Light is a form of energy and, according to theory, it travels in waves. The space occupied by a complete wave is the wavelength. Visible light has a wavelength between 400 and 700 nanometers (nm) and each value in this interval corresponds with a color. The human eye has receptors that are sensible to specific wavelengths and in this way we see colors.
Examples:

• RED: 622-670 nm
• GREEN: 520 nm
• BLUE: 425-492 nm
• YELLOW: 590 nm
  
  

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.

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.

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.

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 meter at the distance of 1 meter (see figure to the right ).
Examples:

• 5mm/Superflux LED: 5lm(max)
  
• Luxeon Rebel white LED: 145lm
  
• Seoul P4 white LED: 240lm@1000mA
• Cree X-RE R2 white LED: 242lm@1000mA
  
• 25 Watt halogen: 260lm
• Cree M-CE white LED: 752lm@2800mA
• 100 Watt incandescent: 1200lm
• T8 15 Watt tubular neon: 1350lm
  

Very simple: how many lumens are produced for one watt of power. The result shows the overall efficiency of the light source. At the bottom of the efficiency scale is the incandescent light bulb.

• CREE X-RE R2 LED: 92lm/W
  
• Seoul P4 white LED: 88 lm/W
• 5mm/Superflux LED: 70 lm/W(max)
  
• Fluorescent Tube (neon): 60-90 lm/W
• 60 Watt incandescent light bulb: 11lm/W
• 25 Watt halogen: 9lm/W

Mainly used for white light. At low color temperature the white light has a yellowish tint while at high temperature it has a bluish tint.

Examples:

• Candle: 1800K
  
• Incandescent light bulb(warm white): 2800K
  
• Halogen(Warm White): 3400K
  
• Warm white LED: 2600-3500K
  
• Sun light (pure white color): 5500-6500K
• Neutral white LED: 3700-5000K
• Fluorescent Tube: 6000-8000K
• Cool White LED: 5000-10000K
  
  

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 but not the luminous flux.

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:

*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.

The reason?  Bad lighting design and use of sources with ultra wide view angles creates light pollution, light that is wasted in the sky. The effects of such pollution are wide, they affect animals and insects and also our sleep.

The voltage (V) that the product requires in order to function correctly.Voltage higher than the recommended value will lower the lifetime of the product or destroy it. Lower voltage will affect the performance. If your initial voltage is higher (e.g. Car-voltage or adaptor) you will need a resistor, if is lower you need to either increase it, use a constant current powersupply or a voltage booster.

The current level which creates the highest performance on your product, without damaging it. If the current will be decreased, also the performance of your product decreases. At high current the performance can as well increase, however the lifetime decreases or the product can be destroyed.

Lens are used to effectively focus and direct the light of an LED. They are much more efficient than reflector cases used for other lightsources

Lens can be clear, white diffuse or diffuse in terms of color. Diffuse lens are used in order to produce even illumination. Clear (or water clear) lens don’t affect the light output of the LED. Sometimes the lens have the same color and the LED in order to make the LED more visible.

The LED chip is the actual light source and has the greatest importance in determining the physical limits, performance, lifetime and possibilities of a LED. Most of the cost of an LED is based on the cost of its chip.