Language of Light
3.Instrumentation
3.2 PHOTOMETER
3.2.2 CALIBRATION METHOD
Beside f1 value, the calibration method of the photometer is also an important factor when deciding its suitability to a specific application. For example, a photometer with a relative large f1 value can still achieve good accuracy when the measured light source and the standard lamp used during the calibration process is similar.
There are two basic methods of calibrating photometers. The first and the most common method is using a standard lamp (usually tungsten lamp). These lamps are certified and traceable to national standard laboratories/institutions. The photometers will be adjusted until the measurement reading matches the certified output of the standard lamp. The second calibration method is to use standard detectors. Such detectors have built-in sensors where the spectral responses perfectly match the CIE V λ curve. In such calibrations, a lamp is still required but output can be varied but must be stable. The standard detector first measures the output of the lamp, and is substituted by the photometer and will be adjusted until the measurement give similar readings as the standard detector. Such detectors can also be certified and traceable to national standards.
3.2.2.1 COLOUR CORRECTION FACTOR
The correction of the detector-filter combination to the CIE V λ curve is generally poor at the end of the visible spectral range. Hence, the colour temperature of the lamp used during calibration is critical. As most of the photometers are calibrated by a tungsten lamp, measurement of incandescent, halogen searchlights and sunlight generally give good accuracy. However, these photometers are not suitable for measurement of monochromatic light or narrowband emitters, e.g., blue and white LEDs. Measurement error will also be significant in discharge lamps, e.g., luminescent tubes, which show clear peaks (i.e. spectral lines) in the visible spectrum.
For this reason, modern photometers have incorporated a Colour Correction Factor feature to compensate the error caused by this spectral response difference between the sensor and the CIE Vλ curve. The CCF value can be calculated when both the spectral response of the sensor and the spectral power distribution of the light source is known. An alternate and easier method is to transfer the measurement data of a primary standard (for example, data taken from a spectroradiometer) to the photometer is by varying the CCF value. CCF can also be used as a user-calibration feature, which is particularly useful if in-house standards' traceability is necessary.
