Radiometric references for optical radiation measurements

The subject, as presented in the title, is very broad, and needs to be clarified: it is essentially the energetic character of radiation that will be considered here, with its important application in photometry. Only detectors used as reference will be considered, and their application to lamp calibration with improved uncertainties.

The quantity to be quantified is therefore the power of the radiation, called "flux", expressed in watts, measured by a detector placed in the path of the radiation. The spectral range usually included in what is somewhat improperly termed "optical radiation" extends from the ultraviolet, 200 nm, to the infrared, 2,500 nm.

The primary detector is an instrument capable of making measurements using only the definition of the International System of Units (SI). Two such instruments are the main focus of this article. Since the late 1980s, the cryogenic radiometer has been the best reference, both for its low uncertainty and for its wide spectral range (200 nm to 15 µm, and even beyond), which largely covers the field of optical radiation. A new generation of photodiodes promises to be just as accurate a competitor, and much easier to use, in a smaller spectral range from 400 nm to 900 nm.

As these metrology laboratory items are difficult to use, they are replaced by transfer instruments and specialized instruments adapted to the wide variety of quantities used in radiation measurement. Some methods for calibrating other detectors, measuring quantities other than flux (illuminance, luminance) and applications to radiation source calibration measurements are presented, at least in principle.

In metrology, it is impossible to talk about a "reference" without mentioning the question of the confidence to be attributed to it. Uncertainties will therefore be studied for each reference instrument and transfer method.

Readers can refer to a general article [R 6 410] Radiometry, photometry in Techniques de l'Ingénieur.