**5.4.5 Recalibration**

248 Modern Metrology Concerns

The lamps used for total flux in integrating spheres are usually designed with spherical bulbs and circular or distributed filament shapes to provide a uniform spatial output for a

The lamps used for intensity and illuminance are designed with planar filaments that enables the distance between the lamp and detector to be determined reproducibly and accurately. The bulb of the lamp is also shaped, such as a triangular shape, to reduce the inter-reflections inside the bulb that will cause scattered light errors in the measured output of the lamp. Spectral irradiance lamps are usually of the tungsten-halogen design operating with a CCT of approximately 3200 K to increase the amount of UV radiation. The deviation of the irradiance from these lamps from the inverse-square-law should be measured if they

Sources used for luminance and radiance standards are often the irradiance/diffuser combination described in Section 5.3. If a higher radiance is required, ribbon or stripfilament lamps can be used. Present lamps of this type are limited to approximately 2900 K.

The filament of the incandescent lamp is heated by passing an electrical current through it. For measurement standard lamps, this is usually a direct current. The same polarity of the electrical current must be used each time the lamp is operated. To avoid thermal shock to the filament, this current should be applied gradually, over times on the order of a minute or more. When the lamp is calibrated, the current or voltage is adjusted until the spectral radiant output of the lamp reaches the desired operating CCT. This electrical operating point, either a defined operating current or voltage, must be applied whenever the lamp is used to obtain the same radiant output each time the lamp is used. Since all incandescent lamps will age with use, the second of the two electrical quantities (voltage, if the lamp current is the defined operating variable) may be used as a monitor of the ageing of the

The electrical quantities must be measured accurately since the radiant output of an incandescent lamp depends strongly upon the electrical power applied to the lamp. Since the electrical power causes a change in temperature, the spectral distribution of the radiant output changes as well as the absolute amount of output. For lamps operating with a CCT of approximately 2856 K, it has been observed (CIE 149:2002) that the luminous output of the lamp changes approximately 4% for a 1% change in lamp operating voltage and

Incandescent lamps are sensitive to vibration and shock. In addition to breakage of the glass envelope or the electrical feed-throughs, the filament structure is particularly fragile. As indicated above, electrical power should be applied gradually to avoid thermal shock.

The electrical and mechanical properties of a lamp change rapidly when a lamp is first used. Before a lamp is calibrated, it is usually aged by operating at the desired CCT for a period of time to enable the components of the lamp to stabilise, and to determine when the aging rate has stabilised. If measurements are made of the lamp output over time at a constant

are to be used at different distances from that at which they were calibrated.

filament and consequent degradation of the lamp output calibration.

approximately 8% for a 1% change in lamp operating current.

uniform illumination of the sphere walls.

**5.4.3 Electrical characteristics** 

**5.4.4 Mechanical characteristics** 

Every lamp is an individual. Although an estimated aging rate may be determined as indicated above, the lamp may change suddenly for no apparent reason. The best method to monitor the behaviour of a single lamp is to keep records of every use and to check for any change in the electrical behaviour of the lamp. In a laboratory where accuracy is paramount, measurements should be made with more than one standard to ensure agreement between measurements. When the difference between standards becomes larger than acceptable, consideration must be given to a recalibration of the standards. If the lamps have been used extensively and are changing rapidly, it may be necessary to discard the lamp. However, since standard lamps are costly, sometimes difficult to obtain, and the calibration process is costly, a recalibration of existing lamps should be a first consideration.

If the standard lamps are extensively used in the measurement laboratory, consideration should be given to using the purchased calibrated sources as reference measurement standards to calibrate a set of working measurement standards that can be calibrated in the measurement laboratory and used for internal calibrations.
