**5.4.3 Electrical characteristics**

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 filament and consequent degradation of the lamp output calibration.

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