Anna Vaskuri

Luminous efficacy measurement of solid-state lamps

We present a system for luminous efficacy measurements of solid-state lamps (SSLs). It is based on a 1.65 m integrating sphere, originally designed for luminous flux measurements of dc-operated incandescent standard lamps. The sphere was slightly modified, and equipped with a programmable ac-power supply, as well as equipment for electrical power measurement of the lamps. The waveforms of both the electrical power and the luminous flux can be measured for analysing the quality of the electronics and the light produced by the SSLs. A goniospectrometer is used for angular characterization of the lamp intensity. Luminous efficacy test measurements were conducted for a group of 25 SSLs of different types. Special attention was paid to the spectral, spatial and self-absorption corrections to obtain low measurement uncertainty. The luminous efficacies were between 25 lm W−1 and 64 lm W−1, and the total harmonic distortions of the current waveforms were in the range of 30% to 280%. Depending on the lamp, the maximum deviation of the flux from its mean value was 0.06% to 105%. The relative expanded uncertainty obtainable for the luminous efficacy of a typical SSL with stable built-in electronics is 1.2% (k = 2).

Spectrally adjustable quasi-monochromatic radiance source based on LEDs and its application for measuring spectral responsivity of a luminance meter

A spectrally adjustable radiance source based on light-emitting diodes (LEDs) has been constructed for spectral responsivity measurements of radiance and luminance meters. A 300?mm integrating sphere source with adjustable output port is illuminated using 30 thermally stabilized narrow-band LEDs covering the visible wavelength range of 380?780?nm. The functionality of the measurement setup is demonstrated by measuring the relative spectral responsivities of a luminance meter and a photometer head with cosine-corrected input optics.

High-resolution setup for measuring wavelength sensitivity of photoyellowing of translucent materials

Polystyrene and many other materials turn yellow when exposed to ultraviolet (UV) radiation. All photodegradation mechanisms including photoyellowing are functions of the exposure wavelength, which can be described with an action spectrum. In this work, a new high-resolution transmittance measurement setup based on lasers has been developed for measuring color changes, such as the photoyellowing of translucent materials aged with a spectrograph. The measurement setup includes 14 power-stabilized laser lines between 325 nm and 933 nm wavelengths, of which one at a time is directed on to the aged sample. The power transmitted through the sample is measured with a silicon detector utilizing an integrating sphere. The sample is mounted on a high-resolution XY translation stage. Measurement at various locations aged with different wavelengths of exposure radiation gives the transmittance data required for acquiring the action spectrum. The combination of a UV spectrograph and the new high-resolution transmittance measurement setup enables a novel method for studying the UV-induced ageing of translucent materials with a spectral resolution of 3-8 nm, limited by the adjustable spectral bandwidth range of the spectrograph. These achievements form a significant improvement over earlier methods.

New source and detector technology for the realization of photometric units

The production of incandescent light bulbs is bound to end, as incandescent lighting is being phased out globally in favour of more energy-efficient and sustainable solutions. Temporally stable light-emitting diodes (LEDs) are potential candidates to replace incandescent lamps as photometric source standards. However, traditional V(λ) filter based photometers may have large uncertainty when LEDs are measured instead of incandescent lamps. This is due to the narrow and complicated spectra of LEDs. When the spectra of LEDs are limited to the visible wavelength range, new silicon detector technology can be advantageously exploited in photometry. We present a novel method—based on the recently introduced Predictable Quantum Efficient Detector (PQED)—for the realization of photometric units which completely eliminates the need to use V(λ) filters. Instead, the photometric weighting is taken into account numerically by measuring the relative spectral irradiance. The illuminance values of a blue and a red LED were determined using the new method and a conventional reference photometer. The values obtained by the two methods deviated from each other by −0.06% and 0.48% for the blue and red LED, respectively. The PQED-based values have much lower standard uncertainty (0.17% to 0.18%) than the uncertainty of the values based on the conventional photometer (0.46% to 0.51%).

Monte Carlo analysis of uncertainty of total atmospheric ozone derived from measured spectra

We present a Monte Carlo based model to study effects that possible correlations in spectral irradiance data may have on the derived total ozone column values. Correlations may produce systematic errors in the spectral irradiance which behave differently from uncorrelated data. The effects are demonstrated by analyzing the data of one day’s measurements.

Out-of-Range Stray Light Characterization of Single-Monochromator Brewer Spectrophotometers

ABSTRACT Stray light in single-monochromator Brewer instruments increases the uncertainty of solar ultraviolet spectral irradiance measurements and ozone retrievals. To study how spectral irradiance within and outside the measurement ranges of the instruments affects stray light, two Brewer MKII instruments were characterized for the level of in- and out-of-range stray light at multiple laser wavelengths. In addition, several solar-blind filters utilized in single-monochromator Brewers to limit out-of-range stray light were characterized for spectral and spatial transmittances. Finally, the measurement results were used to simulate the effect of stray light and stray light correction on spectral irradiance and ozone measurements at different wavelength regions. The effect of stray light from wavelengths above 340 nm was found to be negligible compared with other sources of uncertainty. On the other hand, contributions from wavelengths between 325 and 340 nm can form a significant portion of the overall stray light of the instrument, with 325 nm being the upper limit of the nominal measurement range of the instrument.

Modeling the spectral shape of InGaAlP-based red light-emitting diodes

We have developed a spectral model for describing the shape of the emission spectrum of InGaAlP-based red light-emitting diodes (LEDs) with quantum-well structure. The model is based on Maxwell-Boltzmann distribution with junction temperature Tj and an experimental two-dimensional joint density of states (DOS). We model the DOS with a sum of two exponentially broadened step functions describing the two lowest sub-bands in semiconductor quantum well. The relative locations ΔE1 = 0 meV and ΔE2 = 112.7 meV above the band gap energy Eg = 1.983 eV and the ratio 2.13 of the step heights were fixed using an experimental DOS extracted from a LED spectrum measured at known Tj and driving current I. The model can then be fitted to other spectra of other LED samples at varied Tj and I by varying the fitting parameters Eg, Tj, and the broadening of the sub-band edges. The model was tested for three LED samples over I = 200–370 mA and Tj = 303–398 K. Junction temperatures obtained by modeling were compared with calibr...

Key comparison CCPR-K1.a as an interlaboratory comparison of correlated color temperature

We analyze the results of spectral irradiance key comparison CCPR-K1.a for correlated color temperature (CCT). For four participants out of 13, the uncertainties of CCT, calculated using traditional methods, not accounting for correlations, would be too small. The reason for the failure of traditional uncertainty calculation is spectral correlations, producing systematic deviations of the same sign over certain wavelength regions. The results highlight the importance of accounting for such correlations when calculating uncertainties of spectrally integrated quantities.

UV exposure in artificial and natural weathering: A comparative study

We report on a study focusing on UV exposure conditions in three different types of chambers used for accelerated ageing of materials. The first chamber is equipped with four 300-W UVA/UVB mercury vapour lamps (Ultra-Vitalux/Osram). The second chamber uses four 40-W UVA fluorescent lamps (QUV-340/Q-Lab). The third chamber is Weather-Ometer Ci3000+ from Atlas with a 4500-W xenon arc lamp. UV irradiance prevailing in each chamber was measured using Bentham DM150 double monochromator spectroradiometer. The results were compared to measurements of solar spectral UV irradiance at Jokioinen, Finland, with a Brewer MkIII double monochromator spectrophotometer. The spectral shapes of the exposing UV radiation in the different chambers were found to notably differ from each other and from the solar UV spectrum. Both spatial inhomogeneities and temporal variability caused by various factors, like the ageing of the lamps, were detected. The effects were found to strongly depend on wavelength of the exposing UV radia...

Facility for determining action spectra of UV photodegradation

We have developed an ultraviolet (UV) ageing facility consisting of a spectrograph which can be used to expose material samples with spectrally dispersed UV radiation at the wavelength range 273 – 423 nm, and an automated, high-resolution laser-based setup for measuring the induced color changes in the transmittances of the exposed samples. The samples are scanned using a suitable laser wavelength to obtain the wavelength dependence on the color change. The locations on the samples are converted to exposure wavelengths with the known dispersion of the spectrograph. We demonstrate the facility by exposing and measuring a set of atactic polystyrene sheets. By studying samples aged for different time periods we derive a complex action spectrum for the photoyellowing that is nonlinear with respect to the radiant exposure. We further derive linear activation spectra for the photoyellowing demonstrating how they depend on the selected threshold level.