Daylight's great for as long as it lasts, and then we switch on electric lights. But for lighting up the insides of trailers, forget old-fashioned incandescent and even late-model fluorescent types.

The in thing today is the light-emitting diode, which in multiples spreads bright-white light throughout a trailer's interior at low overall cost. White LEDs - the last of the principal automotive colors to be developed - are taking over this market, just as they did for amber and red clearance, marker and tail/stop lamps on trailers' exteriors.

LED interior lamps can cost more than the other types to buy, but they provide more illumination, draw less current, last far longer and are virtually trouble-free, according to three main suppliers, Grote, Phillips and Truck-Lite. These companies also claim that higher light output makes drivers and dock workers safer and more productive because they're less likely to trip over things and can read shipping documents much easier and faster.

LEDs now go into 90 percent or more of new refrigerated trailers, Truck-Lite estimates. In vans, customers tend to stay with what they know and fluorescents and incandescents still sell, though the market is dimming for them. LEDs can easily replace older lamps in vans or reefers, and fleet managers who know their costs are making the switch because once LEDs are installed, maintenance savings quickly begin, the suppliers say.

All have had LED products for some time, but have recently introduced new fixtures and advanced switching that make the use of LEDs more economical.

Fluorescents: A step up

Incandescent lights, which have been around since Thomas Edison and almost since the advent of motor trucks, did a tremendous job when the alternative was cursing the darkness or lighting a candle. Fluorescent interior lights entered trailers in the 1990s, and at first fleet managers liked them for their stronger and brighter light signature. But the typical fluorescent tube often fails prematurely, especially in refrigerated trailers, where cold temps keep ballast from firing completely and the tubes glow only dimly. Low voltage quickly kills tubes. Also, road vibration causes the phosphorescent coating inside the glass cylinders to flake off, dimming and eventually killing the tubes.

There's also a bit of mercury in each tube, and if it shatters, traces of the toxic compound can contaminate food shipments that must be discarded, safety experts insist. (Never mind that as kids some of us occasionally played with drops of the fascinating, silver-colored liquid metal that we got by smashing cheap thermometers; we pushed it across tables and rolled it around in our fingers, and they didn't fall off and our insides didn't rot and we're still here to tell about it.)

Fluorescent tubes aren't cheap, but the true cost of maintenance includes the time it takes for a mechanic to walk to the trailer with a ladder, climb up and in, determine which tube is shot, position the ladder and climb up and remove the bad one and pop in the new one, which he's been juggling along with the ladder. And he'd better not drop either tube, or the shattered glass - thin, tiny pieces that cling to a floor, especially in a reefer floor's grooves - will cause him grief and add cleanup time. (And don't forget about that mercury.)

Yet compared to incandescents, fluorescents produce far more light, even if they use more current. Drawing 2.7 amperes at start-up, a fluorescent lamp will produce 1,000 lumens versus a common incandescent light's 1.44 amps and 150 lumens (with a 21-candlepower bulb).

Next generation

But LEDs are better still. Grote's most popular LED makes 800 lumens and draws 1.75 amps; a brand-new model makes 1,200 lumens at 2.1 amps.

Put another way, a single 800-lumen LED lamp is more than seven times as bright as an incandescent while using only an extra 0.3 amp. A 1,200-lumen LED is 20 percent brighter than a common fluorescent while using 0.6 amp less current. The LED lamp will operate at 9 to 12 volts, so isn't nearly as sensitive to low voltage as a fluorescent. LED lamps are also far superior in rated life - 50,000 hours, versus 12,000 for a fluorescent (barring early failure, as we've discussed) and 300 hours for an incandescent bulb.

Grote, Phillips and Truck-Lite all make replacement LEDs to mimic the shape of existing incandescent and fluorescent fixtures. Elongated bolt holes cover many specific bolt patterns in the old fixtures, so the new LED fixtures mount easily and wire right up.

Watch the power

Power draw can be a serious consideration for trailer lighting, especially with reefer trailers that make multiple stops on a shift. Lights take power from on-board batteries, which can run down because the tractor's engine-driven alternator doesn't run enough to recharge them. If the fleet has to follow strict engine-off regulations, the batteries on are their own. The situation is worse if the driver forgets to shut off the interior lights and they continue to draw power between stops. This is a problem even with LEDs.

The answer is special switching. Most trailers have a manual switch near each entrance so drivers can flick on the lights as they enter. Timers on the light circuit cut off power after a pre-set time - 30 or 60 minutes is typical. If the driver's still working he can hit the switch again, but chances are he's done because his boss has chosen a shut-off time to suit the operation. That means he's back in the cab and driving, and the batteries are being recharged even if he's forgotten to shut off the lights in the trailer.

Phillips also offers its Permalogic mechanism that mounts over the seven-way socket in the trailer's nose. This works with the interior lights' manual switches, even if one is left on and the lights inside are still shining. When the driver steps on the brakes, Permalogic automatically shuts off the light by cutting power in the system's circuit.

Permalogic also monitors the power source and disables the lighting circuit if batteries are low; when this happens, a red light glows on the box. The driver sees it as he passes by, so before he's walked all the way to the trailer's door, only to find that the lights won't switch on, he can immediately return to the cab and restart the engine so the alternator charges the batteries which then can light the lights. Phillips is also working on a switch with a low-voltage disconnect.

Grote is now offering a motion-sensing switch that turns on the interior lights when it sees movement and shuts them off soon after that movement has stopped. It has to be human movement, though, because the sensor looks for the heat signature emitted by the human body. That way, the in-transit shifting of cargo and falling boxes won't trigger the sensor and switch on the lights. The optional motion detection function can also eliminate manual switches and all their wiring, and all their failures from the high humidity inside reefer trailers.

Phillips, however, believes that a motion detector creates its own problems, including extra wiring if it's mounted remotely, close to a door so it switches on the lights when the driver enters. And if the detector doesn't work, it's hard to figure out why. That's why Phillips is staying with manual switches and timers. So is Truck-Lite, for now.

Van trailers usually have translucent fiberglass roof panels that allow daylight in, so lighting isn't as important as with reefers. But of course it's a concern at night. Work lamps at loading docks then come on, and night-time pick-ups and deliveries rely on dome lamps. Many fleets buy on price, and incandescents are still the lowest. But as with reefers, managers who know their total costs are following the switch to LEDs for their vans. You might say they've seen the light.

From the November 2009 issue of Heavy Duty Trucking.