HID Fixture Performance It’s Not All About The Lamp
Lighting Science April The Scoop
Operating a great-performing light fixture can be likened to having a V8 under the hood. . . it doesn’t have to look like a Maserati on the outside as long as you have a powerful core. That being said, car enthusiasts fully understand that, while a great engine is an absolute ‘must’ key feature, there are many supporting design elements that are critical in supporting a fast car. Efficient light fixtures are no different, as high-output lamps rely on efficient reflector and ballast designs in order to reach peak potential.
Amazingly there are over 30 different brands of 1000-watt double-ended HID fixtures sold into the North American market today, each touting its own set of features and benefits. Testing a number of these fixtures, SUNMASTER has recently confirmed that there is a wide disparity with regard to the optical performance of these fixtures, simply by measuring the number of µmol at the plant canopy as well as measuring the distribution of the light output across the plant canopy.
The reflector profile and its material components dramatically influence the overall performance of an HID lighting system. For example, when operating a lamp that produces approximately 2,000 µmol, the net output of some HID fixtures is less than 1,500 µmol. In an increasingly competitive marketplace where supply is catching demand, optimizing the amount of light energy per watt consumed should be a primary consideration for commercial growers and hobbyists alike.
Some useful tips when choosing and optimizing your HID light system include:
1. Assure that the brand you choose backs their performance. Look for or ask for an optical characterization, or profile, of the fixture output. If none are available, a fairly simple test can be conducted by hanging the fixture over a 5’ x 5’ area and measuring the µmol at every one square foot at ground level below the fixture. Light uniformity over this 5’ x 5’ area can be easily compared in this way, from one brand/model to another.
2. Assure good reflector specifications. Good polished aluminum has a reflectivity of approximately 95%. Superior reflectors are optically modeled given a particular light source (e.g. 1000W double-end lamp) to bend the light around the back side of the lamp rather than reflect the light back into the lamp.
3. Keep the reflector clean. Dust and dirt can reduce reflector efficiency by 15% or more. For dust and light debris, simply blowing out the reflector out may significantly improve the reflector efficiency. For finger prints and stubborn dirt, water and a micro fiber cloth may be used to polish aluminum reflectors.
4. Minimize the distance from the plant canopy. Surprisingly, some indoor growers mount their light fixtures as high as 12’ to 15’ above the plant canopy. Light diminishes at an exponential rate compared to its distance from the target object (1/distance2 or inverse square law). So when moving a fixture that is 3’ from its target to 4’ from its target (25%), the light reaching the canopy will actually decrease by 46%. To reduce fixture count or simply increase light intensity, one should keep the fixture as close to the plant canopy as possible while managing the related heat emanating from the fixture.
5. Consider open fixtures where applicable. While many enclosed fixtures incorporate a glass lens having an AR (Anti-Reflective) coating, typically most flat glass covers will typically reduce the amount of light hitting the plant canopy by 2% or more due to reflection and/or refraction. In addition, glass lenses are a great flat surface for dust and debris to collect, further reducing light output.
While LED fixtures eliminate many of the potential obstacles listed above, HPS and ceramic metal halide systems continue to flourish in both hobby and commercial applications. Given the vast majority of horticultural lighting is currently HID-based, there are ample opportunities to optimize both existing and new HID lighting systems.