Characterization of diffusers and light-emitting diodes using radiometric measurements and mathematical modeling

Entrance diffusers enable good cosinusoidal angular responsivities of optical detectors. However, they also affect the position of the apparent detector receiving plane (reference plane). In this thesis, the reference planes of four spectroradiometer diffusers have been investigated and found to be located, in most cases, inside the diffuser. The reference plane shift should be taken into account in the calibration of the spectroradiometer equipped with such a diffuser. If the reference plane shifts are omitted, the measurement results may become systematically erroneous. The reference plane distance offsets correlate with their angular responsivities. Thus, with available angular responsivity data of a diffuser, the need to measure accurately the reference plane offset of the diffuser can be evaluated. The results obtained can be utilized in solar ultraviolet irradiance measurements. In the thesis, new kinds of measurement methods for characterizing photometric properties of light-emitting diodes (LEDs) have also been developed. The modified inverse-square law method has been developed for analyzing LED illuminance at varying distances. The method has been tested for several LEDs with different packages, colors and power levels. The method gives information about the LED luminous intensity, the LED directivity and the size and location of the LED virtual image source. The new method developed is useful in designing LED-based luminaires when photometric characteristics of LEDs become more reliably predicted. Pulse-width modulation (PWM) is often used to dim LEDs. An LED is driven at pulsed current and by changing the duty cycle of the current the LED brightness can be controlled. The influence of the PWM on spectral, thermal, and colorimetric properties of low-power, epoxy-encapsulated LEDs have also been investigated. In the thesis, a measurement system, with which the LED temperature at different duty cycles could be determined, was constructed. Using the experimental set-up, the peak-wavelengths and bandwidths of LEDs under the PWM control were evaluated. A blueshift of the peak wavelength and bandwidth narrowing of the LEDs were found. The changes in the emission spectra of the LEDs were concluded to be caused by the changes in the LED temperature. Apparent color shifts, which can be perceived by the human eye, were observed in emitted colors of two LEDs. The results indicate that it would be useful to include information on optical and thermal properties of pulsed LEDs more in detail in the datasheets by the LED manufacturers