Pasi Manninen

On spectral and thermal behaviors of AlGaInP light-emitting diodes under pulse-width modulation

Behavior of the emission spectrum, junction temperature, and charge carrier temperature of low-power AlGaInP light-emitting diodes (LEDs) with different colors under pulse-width-modulation (PWM) dimming is investigated. The blueshift of the peak wavelength and the bandwidth narrowing in the emission spectra of the studied LEDs with shortening pulse are found. A linear relation of the junction temperature and carrier temperature of the studied LEDs to their duty cycle is detected. Perceivable changes in color of AlGaInP LEDs under the PWM scheme are observed.

Method for analysing luminous intensity of light-emitting diodes

Light-emitting diodes (LEDs) have built-in lenses that enable spatially limited light beams. The use of lenses increases the luminous intensity levels, but complicates accurate LED intensity measurements. A novel method for determining the luminous intensity of the LED is proposed. The method based on a modified inverse-square law describes the behaviour of an LED in terms of its luminous intensity, the radius of the virtual source and the location of the virtual source. The applicability of the method was tested for 17 LED types with different packages, angular intensity distributions and power levels. When applying the new method to the measurement data, instead of the inverse-square law of the point source, the distance dependence of apparent LED luminous intensity of up to 47% reduced to statistical variation of less than 1%.

Determination of distance offsets of diffusers for accurate radiometric measurements

A method for the determination of the effective measurement plane of spectroradiometer diffusers at various wavelength regions is described. The method is based on the inverse-square law of the distance dependence of the measured signal. The scheme is tested with three planar and one dome-shaped spectroradiometer diffuser at four wavelength bands. The distance offsets of the diffusers determined in the UVA region are from 0 mm to 2.1 mm for the planar diffusers and 6.4 mm for the dome diffuser, whereas the corresponding values in the NIR region are from 0 mm to 7.7 mm and 8.2 mm. The uncertainties of the measured reference plane positions of the diffusers are estimated to be 0.3 mm. If the reference plane position is not properly taken into account in calibration measurements with lamps, large systematic errors may appear when measuring radiation from distant sources. We also investigate the wavelength dependence of the angular responsivity of the diffusers. A clear correlation appears between the wavelength dependences of the distance offsets and the angular responsivity curves.

Multifunctional integrating sphere setup for luminous flux measurements of light emitting diodes

A multifunctional setup based on the absolute integrating sphere method for measuring luminous flux of light emitting diodes (LEDs) is presented. The total luminous flux in 2pi and 4pi geometries and partial luminous flux with variable cone angle can be measured with the same custom-made integrating sphere. The number and area of ports and baffles of the sphere was minimized. The sphere has three ports: a main port, a detector port, and an auxiliary port, located in the same hemisphere. The other hemisphere is free of ports. The main port is used for the calibration of the sphere as well as for the LED under test. Only one absolute calibration of the integrating sphere photometer is needed for measuring LEDs in all three geometries. The spatial nonuniformity correction is needed only for LEDs with low directivity or having significant minor beams. The expanded uncertainty (k=2) for the measurement setup varies between 1.2% and 4.6% depending on the measurement geometry, color, and the angular spread of the LED light beam. A complete calibration procedure of the constructed integrating sphere photometer is presented as well as comparison measurements with a goniophotometer.

Uncertainty analysis of photometer quality factor f_1

The applicability of biased and random error models for determining the uncertainty of photometer quality factor using Monte Carlo simulations is investigated. In the biased error model the contribution to the measured value has the same sign and almost the same magnitude at neighbouring wavelengths, whereas in the random error model contributions of varying signs appear at the neighbouring wavelengths. Both error models are used with real spectral responsivity data and uncertainty budgets of two photometers. Analytical considerations supported by simulation results show that the random error model alone may easily underestimate the uncertainty of . The biased error model is recommended as the basis of the uncertainty evaluation of . It is also demonstrated by real photometer data that the uncertainty of can be highly sensitive to fine details of the photometer spectral responsivity when the biased error model is used.

Determining the irradiance signal from an asymmetric source with directional detectors: application to calibrations of radiometers with diffusers

The energy transfer integral between radiating rectangular and detecting circular parallel plates having nonideal angular characteristics is solved for modeling the distance dependence of the irradiance signal. The equation derived for the irradiance signal, which is called the modified inverse-square law, depends on the position, shape, size, and angular characteristics of the light source and the detector. We apply the new model equation to the calibration of a spectroradiometer to determine accurately the distance offsets, which fix the positions of the effective receiving apertures of diffusers used in the entrance optics of spectroradiometers. Earlier measurement results, e.g., for solar UV irradiance, may include uncorrected effects and can be corrected reliably as diffuser offsets and other correction factors are determined with the modified inverse-square law. Simplifications of the modified inverse-square law for analyzing the distance offsets and the correction factors are studied. Simplified equations for the diffuser offset analysis may be used without losing the accuracy when the cosine response of the diffuser is reasonably good. However, for diffusers whose angular responsivities deviate much from the cosinusoidal angular responsivity, large approximation errors in the diffuser offset values may appear if the angular effects are not properly taken into account.

Estimation of the optical receiving plane positions of solar spectroradiometers with spherical diffusers on the basis of spatial responsivity data

A straightforward method for estimating the position of the optical receiving plane of a spherical, dome-shaped diffuser from its spatial responsivity data is presented. The method is tested with two diffusers, types J1002 and J1015 from CMS Schreder, commonly used in solar UV spectroradiometers. The shift of the receiving plane from its nominal position determines a potential measurement error that occurs when measurements and calibrations are carried out with sources at different distances from the diffuser. Such information is particularly valuable for voluminous solar UV monitoring spectroradiometers that cannot easily be transported to laboratory calibrations. The results suggest that systematic measurement errors are at least of the order of 1%, if the position of the receiving plane is not properly taken into account, thus indicating a need to study the effect more carefully. This method can also be used to minimize measurement errors when designing diffusers.