When to Replace High Intensity Discharge Light Bulbs

What is a High Intensity Discharge (HID) Light:

High Intensity Discharge is an overarching term for a gas-discharge light. They are the oldest type of electrical light Among the common types of HID lights are mercury vapor, low and high pressure sodium and metal halide lamps. Other less common variants include ceramic metal halide and xenon short-arc lamps. HID lamps produce light by sending an electrical charge or "arc" between two tungsten electrical conductors (electrodes) and through an ionized gas (also known as "plasma") which is housed inside the bulb. HID lights require ignition which is typically provided by a voltage pulse or a third electrode (an additional metal part) internal to the bulb. Once lit the electrical arc begins to evaporate the metal salts inside the bulb which significantly increases the luminous power of the bulb while simultaneously improving lighting efficiency. HID lighting requires a "warm-up" period because the lighting intensity is dependent on and changes as the material inside the bulb is evaporated into plasma. Additionally, as the light heats up it requires additional voltage to operate. Voltage requirements in HID bulbs are balanced by an electrical ballast (essentially a device that limits electrical current to that required to operate the lamp). As the HID light ages, more and more voltage is required to produce the same amount of light until eventually the voltage exceeds the fixed resistance provided by the ballast and the light goes out (fails). HID lights become less and less efficient over time because they must use more and more voltage to produce the same lumen output as the light degrades.

How Often Should I Replace My Lights?

The following table lists the accepted average life hours of different standard types of light bulbs.



When to Replace HID Light Bulbs

High intensity discharge lighting (HID), such as metal halide (MH) lamps, diminish in PAR, or usable light energy for plants, over time for a few reasons. The main reason is the way they start up.

When an HID lamp is first fired, it receives a huge voltage pulse, much higher than what it uses during normal operation. The initial voltage blast slightly damages the lamp every time the bulb is fired.The initial voltage is the main reason why blackening may be seen within the arc tubes of HID lamps.

Over time (6-12 months), the repetitive process of firing the lamp causes the properties of the arc tube to change, which decreases intensity and causes a color shift in the spectral output.

Another significant factor that affects an HID lamp's output is temperature. HID lamps are designed to operate at high temperatures.Although the normal air-cooling of HID lights will only slightly affect the lamp's output, air-cooling with refrigerated air coming directly from an air conditioner, or drawn from outside when temperatures are below 50°F, can cause a noticeable reduction in bulb brightness.

This decrease in brightness is caused by the lower temperature, which does not allow the lamp to burn the halides and salts hot enough to produce the lamp's rated output.

The age of the lighting system's ballast and the type of ballast being used can also contribute to the lamp's light energy depreciation. This is especially true when older lamps are used. As lamps age, they may require higher operating voltages.

Digital ballasts sense when a lamp has not obtained full brightness and will increase the power to the lamp. Although this helps maintain a consistent brightness, it is no cure for PAR depreciation, as the internal components in the bulb are still degrading.

Magnetic ballasts will keep the voltage to an aged lamp consistent, which means the lamp brightness will automatically decrease as the lamp continues to age.

Due to the specific metals and components within an MH bulb, the rate of depreciation is faster than that of a high pressure sodium (HPS) bulb.

In fact, after 20,000 hours of use, an MH bulb will have depreciated twice as much as an HPS bulb. If you use MH bulbs, plan to replace them after 6-10 months of continuous use.As with MH bulbs, HPS bulbs depreciate because of the initial voltage spike, operation temperature, and the age and type of ballast being used.HPS bulbs enhanced with the blue spectrum will usually degrade in the blue spectrum output first, due to the specific compounds used. Blue spectrum aside, HPS grow lights degrade a bit slower than MH lights. If you use HPS bulbs, plan to replace them after 10-14 months of use.
single end HID lamps





When used as a sole source of crop lighting indoors, single end HID lamps like MH (Metal Halide) and HPS (High Pressure Sodium) are best replaced every 9-12 months.
DE (Double Ended) HID lamps



DE (Double Ended) HID lamps are a newer technology and give brighter light for longer-they are also more expensive to replace, typically. To balance economy and performance, these lamps can be replaced every 12-18 months.

Note: the above suggestions re grow lamp replacements assume an average of 12 ON hours per 24 hour period, so running them 18-to 24 hours a day further decreases suggested time between replacement. Very frequent ON and OFFs may also reduce optimal working life further.