Photopic lux describes an average response of the three color vision receptors, (cones), and it has been used for years to describe light therapy devices.
A candle at 1 meter distance gives 1 photopic lux of light. Typical room illumination is in the order of 300-500 lux, whereas outdoor light varies from 1500 lux on a cloudy day to 100000 lux on a sunny day.
Under dark conditions of far below 1 lux, we cannot see colors but are still able to distinguish objects thanks to rod receptors. Under such conditions, it makes sense to use scotopic lux, which is the measure for the response of rod receptors. Scotopic lux is not relevant for bright light conditions, such as the ones that exist when the EnergyLight is used.