:: : You can " approximately " convert electric field strength ( volts / meter ) to radiant intensity ( watts per square meter ) by squaring and dividing by the antenna impedance; in practice, you need to account for the antenna matching properties, impedance of free air, antenna directivity, and so on.
22.
The lumen is defined as amount of light given into one steradian by a point source of one candela strength; while the candela, a base SI unit, is defined as the luminous intensity of a source of monochromatic radiation, of frequency 540 terahertz, and a radiant intensity of 1 / 683 watts per steradian . ( 540 THz corresponds to about 555 nanometres, the wavelength, in the green, to which the human eye is most sensitive.
