Development and Characterization of the Predictable Quantum Efficient Detector, and Its Applications in LED Photometry

Radiometry is the science of studying and measuring electromagnetic radiation. It has many uses ranging from industry applications to fundamental research. Recently, the Predictable Quantum Efficient Detector (PQED) was introduced as a convenient method to quantify radiant flux, i.e. optical power, in the visible wavelength range. The PQED enables absolute measurements with an uncertainty around 0.01% even at room temperature. The high quantum efficiency of the PQED is achieved with the use of custom-made photodiodes that are mounted into a trapping configuration. The PQEDs currently in use have photodiodes that have a thermally grown SiO2 layer on a p-type silicon substrate. Such detectors were characterized for spatial uniformity, reflectance, responsivity and dark current properties. The obtained results show that the PQED can serve as the primary standard of optical power and even has the potential to replace more expensive and cumbersome cryogenic radiometers in the visible wavelength range. A new type of PQED photodiode was developed, where atomic layer deposition is used to grow an Al2O3 layer on a n-type silicon substrate. Two sets of photodiodes with varying doping concentrations were manufactured. Their responsivity was modelled using a 3D model of the photodiode. A novel method to obtain input parameters for the modelling was developed. The n-type PQEDs were characterized similarly as the p-type detectors. Results indicated that the n-type PQED is a promising alternative for the p-type. Thus, the manufacturing of PQEDs is no longer dependent on the availability of particular materials and processing. The PQED can be used to calibrate conventional reference photometers. A new method for the realization of photometric units was developed, which exploits the PQED more directly. In the method, the PQED is used together with a precision aperture, but the traditionally used photometric filter is omitted. Instead, the photometric weighting is done numerically. The method is applicable to sources that emit only little light outside the visible wavelength range, such as white LED lamps. The main advantages of the new method are the reduced uncertainty and simplified traceability chain. In many of the applications, the PQED is used together with a precision aperture. A new method was developed to determine the area of an aperture that is mounted to the PQED – without dismantling the assembly. It exploits previously developed method where the aperture is scanned in front of a Gaussian laser beam and the light passing through is measured at with an integrating sphere. In the new method, PQED itself is used to measure the flux passing through the aperture. The diffraction properties of the PQED and aperture assembled were studied using numerical calculations. The results indicated negligible effects due to diffraction.