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Comparing Performance: LED vs. Fluorescent

Light Emitting Diode (LED) raw lumen output continues to improve and has now reached a point where replacement of fluorescent lamps is practical. When comparing LED to fluorescent, considering raw lumen output is not the critical parameter; effective lumens for a given population of fixtures over time is the most important factor and must be considered.

Calculating Effective Lumens

A popular T8 fluorescent tube is rated at 2850 lumens, but what is the true output and where does that light go? Effective lumens are a measure of true output to accurately compare a fluorescent to an LED fixture. Effective lumens can be determined using the following equation:

Effective Lumens = RL * CU * BF * LLD * LDD * LBO

RL = Rated Lumens LLD = Lamp Lumen Depreciation
CU = Coefficient of Utilization LDD = Luminaire Dirt Depreciation
BF = Ballast Factor LBO = Lamp Burnout Factor

The coefficient of utilization is defined as the percentage of lamp lumens reaching a work surface. Due to the cylindrical design of a fluorescent tube lamp, only ~1/3 of the light output directly illuminates the intended surface. In a fluorescent reflective fixture, known as a troffer, a large portion is reflected; thus the total zonal lumens from 0 to 120 degrees averages 65 percent.

T8 fluorescent troffer
0 to 120° zonal lumens
CU = 65% average

An LED is a directional light source. Most of the light is contained within a beam angle of 120 degrees, providing light where it’s needed. A comparison of photometric data validates this effect.

LED tubes installed in a troffer
0 to 120° zonal lumen
CU = 86% average

In a channel fixture, a small portion of the fluorescent tube light is reflected. The resulting 0 to 120 degree zonal lumens is around 50%. Again, the LED product is directional.

Fluorescent channel
0 to 120° zonal lumens
CU = 50% average
LED Tubes
0 to 120° zonal lumen
CU = 86% average

A typical ballast factor for fluorescent fixtures is 0.9. With LED fixtures, there is no such factor associated with a low voltage transformer; thus, the comparative value is 1.0. Lamp Lumen Depreciation is the most difficult comparison because the products have such different lifetimes, lumen output curves, and failure mechanisms. Average fluorescent depreciation is determined at 40% the rated life, or 8,000 hours. A factor of 0.9 gives the mean lumen output for T8 bulbs, and then the lamp completely dies after a 20,000 hour life expectancy. An LED does not die catastrophically; it is simply considered to reach end of life when the total lumen output is less than 70%. However, on average, the initial lumen output is maintained above 80% up to 50,000 hours.

Therefore, LEDs are assigned an average LLD factor of 0.9 for rated life. Luminaire Dirt Depreciation factors can be found on IESNA graphs. Even in a clean environment, LDD is 0.9 after 12 months. LED fixtures do not rely on reflected light and the LED is pointed down, so the comparative value is 1.0.

The Lamp Burnout factor is 1.0 minus the percentage of allowed failed lamps without replacement. A conservative estimate for spot relamping is 0.95. LEDs do not burn out, so the corresponding factor is always 1.0.

Quantifying the Difference

Fluorescent troffer effective lumens = RL * 0.65 * 0.9 * 0.9 * 0.9 * 0.95 = 45% rated lumens (RL)
Fluorescent channel effective lumens = RL * 0.5 * 0.9 * 0.9 * 0.9 * 0.95 = 35% rated lumens (RL)
LED fixtures effective lumens = RL* 0.86 * 1.0 * 0.9 * 1.0 * 1.0 = 77% rated lumens (RL)

Practical Application

A three lamp fluorescent troffer can be replaced by a two tube LED troffer.
A two lamp fluorescent channel can be replaced by a single tube LED linear fixture.

Conclusions

Comparing Performance: LED vs. Fluorescent

Fluorescent channel LED tubes or Panels
0 to 120° zonal lumens 0 to 120° zonal lumens
CU = 50% average CU = 86% average

White LEDs are now bright enough to replace fluorescent bulbs.
LEDs put light where it’s needed, greatly increasing efficiency.

When specifying, compare effective lumens rather than raw lumens.

When it comes to the environment, there is simply no question. LED lighting technology is the only alternative. Fluorescent tubes contain Mercury, Lead, and other hard to recycle, or harmful materials. LED lights are 100% ROHS compliant, making them recycle-able anywhere that handles electronics. They contain no harmful chemicals. Save money and the environment.

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