Melcon

Useful Definations

Luminous Intensity (Candela-cd=1000, milicandela-mcd)

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 (yellowish-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 without the 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 intensity of nonfocusing light sources but this can lead confusion . For example: a LED made by CREE can have up to 100 Cd while a 100 Watt incandescent light bulb has around 105 Cd. Does that mean that the LED is a bright as a 100 W light bulb? The answer comes from view angle: Cree 80 deg, light bulb: 360 deg. vs

To clarify this problem we will give some examples of light intensity in day to day life (in candelas per square meter):

Good street light 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 which we explain below.

Illuminance (lux)

Illuminance is the total luminous flux which falls on a surface and it shows the intensity of the incident light. The value is affected by the wavelength of the emitted light and the distance between the light source and the illuminated area.
The human eye is most sensitive to light that has a wavelength of around 550nm (amber) and thus an amber light source will have more lux than a blue one (for example). This is called the luminosity function.
The larger the distance between the light source and the illuminated area the lower the illuminance will be. Below we show some examples of optimal illuminace for day to day activities :

Activity	Illumination on task surface(lux)
Public areas with dark surroundings	20 - 50
Working areas where visual tasks are only occasionally performed (storage rooms)	100 - 150
Easy Office Work, Classes	250
Normal Office Work, PC Work, Study Library, Groceries, Show Rooms, Laboratories	500
Supermarkets, Mechanical Workshops, Office Landscapes	750
Normal Drawing Work, Quality control	1,000
Detailed Drawing Work, Very Detailed Mechanical Works (watchmaking)	1500 - 2000
Performance of visual tasks of low contrast and very small size for prolonged periods of time	2000 - 5000
Performance of very prolonged and exacting visual tasks	5000 - 10000
Performance of very special visual tasks of extremely low contrast and small size	10000 - 20000

Viewangle

The viewangle shows the angle of radiation for a light source. For example a view angle on 30 deg means the light has a shape of a cone and its borders form a 15 deg angle with the center axis. A change in the view angle affects the luminous intensity of an LED (candela) but not the luminous flux (lumen).

Viewing angle is actually the most important element in the performance of a light source. A very wide viewing angle means that a large percentage of light ends up going in non-useful directions ( for example: up). Incasing the light source and using a reflector has limited efficiency.

By taking into the account both viewing angles and lumen output the comparison between LEDs and other light sources looks like this:

Light source	Lm	Lm/W	Viewing angle	Usefull viewing angle*	Useful lm
CREE X-RE R2 White LED	242	92	90	100%	242
100 Watt incandescent	1200	12	360	33%	396
25 Watt Halogen	260	9	360	33%	85.8
15 W T8 neon	1350	90	360	33%	445.5

^* without case or reflector

Viewing angle and the environment

Next time when you go out at night look up in the sky. Do you see stars, or the Milky Way? In you live in a big city the answer is: only a white glow. For people that live outside a big city in North America, Europe, India, Japan or China the answer will be: a white glow and a few stars. The Milky Way can only be seen from remote regions or non-developed countries.