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
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.
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).