LIGHT SCIENCE March 2018 The perils and pitfalls of dimming HID lamps

The perils and pitfalls of dimming and overdriving HID lamps

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 is 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. Adverse effects when dimming HID lamps can include the following:

• Decreased efficacy: µmol per watt decreases as lamp is not operated at its designed
• Changed spectral quality: 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
• Decreased light outpu: 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. When considering these systems, one should understand the potential adverse effects when overdriving HID lamps:

• Shortens lamp life: Lamp life can be shortened to ½ rated life or more. Light output typically depreciates at an accelerated rate when lamps are overdriven.
• Risk of rupture: Overdriven lamps operate at a higher temperature which significantly increases the risk of non-passive failure.

A few basic guidelines when dimming HID ballasts are as follows:

• The HID lamp should warm up to full brightness for approximately 5 minutes prior to dimming.
• The ballast must provide sufficient voltage to sustain lamps throughout their rated life in both the full and dimmed wattage modes.
• On high frequency ballasts the dimming frequencies must be higher than the nominal wattage operating frequency.
• Dimming should be accomplished by line voltage reduction only, not by waveform chopping (i.e. phase control)

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!