The journey of light :
Wavelenght :
Light is a form of energy and , acording to theory , it travels in waves . The space occupied by a complete wave is the wave length . Visible light has a wavelength between 400 and 700 nanometres (nm) and each value in this interval coresponds with a color . The human eye has receptors that are sensible to specific wavelentghs and in this way we see colors.
As it can be seen in the image to the right , RED has a wavelenghts between 780 and 622 nm , BLUE between 492 and 425 and so on .
What is a steradian ?
In simple terms a steradian represents an area of 1 m² on the surface of a sphere that has a radius of 1 m or , in other words , an area of 1 m² at a distance of 1 m ( see the figure to the right ).
Candela :
The candela is the foundation unit for the measurement of visible light. It is one of the seven foundation SI units. It's formal definition is:
The candela is the luminous intensity, in a given direction. of a source that emits light that has a wavelength of aprox. 555 nm (yelowish-green ) and that has a radiant intensity in that direction of 1/683 watt per steradian. One candela is 1000 milicandelas or 1 cd = 1000 mcd .
The candela value is independent of distance. One can think of it as the emission from the lamp, which then loses interest in what happens to the photons it has ejected. The candela is mostly used when dealing with focused light - for LEDs , flashlights or spots .
From candelas to lumens :
The candela can be used for measuring the light intensitiy of light sources that don't focus but this fact can create confusions . For exemple : a LED made by Lumileds can have up to 800.000 mcd (800 cd ) and a 100 w lightbulb has only around 150.000 mcd (150 cd ) . Does that mean that LEDs can be five times brighter than a 100 W light bulb ? No way , at least , not for a few years .
To clarify this problem we will give some examples of light intensity in day to day life ( in candelas per square metre ) : good street light 1,2 CD , corridors 3-6 cd , living rooms 3-12 cd , office 12-18 cd , drawing office 18-30 cd , shop windows 60-300 cd .
Except for focusing light sources light is measured in lumens :
What is a lumen ?
The lumen can be defined as the luminous flux emitted into one steradian by an isotropic point source having a luminous intensity of 1 candela . In other words a 1 candela (1 lm/sr) light source will produce 1 lumen per square metre at the distance of 1 metre (see figure to the right ) .
What is viewangle ?
For example a viewangle of 60º is the orange surface in the left figure .
What does lumens/watt mean ?
Very simple : how many lumens are produced for one watt of power. For example an 100 W incandescent lightbulb has 1700 lumens or 17 lumens per watt (lumens/watt) and a LUXEON LED produced by Lumileds has between 25 and 80 lumens/watt.
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