HID systems for plant growth are dimmed for a variety of reasons. Whether to simulate sun-up / sun-down or to reduce light levels while saving energy, some brands advertise dimming as a significant feature/benefit for their system. Unfortunately, dimming HID is not as simple as incandescent or LED and, when not done properly, can compromise your lighting system or even cause harm to you and your plants.
The µmol per watt decreases as lamp is not operated at its designed.
As CMH and MH lamps are dimmed, the capsule (arc tube) within the lamp operates cooler. In many cases, a cooler arc tube will not keep the salts (halides) engaged which causes the spectrum to become less broad and more pronounced in the green area of the spectrum.
Lamps dimmed during start-up may cause tungsten to sputter on the arc tube wall, causing blackening and less light output
Other HID light systems are marketed with a ‘turbo charge’ or similar setting where the lamp is slightly or more-than-slightly overdriven but this can create some problems.
Lamp life can be shortened to ½ rated life or more. Light output typically depreciates at an accelerated rate when lamps are overdriven.
Overdriven lamps operate at a higher temperature which significantly increases the risk of non-passive failure.
In general, dimming HID is possible but not a preferred choice. Alternatively, one might simply consider raising the fixtures where/when possible as light rapidly decreases at a rate of 1/(distance)2.
The image and background information for this article was provided by Sunmaster. Visit their website at SunMasterGrowLamps.com.