Is it possible that some T5 lighting fixtures are perfectly efficient? One might get that idea based on some efficiency ratings of 98–100 percent, although it might not seem possible. Such a rating must be a mistake, right? But it’s not.

We’re talking about luminaire (lighting fixture) efficiency, of course—the percentage of lumens produced by the lamp that exit the fixture. In contrast, energy efficiency, or efficacy, is a measurement of lumens per watt.

Well-designed optical systems can be very efficient—dedicated T5 fixtures in particular, as the slimmer lamps enable greater efficiency through enhanced optical control. For this reason, some two-lamp fixtures are promoted as being able to replace similar three-lamp T8 fixtures.

Nothing is perfect, however. Some light is always trapped inside the fixture, so how can T5 lighting fixtures achieve luminaire efficiencies as high as 98–100 percent? The answer lies not in what they can certifiably do—they are actually not as efficient as these high ratings suggest—but in how efficiency is determined.

First, let’s look at legacy systems. Fluorescent lamps are sensitive to ambient temperature. T8 and T12 lamps are designed to produce their highest amount of light output at 25ºC (77ºF). As temperature varies, light output diminishes.

T5 lamps, in contrast, are much slimmer than T8 and T12. As it is assumed they will be used in more compact fixtures in which greater heat buildup can be expected, T5 lamps are designed to produce their peak light output at 35°C (95ºF). Again, as ambient temperature varies, light output drops.

The key to higher efficiency ratings for T5 lighting fixtures lies in this difference.

Now we must understand how luminaire efficiency is determined. For linear fluorescent fixtures, manufacturers use the relative photometry process, in which the quantity, intensity and direction of the light emission is determined relative to the rated output of the light source. Manufacturers use IES-LM-41-98, a common photometric testing standard for indoor fixtures with fluorescent lamps, ensuring they all measure efficiency basically the same way, allowing us to compare different products with some confidence.

The testing standard involves operating a bare lamp on a reference ballast in a lab at a constant room temperature of 25ºC. This test lamp is very unlikely to produce the exact rated lumens one would find in its listing in a manufacturer’s catalog, as the rating is an average for a large population. (Lamp output is calibrated to rated output using a special factor.) After this measurement is complete, the test lamp is placed inside the fixture, and new measurements are taken. Luminaire efficiency—the ratio of bare lamp lumens to lumens emitted by the fixture, including both optical and thermal effects—can then be calculated.

Again, the room is maintained at a constant temperature of 25ºC, but we can expect temperatures inside the fixture to be higher. So for a T8 lamp, designed to produce peak light output at 25ºC, with light output dropping as temperature increases, we can expect the T8 lamp will produce higher light output outside the fixture than in it.

T5 lamps, on the other hand, are designed to produce peak light output at 35ºC. As temperature increases above the room temperature of 25ºC, T5 lamp light output will actually increase where T8 light output decreases. So unlike T8 lamps, during this test, we can expect the T5 lamp will produce higher light output inside the fixture than outside it. The increase in lumens from the lamp offset the decrease in lumens due to optical losses, and, as a result, efficiencies as high as 100 percent are possible.

Bob Davis, director, product innovation and marketing for Litecontrol, said these ratings can be useful as long as one compares very similar lighting fixtures, since luminaire efficiency is not a function of the fixture itself but, instead, a combination of the fixture plus its lamping and ballast. He cautioned against choosing fixtures with the highest luminaire efficiency as an end in itself because it does not directly correlate to energy efficiency, despite some utility rebate programs setting a minimum efficiency level. Davis further advised that, if the photometric files (IES files) for a given product are derived using a simulation program rather than physical testing, they should be particularly cautious about what they are seeing, as these programs only simulate the optical performance of the fixture and often do not account for thermal effects that produce light output reductions for T8 or additions for T5 lamps operating inside fixtures.

Peter Ngai, vice president, research and development, Acuity Brands Lighting, regards the situation as a problem that the industry should address through a new standard issued by the Illuminating Engineering Society. He recently proposed a new photometric testing method for T5HO lighting fixtures in which bare lamp measurements are made at the operating temperature inside the fixture to determine true fixture efficiency. This method would uncouple thermal effects from evaluating optical efficiency.

DILOUIE, L.C., a lighting industry journalist, analyst and marketing consultant, is principal of ZING Communications. He can be reached at