"NIST-traceable" metrology labs purchase calibrated transfer standard detectors directly from the National Institute of Standards and Technology (NIST) in Gaithersburg, MD. From 400 to 1100 nm, this transfer standard is a Hamamatsu S1337-1010BQ photodiode, a 10 x 10 mm planar silicon cell coated with synthetic quartz. The photodiode is mounted behind a precisely measured 7.98 mm diameter circular aperture, yielding an active area of 0.5 cm2. The responsivity is usually given every 5 nanometers.
The calibration labs then use this transfer standard to calibrate their intercomparison working standards using a monochromatic light source. These working standards are typically identical to the equipment that will be calibrated. The standards are rotated in the lab, tracked over time to monitor stability, and periodically recalibrated.
Detectors are most often calibrated at the peak wavelength of the detector / filter / diffuser combination using identical optics for the intended application. The key to this calibration transfer is a reliable kinematic mount that allows exchangeability of detectors in the optical path, and a stable, power regulated light source. Complete spectroradiometric responsivity scans or calibration at an alternate wavelength may be preferred in certain circumstances.
Although the working standard and the unknown detector are fixed in precise kinematic mounts in front of carefully regulated light sources, slight errors are expected due to transfer error and manufacturing tolerances. An overall uncertainty to absolute of 10% or less is considered very good for radiometry equipment, and is usually only achievable by certified metrology labs. An uncertainty of 1% is considered state of the art, and can only be achieved by NIST itself.
Expanded uncertainties of NIST photodiode standards: