Photometry Calibration

Photometric calibration compares instruments that measure light according to the sensitivity of the human eye against a reference with documented traceability. Photometric quantities differ from radiometric ones: the measurement is weighted by the eye's spectral sensitivity curve (V-lambda).

The aim is for lighting measurements to be reliable. Workplace lighting levels, road and tunnel lighting, screen brightness and product lighting declarations rest on this measurement.

• Lux meters (illuminance meters)
• Luminance meters
• Spectroradiometers
• UV radiometers
• Light-source test systems and integrating spheres
• Colour-measuring instruments (colorimeters)

• Illuminance (lux) accuracy
• Luminance (cd/m²) accuracy
• Spectral-sensitivity mismatch (V-lambda match, f1' error)
• Cosine-response error (f2)
• Linearity
• Behaviour dependent on colour temperature
• Dark current and zero drift

The calibration interval is not fixed by any standard and there is no general rule such as "once a year". The interval is set by weighing together the manufacturer's recommendation, how often and under what conditions the instrument is used, how critical the measurement is, the drift shown by the instrument's previous calibration results, and any legal requirement. ILAC-G24 / OIML D10 describe the methods for setting it. The sound approach is to start with a reasonable interval for the first period and then lengthen or shorten it in the light of subsequent results: an instrument that proves stable can have its interval extended, one that drifts has it shortened.

• ISO/IEC 17025
• CIE S 023 / ISO/CIE 19476 (photometer characterisation)
• CIE 69 (photometer heads)
• DIN 5032-7 (photometer classes)
• JCGM 100 (GUM — uncertainty)

• ISO/IEC 17025
• CIE S 023 / ISO/CIE 19476 (photometer characterisation)
• CIE 69 (photometer heads)
• DIN 5032-7 (photometer classes)
• JCGM 100 (GUM — uncertainty)

1. 1Receipt of the Instrument and Initial InspectionThe cleanliness and physical condition of the instrument are checked. On volumetric glassware, cracks and wear are examined; on flow meters, residues.
2. 2Conditioning of Environment and FluidThe instrument and the fluid to be used are brought to equilibrium under laboratory conditions. Temperature directly affects the density of the fluid and therefore the result.
3. 3Preparation of the Reference SystemA reference flow meter, gravimetric rig or acoustic calibrator with documented traceability is put into service.
4. 4Selection of the Measurement PointsPoints representing the instrument's operating range are selected; in flow calibration the low-flow region is addressed separately because error grows there.
5. 5Taking the MeasurementsAt each point, repeated measurements are taken once a steady flow or steady signal has been established.
6. 6Application of CorrectionsTemperature, pressure and density corrections are applied; in the gravimetric method the air buoyancy correction is taken into account as well.
7. 7Uncertainty BudgetThe reference uncertainty, repeatability, temperature stability and reading resolution are combined.
8. 8Issuing the CertificateThe results and uncertainty values are entered on the certificate; a statement of conformity with the decision rule is added if requested.

Every calibration result is given together with a measurement uncertainty; a result reported without its uncertainty is incomplete. The uncertainty is budgeted following the GUM (JCGM 100) approach: the uncertainty of the reference, repeatability, resolution, environmental effects and the stability of the instrument are combined. The certificate states the expanded uncertainty with the coverage factor (usually k = 2) and the approximate level of confidence. The smallest uncertainty a laboratory can report is limited by its calibration and measurement capability (CMC) in the accredited scope; no better uncertainty may be declared.

• The uncertainty of the reference light source and the photometer
• Spectral mismatch (the instrument's deviation from the V-lambda curve)
• The colour-temperature difference between the calibration source and the source in use
• Cosine-response error
• Distance-measurement uncertainty (the inverse-square law)
• Ambient temperature and detector stability
• Stray light and reflections

• Laboratory details and certificate number
• The customer and identification of the instrument calibrated: make, model, serial number
• Date of calibration and date of issue of the certificate
• The calibration method applied or reference to the procedure
• Reference standards used and traceability information
• Ambient conditions during calibration (temperature, humidity and, where relevant, pressure)
• Measurement points, reference values and instrument readings
• The deviations calculated
• Expanded measurement uncertainty and coverage factor
• The personnel who performed and who approved the calibration
• The colour temperature of the light source used in calibration
• The measurement points and distances
• The spectral-mismatch (f1') value, if any
• The instrument's photometer class

A calibration certificate is NOT a certificate of conformity. Calibration measures and reports the deviation of the instrument; it does not correct that deviation and does not say that the instrument is "correct". To decide whether an instrument is fit for use, a tolerance (acceptance criterion) must be defined — and it is the user who sets that tolerance, not the laboratory. If a statement of conformity is requested, ISO/IEC 17025 clause 7.8.6 requires a DECISION RULE to be agreed in advance, defining how measurement uncertainty is taken into account in the decision; a conformity statement made without regard to uncertainty is misleading. The certificate also shows the condition AT THE TIME of calibration: later use, transport or shock can change the instrument's condition, and the certificate does not cover that. Where a next calibration date appears on the certificate, it is not the laboratory's decision but a reminder entered according to the interval stated by the customer.

Calibration is carried out under ISO/IEC 17025, which requires the laboratory's technical competence, impartiality and metrological traceability. Accreditation is granted per METHOD and per MEASUREMENT RANGE: a quantity or range that does not appear in the laboratory's scope annex is not accredited, and no certificate bearing the accreditation mark may be issued for that measurement. The scope also states the smallest uncertainty the laboratory can offer (CMC); no better uncertainty may be declared. EUROLAB holds internationally recognised accreditation under ISO/IEC 17025, granted by accreditation bodies that are signatories to the ILAC mutual recognition arrangement; whether your instrument can be calibrated within accredited scope is confirmed in writing at the quotation stage.

• Volume Calibration
• Flow Calibration
• Acoustic Calibration