The most efficient light source is the LED type. It has an efficiency of about 80% of transforming electrical energy into luminous energy compared to about 10% efficiency for CFL (compact fluorescent lamps) and about 2% efficiency for normal incandescent bulbs. However the price of these light sources is inversely proportional to their efficacy so that incandescent lighting is still preferred. CFL have yet another drawback in front of normal incandescent bulbs in what it concerns the spectrum of the light emitted which is still unpleasant for many.

The total amount of emitted light is measured in lumen (a SI, or international system unit), and in relation with the electrical power consumed to generate the light is an indication of the efficacy of the device. Lux (lx) is the SI (international system) unit for measuring the brightness intensity of the light on a certain surface, and represents the amount of light that falls on to that surface.

`1 lx = 1(lm)/m^2`

Although usually when a light bulb is sold it comes only with the electrical power consumption (the so called Wattage), there are cases when their efficacy or their lumens are specified. This happens only because although the efficacy is  much higher for one particular device, its cost is still high (in the case of CFL) or even prohibitive (in the case of LED bulbs).