaN with the molar fraction of Al of 9% and two samples with the lattice mismatch reduced by partial substitution of Al by 1% and 2% of In were investigated. In AlGaN, the luminescence excited resonantly with the exciton position is red shifted. The photoluminescence excitation spectra indicate that the mobility edge is above the optical band gap, and the localization vanishes. These results show that the incorporation of approximately equals 2% indium into AlGaN leads to the disappearance of the band tail states and smoothing of the potential profile.
Near-to-bandgap luminescence has been studied in bulk-like CdS nanocrystals as a function of the average radii ranging above the exciton Bohr radius (from 3.3 nm to 100 nm). The surface recombination was shown to be the main recombination route under experimental conditions. Comparing the size dependences of the luminescence under high and low excitation, a striking increase in the luminescence intensity of nanocrystals with one excited electron-hole pair was discovered for average radii below 10 nm. The small-volume enhanced bimolecular recombination was shown to account for the observed data.
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