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LED v Halogen

LED V Halogen Lighting

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LED street/parking lot lights offer substantial energy savings compared to High Pressure Sodium (the common type of HID lights used for street lighting) However, as explained below, calculating LED efficiency for this type of lighting requires a clear understanding to what is important when it comes to area illumination of this nature.  System efficiency is measured with the lamp installed in a luminaire (which is, essentially, the entire lamp).   It is based on how many lumens actually reach the target area, not simply how many lumens are emitted from the lamp.  System efficiency is always lower than source efficiency due to three primary sources of light and power loss when a lamp is installed in a luminaire:

 

Trapped Light – Traditional lamps, which emit light in all directions, require reflectors inside the luminaires  to “bounce” as much light as possible away from the lamp and fixture components to the target area.  However, not all light can be effectively redirected.  Typically, 40% or more of the light emitted from the lamp is trapped within the luminaire and does not reach the target area.

Protective Covers and Lenses – Luminaires almost always use some sort of secondary optics (such as tempered glass or acrylic) to help focus or redirect light, and to protect the lamps or  LEDs from the environment.  These materials do not allow light to pass with 100% efficiency, so system efficiency is reduced.

Power Supplies – Most light sources require a ballast or driver to convert line power into a form which can be used by the lamp.  These losses can vary from 5-25%, depending on the power supply used. This also results in lower system efficiency.

 

When considering all of the factors that affect system efficiency, LEDs provide a clear advantage over HID lamps.

Here is a summary comparison:

 

HID lamp:  Source efficiency is typically 120 lumens/watt or higher.  However, losses from trapped light, protective covers and lenses, inefficient ballasts and unfavourable operating temperature typically result in a measured system efficiency of 30 lumens/watt or less.

LEDs:  Source efficiency can be 100 lumens/watt or higher, depending on the type and manufacturer of the LED used.  There are no losses from trapped light due to the focused output of the LED, and secondary optics and protective covers generally reduce output by only 25% or less.  LED power drivers are also more generally efficient than HID ballasts, and reduce system efficiency by 10-15%.  However, even when efficiency reductions due to high operating temperatures are included, LED system efficiency often remains above 50 lumens/watt or higher.

 

LED street lighting does, of course, possess the usual benefits of reduced power consumption cost (at least 50%), reduced bulb cost (over 50%) reduced maintenance costs (over 50%), immediate illumination without a warming up period, and cleaner, better illumination in terms of light quality in comparison to HID lights.

 

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County Park Trading Estate
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Cumbria
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