This paper investigates the technological challenges and solutions to defining a reproducible standard for light together with its measurement, from the 18th to 21st century. The arrival and subsequent need to quantify gas lighting led to development of improved measurement methods, which in turn demanded more reliable standards of light, evolving from candle and flame standards to incandescence, and eventually the current radiometric definition. The progression is similarly traced from the plethora of early comparative visual photometry methods through to modern semiconductor instrumentation. The problem of heterochromatic photometry is also examined.