Metrological traceability of light measurement
In practice, SI units are disseminated with a guarantee of metrological traceability by calibration. The dissemination is based on a chain of measurements that can be clearly traced back to a primary representation of SI units (more about Metrology). Since the beginnings of light measurement technology, reference light sources (so-called calibration standards) have been almost exclusively used, both for the dissemination of units and for the calibration of measurement systems. In most applications their spectral distribution corresponds approximately to that of a Planck radiator at temperature 2,856 K, “standard illuminant A”.
In light measurement technology, measuring instruments and light sources are calibrated by two basic methods:
- Direct calibration: a detector is calibrated with a reference source (standard), or conversely a light source with a reference detector.
- Indirect calibration: a detector is calibrated with another detector by comparative measurement to the same light source (transfer standard) by the substitution method.
Comparability due to unbroken chain of calibration
To ensure the comparability of measurement results it is important to ensure an unbroken calibration chain, from the standard used to a primary standard. For this reason, Instrument Systems uses so-called reference standards calibrated by a metrology institute (e.g. PTB). Since the number and service life of reference standards is limited, for daily lab operations Instrument Systems regularly creates so-called working standards. These are used routinely in the test lab for the calibration and testing of our products (e.g. spectroradiometers and light sources) (Figure: Traceability).
Calibration versus testing
In a calibration the measurement result of a device is set in relation to a traceable standard for the relevant measurand by comparison, and the deviation and respective measurement uncertainty determined. The deviation can be reduced by means of an adjustment. A calibrated instrument may be named in a continued traceability chain.
In a test the correctness of the measurement result of an instrument is verified by comparison with a traceable standard for the relevant measurand. The correctness applies to the measurement method used within the limits of the measurement uncertainty determined for the measurement setup. A tested measuring instrument delivers traceable, comparable results.
Reliable results over a long period of time
Due to environmental influences, measuring instruments are subject to changes that can affect the measurement results. For this reason, each instrument should regularly go through an audit in which compliance with the specified tolerance interval is checked. Such an audit is carried out, for example, with a set of calibrated light sources of type ACS at the customer's site (Figure Audit). Based on previously defined test criteria, the right time for recalibration at the manufacturer's can thus be precisely determined. High-quality and stable measuring devices achieve reliable results over very long periods of time, which can be assured by means of regular audits.
