The intensity of light is measured in different ways. It can be measured in terms of the flow of light going out from a lamp, or the light which falls on a surface. Among the units for these measurements are the candela and the lumen.
Candela as Light Measure
The candela is a unit in the International System of Units which measures the power of the light that is emitted by a light source in one direction. This is known as the luminous intensity. The light from the light source is weighted by a standard model for the sensitivity of the human eye to different wavelengths of light. This function is known as the luminous efficiency function. As a rule of thumb, the light from an ordinary table candle has the luminosity of one candela in all directions, and the unit is sometimes known as one candle-power, or one candle.
Lumen as Light Measure
The lumen is a directional measure of light. Formally, it is defined as the luminous intensity from a light source of one candela into the angle of one steradian (a cone radiating out from a sphere). That is, it is the intensity of a cone of light radiating out from the light source in one direction.
Comparing Light Sources
The candela is a unit for radiation in all directions. The lumen is a unit that measures the light in one specific direction. The amount of light from a light source can be the same, but when measured in candela, the number is the average of light at all points across a sphere surrounding the light source. For a directional light source like a spotlight, the lumen measurement makes more sense.
3 Million Candela vs. 600 Lumens
So how is 600 lumens different from 3 million candelas? One lumen is one candela times one steradian, so the amount of candela divided by the number of steradians gives the directional light intensity. Since there are 12.56637 steradians in a sphere, 6 million candelas equal 238,732.42 lumens per steradian. This means a light source which emits 6 million candelas is much stronger than one which emits 600 lumens. Lightbulbs and LEDs for illumination carry ratings in lumens, according to a recent rule in the European Union. Indicator LEDs are usually rated in candelas.