(d) Total luminous flux entering the eye.Total luminous flux is the total brightness producing capability of a radiating source. Or, it is the measurement of the total energy entering our eyes that produces the sensation of vision. (a) This cannot be the correct answer because all energies cannot be sensed by our eyes.(b) This cannot be the correct answer because all wavelengths do not produce any sensation in our eyes.(c) This cannot be the correct answer because all wavelengths contributing the radiant flux are not always visible to our eyes. There may be a large radiant flux yet there may not be any sensation of vision.