A x cos u, where a is the actual area of the source, and u the angle between the normal to the surface and the direction of observation. Iu is the luminous intensity in that direction. The upper limit is determined by the luminance required to damage the retina. The reason that our eyes are so easily damaged by looking at the sun is explained when we see that its luminance is 1000 times greater than this maximum level. This is obviously inefficient in terms of the conversion of electrical energy into light.
Incandescent lamps are, however, cheap and easy to work with.
Most of the energy radiated by a fluorescent lamp, on the other hand, is emitted as visible light. This gives fluorescent lamps a relatively high efficacy and good color rendering properties. They have a long life compared to incandescent lamps but are more expensive and more complicated electronically. The light emitted by a more typical fluorescent tube consists of several prominent spectral lines. Daylight consists of a much more even spread of wavelengths.