The eye is most sensitive to light in the yellow and green area of the spectrum, specifically 555 nm, and is least sensitive in the blue and red regions of the spectrum. This response is known as the photopic response. The scotopic response is the response of human dark-adapted vision to light. (See figure 1.)

Figure 1. Photopic and scotopic response of the human eye.

Therefore, in order to convert a radiometric measurement into a photometric value it is necessary to correct the spectrum by the photopic curve and multiply the result by 683. The total light output from a light source is called the total radiant flux and is measured in Watts.