LED Street Lights are expected to revolutionize illumination of streets and common areas. They consume less than 15% electricity as compared to Incandescent lights, and have better life and light quality compared to High Intensity Discharge (HID) lamps like Sodium Vapour (HPS), Mercury Vapour (HBO), or Metal-halide lamps. LED Lights last at least 50,000 hours (13 years @ 12 hours use per day), and provide better control over the illuminated area reducing light pollution. Being solid state devices, they can be constructed in a robust fashion which reduces the risk of glass breakage during maintenance and operation. By switching to LED Street Lights, Municipalities/Institutions can save almost 30-40% of their budget costs which can be put to other good uses.
Why a _GOOD_ LED Street Light?
A badly constructed LED light can fail early, cause damage, or not function as per your requirements. LED Lights can require a higher upfront capital investment with a promise of lower long term running costs; that's why it's very important that you be able to reap the benefits of the long life of the LED Light. This post hopes to equip you with the right questions that you should ask your LED Light supplier to be able to choose a suitable product for your use.
Characteristics of a GOOD LED Street Light
Dust and Water Proof
Your light will be sitting outside exposed to elements. Therefore, the innards of your light (viz. the LED Chips, Driver, etc) must be adequately protected against moisture and dust. This protection is usually described using a rating system called the IP Code or the Ingress Protection code. To a minimum, your light fixture must be IP65 compliant. This means it has dust and water protection. If you stay close to coastal areas, also consider if the light is safe against the corrosive salty sea winds.
The single most important thing that determines the life of your LED in the temperature at which the LED operates. Life of LED means the duration for which the LED generates rated light output. LED which can last more than 50,000 hours at the rated temperature can rapidly reduce in life, say from 10 years to less than 2 years, if operated at a higher temperature. Therefore great care needs to be taken when constructing the LED Light to rapidly dissipate heat away from the fixture. LED Lights can employ either active and passive cooling. Most fixtures have a passive cooling mechanism where LEDs are mounted on Metal Core PCB (MCPCB) which in turn are attached to an Aluminium casing using a good thermal interface material. When selecting a LED light, it's important to choose one which has good thermal management. This can ensure full life of your LED Light.
Robust LED Driver
LEDs are DC devices. That means you need an electrical circuit to convert 230V AC from grid to a DC voltage. This electrical circuit is called a LED Driver. Since LED Street Lights can be installed at remote/rural areas with not-so-reliable-grid-voltages, the driver should be able to withstand sudden voltage surges induced by electrical interference like Lightning strikes OR be able to shutdown if voltage falls over or under an operating range. Good LED drivers typically operate between 150V to 270V with about 4KV surge protection. A good driver will also have a thermal shutdown mechanism that protects the LEDs in case of a malfunction that causes a lot of heat to get generated.
The drivers can come in 2 varieties: Isolated & Non-Isolated. Isolation is basically an electrical barrier between the input and output side, most commonly using a transformer. An Isolated driver protects the end user from an electrical shock and can also protect against reversed polarities. Isolated driver should be preferred over an non-isolated one.
LED can also get damaged due to an excessive current flowing through it above and over its rated value. Therefore, good LED Drivers are Constant Current devices (as against Constant Voltage), which keep the output current constant even if the output load changes due to changes in temperature. As LEDs get hot, their resistance tends to decrease, which can cause an increase in current. Also, not all LEDs have the same resistance. A constant current driver protects against all these conditions and keeps the same current flowing through the LEDs thereby protecting them and increasing their life.
Light emanating from a LED tends to be very directional. Therefore LEDs come attached with special Optics or Lenses that help it dissipate light over a wider area. The shape and the nature of the light coming out of the LED Light will be a topic of a separate blog but it suffices to say that selection of LEDs and Optics plays an important role in deciding how suitable the light is for your specific application. E.g. a light used for general illumination should spread its light over a wide area where as a light meant to illuminate roads should direct most of its light towards the road and very little away from it.
Light Intensity and Color
The total light coming out of a fixture is measured in Lumens, where as the the total light falling on a unit area is called LUX or luminous intensity. There are specific guidelines for LUX based on the nature of work. E.g. total LUX needed at a work desk in an office will be different than that needed on a side walk. When selecting a LED light, make sure that the light satisfies your LUX requirements when mounted at the specified height.
Another parameter is the Color of light produced by the LED. Most common colors are Cool White or Natural White which is closer to daylight. Some applications might prefer Warm white. Color is measured by something called Corelated Color Temperature (CCT) and its unit is degree Kelvin (K).
Yet another parameter is called the Color Rendering Index (CRI), which essentially indicates how accurately the light source can illuminate an object's true color. Yellow light has a very poor CRI where as a white CFL has better CRI. CRI can become very important if the light is to be used where accurate color reproduction is needed, e.g. a shop selling Sarees or Shirts. CRI over 70 is generally desired.
Like a lot of things in life, things are not as simple as they appear at first. Not all LED lights are created equal. A little information can go a long way in helping you select an appropriate LED Light for your specific application. Hope this article will help you with just that. Comments welcome!