If the instrument is the human eye then you need to consider the visible wavelengths and you will need to weigh each of the wavelengths according to the human eye sensitivity curve. If the instrument is a silicon detector then you would use its response curve. When doing research on light you will come across many different units being used by various light manufacturers. All the units are trying to describe how much light their devices emit. Some units refer to the energy of the light source and others to the power.
Many units take only the human eye sensitivity into account.
The light units can be even more confusing when you consider that some light sources, such as a common light bulb, launch light in all directions while others, such as a laser, concentrate the light into narrow beams. Rather than confuse you even more by going into a long discussion of what the various units mean, I'm going to try to simplify the problem. Let's just assume that each light source has a distinctive emission spectrum and a certain emission geometry. You will have to treat each light source differently, according to how it is used with a specific communications system. In optical communications you only need to consider the light that is sent in the direction of the detector.