Four common descriptions of light’s physical quantity:
1. Luminous intensity is a measurement of the wavelength-weighted power emitted by a light source in a given direction. The SI unit of luminous intensity is the candela, abbreviated cd. Some people still used the candle to express the luminous intensity, which was outdated; however, in 1940 (some saying in 1948) the word “candela” has been used. And the “candle”, abolished after 1968, became just a candle.
2. Luminous flux is the quantitative expression of light source emitting per unit time and its standard unit is lumen (Lm).
3. Illuminance is the total luminous flux incident on the 1m2 surfaces, and the standard unit is lux (Lx).
4. Luminance, the luminous intensity of light per unit area in a certain direction, describes that the luminous flux is emitted for a unit solid angle. The unit is called nit (nt).
Generally, the luminous intensity is a way to measure candela of LED flashlights. In early days the brightness of LED was low, so the unit of measurement was micro candela, abbreviated mcd. Later on, many of them came out with over a thousand and over ten thousand mcd’s, therefore 10,000mcd=10cd.
With the same tube core, the mcd of 5mm LED is lower than 10mm LED due to the good concentration and small light spot. In fact, their luminous flux is apparently the same.
A norm of the measurement of luminous efficacy is how many lumens are emitted by electric power per watt. In theory, the maximum possible efficacy is 683lm/W (and it needs the 0.54 micron wavelength light in olivine). An incandescent light bulb is about 10lm/W and a good white LED should be possible to achieve above 200 lm. Due to the phosphor material which is added around the LED, the LED which is originally blue turns into white. Therefore, there are two peaks in the emission spectrum of the white LED. A bad manufacture process could possibly make the LED blue.
Measurement of luminous intensity: Firstly, find a digital light meter and make upward the sensor on the floor. Secondly, prepare a flashlight or a LED to illuminate it. Lastly, the maximum data you get is cd and multiply it by 1,000, which will be mcd number.
The conversion between the wattage and the mcd.
Luminance has the light character and wattage has electric character.
Wattage is the unit of electric power of electromagnetic wave from the visible and invisible light emitted by LED and the lumen refers to the unit of electric power of the visible light. Therefore, luminous efficacy means the ratio of the lumens and the electromagnetic wave emitted by the LED.
The lumen, in the solid angle, is the luminous intensity and its unit is candela. Generally speaking, the luminous intensity of the LED means the maximum of the luminous intensity. In order to describe its character of brightness, there is still a technical parameter, beam divergence.
The candela is the unit for the luminance, which is candela per square metre and is also called nit (nt).
There is no connection between the wattage and the luminance.
With 1W white light, our LED is about 13~18 lumens and the LED of America is 25 lumens. It just sees who can make more light by transforming the electricity. The reason that the LED can economize on electricity is that the LED’s efficiency of changing the electricity into the light is more than other light materials, which can saving the energy source.