The Physikalisch-Technische Bundesanstalt has further improved and extended its spectral responsivity scale based on the laser-operated radiation thermometry cryogenic radiometer. Five laser wavelengths in the ultraviolet (UV) (238 nm to 400 nm) and another fifteen in the visible and near-infrared (NIR) (400 nm to 1015 nm) almost cover the entire spectral range where silicon photodiodes can be used in air. The lasers employed are a Kr+ laser, an Ar+ laser with optional intra-cavity frequency-doubling, and a continuously tuneable Ti:sapphire ring laser. The total relative standard uncertainty of the spectral responsivity at the laser lines is u = 10-4 in the visible and NIR below 950 nm and u = 10-3 in the UV. With an improved model for interpolation between the laser wavelengths, a continuous scale of spectral responsivity has been established in the wavelength range 400 nm to 1015 nm.

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.

The Physikalisch-Technische Bundesanstalt (PTB) has developed a spectral responsivity scale in the spectral range 0.95 µm to 1.6 µm. The responsivity scale was established by calibrating InGaAs photodiodes against the PTB radiation thermometry cryogenic radiometer (RTCR) using several laser sources: a Ti:sapphire laser (0.850 µm to 1.02 µm); a Nd:YAG laser (1.064 µm); and two diode lasers (1.31 µm, 1.55 µm). At intermediate wavelengths the responsivity was interpolated with the help of thermal cavity detectors of high emissivity. In this way the high accuracy of the scale at the laser wavelengths can be maintained over almost the entire spectral region. A detailed discussion is given of the uncertainties achieved in the calibration with the RTCR and the interpolation with the thermal detector.

A cryogenic-radiometer-based calibration facility utilizing monochromatized radiation of an argon arc plasma has been put into operation at the PTB. The high radiance of the plasma source allows us to improve the accuracy of the UV spectral responsivity scale between 200 nm and 410 nm and to achieve relative standard uncertainties of about 0.1% to 0.2%. Comparison with the PTB laser-based cryogenic radiometer at five laser lines in the UV indicated excellent agreement.

The Physikalisch-Technische Bundesanstalt (PTB) has established spectral responsivity standards for photodetectors in the wavelength range between 200 nm and 400 nm based on cryogenic electrical-substitution radiometers. In order to obtain standards over a continuous wavelength range with low uncertainty, combined use is made of both the dispersed synchrotron radiation of the storage ring BESSY I and quasi-monochromatic laser radiation. Taking advantage of the specific properties of the monochromatized synchrotron radiation and the laser radiation, relative standard uncertainties between 10 -3 and 5 10 -3 are achieved in the calibration of trap detectors in the spectral range 400 nm to 200 nm.