LED based lighting is the newest innovation in the lighting industry. It has revolutionized energy-efficient lighting. LED is short for Light Emitting Diode: it is a diode which emits light when current flows through it. These small solid state lights are extremely energy-efficient. LED light assemblies normally combine several such diodes to get the desired brightness. They also consume considerably less electricity for same brightness. The LED technology is still improving very fast and is the lighting solutions of the future.
Low power consumption and long life pays-off in the long-run, apart from being a green technology. Well-designed LED lighting fixtures can retain 70% of their initial output for 50,000 hours or more, depending on operating conditions. LEDs are more efficient than both incandescent and CFL lamps for two reasons: One, they emit light in a targeted direction – instead of scattering it in all directions which is wasteful. Two, they don’t emit great amounts of heat. In comparison, incandescent bulbs and the CFL lamps waste a large portion of power as heat: 70 percent or even higher.
LED’s are now commonly available with efficiencies of 100-120 Lumens per watt compared to 65-80 Lumens/watt of CFL, 45 Lumen/Watt of Mercury vapor and 75 Lumen /watt of metal halide or 94 Lumen / watt of sodium Vapor.
In dozens of nations, green initiatives and energy-efficiency directives are hastening the shift towards LED lighting systems, which have the lowest energy consumption and environmental impact, the longest useful life, and the lowest operational cost. LED lamps are now widely used in solar systems because they offer superior light output per watt which reduces total cost of the system.
Lumens doesn’t describe the quality of the generated light – its color, tone or other variables. People often describe the lights as “warm,” “cool,” “pale,” “dim” and so on The biggest challenge for LED manufacturers is creating lightings that mimic the light quality of conventional lamps. There are two parameters that are often talked about while discussing LED lightings: Color Rendering Index (CRI) and Color Temperature (K)
Color Rendering Index is a subjective indication of how well a lamp will reproduce colors. Lights at the low end of the index, such as low-pressure sodium lamps (CRI 20-30) tend to wash out colors and are best used in applications like street lighting, where accurate color rendering is not important. An incandescent light bulb, on the other hand, is considered to have a “perfect” CRI of 100.
Linear fluorescents and compact fluorescents (CFLs) usually fall in the 80 to 90 range. Metal Halides only has a CRI of about 70, so only 70% of the colors will be rendered correctly. HPS has a CRI of 22. In general, CRI values higher than 80 are considered good for indoor lighting, and higher than 90 are good for visual inspection tasks such as those required in printing or the textile industry.
Color Temperature is not how hot the lamp is. It is the relative whiteness of a piece of tungsten steel heated to that temperature in degrees Kelvin. While CRI represents how accurate a light source is, color temperature (CCT expressed in degrees Kelvin (K)) represents the character of the light source. At the low end, a color temperature of 2600-2700K creates a warm light character like that seen in incandescent bulbs; a higher color temperature of 4100-5500K creates a whiter light like most often seen in office buildings. HPS has a warm (red) color temperature of around 2700K as compared to MH at 4200K, which has a cool (blue) color temperature.
It is important to remember that two light sources can only be compared if their color temperatures are equal. You cannot compare the CRI of HPS (2700K) and Metal Halide (4200K). Also note that at extremely low or high color temperatures, the color rendering can be very poor despite high CRI scores close to 100.
Need to Think Differently
Because of long experience with incandescent light bulbs, most people have learned to correlate brightness with the wattage of the light source: a 100-watt lamp puts out more light than a 60-watt lamp. Thus, when people look for lighting source, they think in terms of 40, 60, or 100-watt bulbs. Generally, incandescent lamps use the same filament material heated to the same temperature, so the only way to increase their light output is to increase the wattage. It is different with LED light assemblies. They involve no filament or heating but merely current is passed through diodes which results in the emission of light. Usually several tiny LED devices are mounted alongside to get the desired illumination. Different LEDs may use different materials, each with its own light emission efficiency. For this and other reasons, two different LED sources can consume the same number of watts but give out different light output.
Another issue is the directionality of light. An incandescent light bulb illuminates in a 360-degree spherical pattern regardless of the shape of the bulb. When we light an area with such a ‘spherical bulb’ source, even with a reflector, only a small proportion (50% or less) of the light is delivered to the surface we are lighting – the rest is lost. Light is wasted by diffusers; or through filtering or lensing; or when directed away from the target area.
The way LEDs are shaped and designed, light is not emitted in all directions; with proper design over 80 percent light can be directed where it is needed. LED fixtures deliver light in the desired direction and create brighter illumination, allowing efficient use of the produced light. Thus, an LED lighting fixture with lower rated Lumens (a measure of illumination explained later) may deliver the same or more useful light to a targeted area than a comparable fluorescent lighting fixture with a higher rated light output.
Thus, people should now learn to think differently when they talk about lighting. With LED lights it is more useful to talk in terms of brightness delivered rather than brightness produced by the fixture.
An Important Note
Heat dissipation is an important design factor for efficient LED performance.
The LED must be connected to a Resistor in series for safe operation, otherwise the LED will “burn” in a short time.
The resistor Value, R is given by R = (VS-VL)/I
VS= the Supply Voltage, VL= the LED Voltage I = the LED current
How to choose the resistors? Choose the nearest standard resistor value which is greater than the calculates values, if the calculate value is not available. The higher resistor value will decrease the LED current and increase the LED life but it will decrease the LED brightness.