Garden & Greenhouse


HID Fixture Performance is Not All About the Lamp

Posted June 21st, 2018 by Garden & Greenhouse in ,

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 has shown with a number of these fixtures 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.

Tips for Choosing and Optimizing HID Light Systems

Assure the Brand 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.

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.

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.

Minimize the Distance from Plant to 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, keep the fixture as close to the plant canopy as possible while managing the related heat emanating from the fixture.

Consider Open Fixtures Where Possible

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.

Background information for this article was provided by Sunmaster. Visit their website at

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