"units m^-3" is another way of saying "units per m^3", i.e. radiation per cubic metre. For comparison, speed is "m s^-1" and acceleration is "m s^-2". Radiatio

"units m^-3" is another way of saying "units per m^3", i.e. radiation per cubic metre. For comparison, speed is "m s^-1" and acceleration is "m s^-2". Radiation per cubic millimetre would be "units mm^-3".