Photometry maps to characterize incandescent and halogen lamps

In lamp development and quality control we are often faced with small differences between lamps in a batch or changes in lamp characteristics during life that we want to understand. The paper describes a practical and powerful tool to guide the interpretation, viz. a photometry map in which the luminous flux of a lamp is plotted versus the electrical power input in a relative way. Due to the physical properties of tungsten, there are only a limited number of directions in such a plot. At constant voltage, two principal directions are shifts related to variations in filament dimensions that go in the direction of +2.5% increase in luminous flux for +1% power increase, and shifts related to variations in convective losses that go in the direction of 8% drop in flux for 1% increase in power. Other relevant directions are related to external resistive circuit elements and blackening. A number of cases in which these plots have been applied are described. They show that the maps give clues to complex lamp phenomena. However, the agreement between the experimentally observed shifts and some predicted values is unsatisfactory.