Often, for legal and safety reasons, it is necessary to specify the amount of chromatic light that is needed. Consider, for example, traffic lights. It is important to ensure that there is an adequate amount of light in the traffic signal so that drivers can easily see the lights, correctly interpret their colors and respond in time to guarantee safety. So now one has the problem of finding an appropriate method of light measurement. Fortunately there are light measuring instruments available.
Radiometers measure the energy emitted or reflected by the source.
This is a strictly physical measurement. Unfortunately the radiometric measurement of light does not represent what people see. All observers would report that the yellow light is much brighter than the red and green. Most would also say that the green is brighter than the red. If our goal is to make the lights approximately equally bright, radiometry is not the way to go.