The design and construction of a predictable quantum efficient detector (PQED), suggested to be capable of measuring optical power with a relative uncertainty of 1 ppm (ppm = parts per million), is presented. The structure and working principle of induced junction silicon photodiodes are described combined with the design of the PQED. The detector uses two custom-made large area photodiodes assembled into a light-trapping configuration, reducing the reflectance down to a few tens of ppm. A liquid nitrogen cryostat is used to cool the induced junction photodiodes to 78 K to improve the mobility of charge carriers and to reduce the dark current. To determine the predicted spectral responsivity, reflectance losses of the PQED were measured at room temperature and at 78 K and also modelled throughout the visible wavelength range from 400 nm to 800 nm. The measured values of reflectance at room temperature were 29.8 ppm, 22.8 ppm and 6.6 ppm at the wavelengths of 476 nm, 488 nm and 532 nm, respectively, whereas the calculated reflectances were about 4 ppm higher. The reflectance at 78 K was measured at the wavelengths of 488 nm and 532 nm over a period of 60 h during which the reflectance changed by about 20 ppm. The main uncertainty components in the predicted internal quantum deficiency (IQD) of the induced junction photodiodes are due to the reliability of the charge-carrier recombination model and the extinction coefficient of silicon at wavelengths longer than 700 nm. The expanded uncertainty of the predicted IQD is 2 ppm at 78 K over a limited spectral range and below 140 ppm at room temperature over the visible wavelength range. All the above factors are combined as the external quantum deficiency (EQD), which is needed for the calculation of the predicted spectral responsivity of the PQED. The values of the predicted EQD are below 70 ppm between the wavelengths of 476 nm and 760 nm, and their expanded uncertainties mostly vary between 10 ppm and 140 ppm, where the lowest uncertainties are obtained at low temperatures.
Light emitting diode (LED) lighting is becoming more and more popular, as incandescent lamps are being phased out globally. LEDs have several advantages over incandescent lamps, including energy efficiency, robustness, long lifetime, and good temporal stability. The three latter features make LEDs attractive candidates as new photometric standards. Because the spectra of white LEDs are limited to the visible wavelength range, a novel method for the realization of photometric units based on the predictable quantum efficient detector (PQED) can be utilized. The method eliminates the need of photometric filters that are traditionally used in photometry, and instead relies on carrying out the photometric weighting numerically based on the measured relative spectrum of the source. The PQED-based realization simplifies the traceability chain of photometric measurements significantly as compared with the traditional filter-based method. The measured illuminance values of a white LED deviate by only 0.03% when determined by the new and the traditional methods. The new PQED method has significantly lower expanded uncertainty of 0.26% (k 5 2) as compared with that of the traditional filter-based method of 0.42% (k 5 2). Furthermore, when filtered photometers that measure LED lighting are calibrated using LED lamps as calibration sources instead of incandescent lamps, a significant decrease in the uncertainty related to the spectral mismatch correction can be obtained. The maximum spectral mismatch errors of LED measurements decreased on average by a factor of 3 when switching from an incandescent lamp to an LED calibration source.
The predictable quantum efficient detector (PQED) is intended to become a new primary standard for radiant power measurements in the wavelength range from 400 nm to 800 nm. Characterization results of custom-made single induced junction photodiodes as they are used in the PQED and of assembled PQEDs are presented. The single photodiodes were tested in terms of linearity and spatial uniformity of the spectral responsivity. The highly uniform photodiodes were proved to be linear over seven orders of magnitude, i.e. in the radiant power range from 100 pW to 400 µW. The assembled PQED has been compared with a cryogenic electrical substitution radiometer with a very low uncertainty of the order of 30 ppm. Experimental results show good agreement with the modelled response of the PQED to optical radiation and prove a near unity external quantum efficiency.
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