Before the first indoor fluorescent lamps were sold in the late 1930s, incandescent light bulbs were, of course, the standard for interior lighting. Incandescent lamps basically convert heat to visible light—very inefficiently. Light output is just 14 lumens per watt.

During and after World War II, fluorescent lighting burgeoned in the United States. In the 1970s, European development led to the compact fluorescent, the direct predecessor of the T5 lamp we know today. Fluorescent lamps work by exciting atoms of mercury with accelerated electrons and emitting ultraviolet (UV) radiation. The fluorescent powder coating converts the UV radiation to visible light. Fluorescent lamps are far more efficient than their incandescent predecessors.

The oldest fluorescent lamp is the T12. Its 1.5-in. diameter has an output of 60 lumens/watt. T12s were eventually replaced by T8s, which have a 1-in. diameter. The T8 boasted an improved fluorescent powder coating and inert gas. Efficiency improved by 33% to 80 lumens/watt. The T8 lamp is currently being replaced by the T5, with just a 5/8-in. diameter. The liquid mercury is replaced by an amalgam, formed by the reaction of another substance with mercury. The newest T5 is even more efficient, with 95 lumens/watt. It can operate at 95˚F, whereas the T12 and T8 max out at 77˚F. This evolution was driven by these factors:

  • Diameter reduction was made possible by the development of tri-phosphorous, which is more efficient and stable.
  • Smaller diameter lamps are less costly to make because they use less glass and less phosphorous.
  • Smaller lamps are more compact and therefore more attractive for the consumer.
  • Smaller lamps are more efficient, with better energy savings.

Not surprisingly, the manufacturing process for T5s is far more complex. The three-band phosphorous has three basic phosphors—red, green and blue. The blue phosphorous is unstable and the light output depreciates quickly if the manufacturing process is not spot-on and if the inert gas is incorrect.

Off-gassing from the glass can react with the phosphorous, resulting in reduced light output. If the amalgam is not right, the T5 lamp will not be able to operate at the higher temperature of 95˚F. The higher-output T5 lamp is dependent on the use of the best blue phosphorous available on the market. A special glass is used to limit the off-gas reaction with the phosphors.

Proprietary amalgams are used to enable the lamp to operate at the higher temperatures of 95˚F and above (these patent-pending components allow for greater lumen output). Each of these components raise the manufacturer's cost for the quality lamps. With proper implementation of these components, the T5 lamp's lumen output and life are extended as wattage is expanded.

Demystifying T5 Lamps

The T5 lamp has undergone quantum leaps technologically with the advent of high-efficiency/high-output designs in the 1990s. Its full-spectrum, super-high intensity is perfect for promoting vegetative growth. Many industry insiders believe the technology represents a paradigm shift in horticultural lighting.

Stephen R. Covey, in his Principles of Centered Leadership, writes, "If you want to make small changes in your life, change your attitude and change your behavior. If you want to make quantum leap changes, then change your paradigm!" With the new full-spectrum, high-intensity fluorescent systems, indoor growing promises healthier plants and more production with less heat and energy.

When T5s were introduced, the first adopters of the technology quickly noticed their plants had thicker stems, carried more fruit and survived outdoor transplanting with less shock at half the energy. Plants were hardier, exhibiting more profuse growth with tight internodal spacing.

Testing and growing have come a long way since then. T5 grow lamps are now available in three outputs: 54-W high output (HO), 95-W very high output (VHO) and 115-W extreme high output (XHO). The latest generation, the XHO, is particularly exciting. These lamps can give off more UV light, which helps keep bugs and mold at bay. Unfortunately, that does include good bugs.

The Present and Future of T5 Lamps for Horticulture

T5 lamps are changing so quickly that it's hard to keep track of the technology. Years ago, when we first worked on T5 lighting, the ballasts were unstable, with high failure rates. Each lamp required a separate ballast, specified as 120 W or 240 W. Even recently, in testing various HO lights, I found big differences in fixtures and lamp performance. In controlled tests of 54-W lamps in different fixtures, several 6.5-K lamps had 28% less lumen output. We found even greater discrepancies with substandard lamps. Not all lamps are created equal nor do all ballast and fixture designs optimize output.

To optimize performance, use a lumen meter and make sure the fixture has the best ballast for the application. If you are unsure, check with the manufacturer on the milliamp (mA). Make sure the fixture uses quality ballasts and are driven at the appropriate milliamp, as in 450 mA for 54 W.

The vast majority of fixtures on the market use 350 mA, which puts out far less lumens with less penetration than 450 mA. If it looks brighter to the human eye, double check with a lumen meter. In multiple tests, the 450 mA put out two times the lumens as the 350 mA when the light meter was set 12 in. from the lamps.

Cheap lamps use low-quality phosphorous or tri-phosphorous, which does not hold the lumen or blue of the 6.5-K lamps. Lumen degradation occurs rapidly. I've tried lamps that start out with high lumens but rapidly drop off by more than 30%. Full lamp lumen output takes 30 minutes with many lamps and fixtures. Lamps may be rated for 10,000 hours but won't last if they use lower-grade phosphorous.

In working with leading lighting scientists, I learned the importance of building lamps to run with specific ballasts. As one physicist colleague stated, "The No. 1 problem with lighting is lamp-to-ballast incompatibility."

Current Testing of T5 Lamps

Last year I completed tests in conjunction with several lighting scientists. We ran controlled tests with multiple light sources–metal halides, high-pressure sodium lights, induction lights, T5 lamps and LEDs–and measured plant growth and quality. We found induction lights underperformed T5s because of the point source and 90% energy factor of the latter. Induction fixtures had to be raised for enough light distribution and, with 80% energy efficiency, plants get less light. Induction lighting has great value with the 100,000-hour lamp life. It is great for tunnels, airports or anyplace lamps are difficult or costly to change.

This year's testing focus is the new HO, VHO and XHO T5s, and LED lights. We are conducting tests right now with the new full-spectrum/enhancement/high output lamps at the 95-W and 115-W levels alongside the newest high-output LEDs.

The plants shown in the photos are the culmination of these tests. These plants were grown under XHO 115-W lamps. The 95-W VHO lamps work well and have even distribution, but are not designed for growing plants with the green spectrum. The current LEDs are prototypes and are in growth trials against a 450 mA HO 54 W.

The LED at 116 W performed admirably against the T5 at 216 W (four lamps at 54 W each). This was the first LED to work out for me in 15 years of testing, and the next prototype arrives soon.

Another plus for LEDs is their low temperature, running around 75˚F. The fixtures could be lowered directly over the plants with no risk of burning. The lamps were kept at the same height above the plants as the T5 for testing purposes. When the T5 and LED fixtures and basil plants were transferred to the greenhouse, all went well, without shock.

With metal-halide lamps, plants need to be acclimated through shade to avoid shock, burning or stunting. Enhanced root development and tighter growth was evident with this LED. Manufacturers, if you think you have the new LED champion, send me one and I'll test it against what has tested the best. The vast majority I've seen and tested seem to put plants in suspended animation, looking nice but bearing little fruit.

In another test with XHO 115-W lamps, a 12-lamp fixture pulled 1,440 W. This had a little help with a fan under the lamps for air and heat circulation. With T5 lamps, I recommend always using a fan under the fixture. The plants enjoy good air circulation and it improves the efficiency of the fixture and lamps. By the way, most VHO lamps I've tested in the past put out 33% less lumens from the start and it diminishes from there.

The pepper plants shown in the photographs were grown hydroponically with organic nutrients, meaning they are 100% organic and 100% hydroponic! They represent the best performance to date, for me, for hydroponic, organic vegetables. I've also had good success with basil grown in rockwool. Interestingly, the new VHO and XHO lamps appear to not only capture but enhance higher elevation lighting conditions.

So get ready, get set—with hydroponic supplies, quality nutrients and appropriate lighting—and grow!

Special thanks to Vee from Grodan, Neil from New Millennium Nutrients, and Ranil from EcoFert for their contributions to this testing.