General V(?) mismatch index: History, current state and new ideas

The general V(?) mismatch index f(1)(') specifies the mismatch between the relative spectral responsivity of a photometer, and the spectral luminous efficiency function for photopic vision, V(?). A short review of its historical development explains the reasons for the current definition and which adjustments may be helpful in the future. The properties of the current definition are described in detail. It is very likely that photometers will be calibrated with a white light-emitting diode (LED) light source as the reference in the future. This might involve the need for a more appropriate definition of the general V(?) mismatch index, either by using a different normalisation in f(1)' for the relative spectral responsivity of the photometer or by introducing a different type of function for assessing the mismatch. On the other hand, the measurement of coloured LEDs is also becoming increasingly important. So, is a single general mismatch index for white and coloured light sources sufficient?