When growing indoors with artificial lighting systems, it is important to know how and when light energy diminishes.

Usually, when growers talk about diminishing light energy, they are referring to the fact that light energy reduces exponentially as it travels away from its source, so plants closer to the light source receive higher amounts of light energy than plants farther away from the light source.

Of all the factors to master in an indoor garden, understanding how light diminishes from its source is the most imperative to maintaining an efficient growroom.

Growers use trellis netting or other plant manipulation techniques, such as topping or super-cropping, to best use the light energy available.

By placing the maximum amount of plant materials as close as possible to the light source, you will automatically make the most of your lighting system. However, the distance light travels is not the only factor to consider in regards to diminishing light energy.

All lighting technologies will depreciate over time, which causes a reduction in usable light energy for plants called photosynthetically active radiation (PAR).

To combat this, you must replace the bulbs in your lighting systems when needed. A better understanding of why and when to replace bulbs in each type of lighting system will help you understand how to maintain consistently high levels of PAR indoors, allowing you to maximize the return on your investment.

When to Replace High Intensity Discharge 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.

Read More: Is It Time to Upgrade Your Magnetic Ballasts to Digital Ballasts?

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 lights degrade a bit slower than MH lights. If you use HPS bulbs, plan to replace them after 10-14 months of use.

When to Replace T5 Fluorescents

As with HID lighting, the PAR output of T5 fluorescents depreciates over time. The main causes are photochemical degradation of the phosphor coating, the loss of gasses through the electrodes and the accumulation of light-absorbing particles within the lamp.

Despite these reasons for PAR depreciation, T5 fluorescent lamps are one of the more efficient lighting systems available to growers because their depreciation is relatively slow.

T5 fluorescents from an indoor garden shop are most likely coated with rare earth phosphors, which means they will only lose 5-10% of their initial lumens after 20,000 hours of operation.

This is quite remarkable when you consider that even compact fluorescents will lose around 20% of their initial lumens after 10,000 hours of operation. If you are using T5 fluorescents, plan to replace the bulbs every 18-24 months.

When to Replace LEDs

One of the biggest benefits of using an LED lighting system is how long it lasts. PAR depreciation in LEDs depends on the way the bulbs are configured and the actual design of the lighting system.

The primary cause of light reduction for LEDs is heat generated at the LED junction. Since LEDs do not emit heat as infrared radiation like other light sources, the heat must be removed via conduction or convection.

If the design of the lighting system has inadequate heat sinking, the temperature of the unit will rise and the result will be diminished light output.

The type of encapsulant used to cover the LED chips can also be a factor in diminished light output. A cloudy, epoxy encapsulant can significantly reduce the light output. High-quality LED lighting systems generally use silicone as the encapsulant to eliminate this problem.

Operating LED chips at their full capacity will increase the rate of their depreciation, which is why so many horticultural LED lighting fixtures are rated by both the full-capacity wattage and the actual wattage draw.

The full-capacity wattage is the full wattage the LED chip can handle. For example, an LED lighting system may contain multiple 5-W LED chips.

However, when that light system is operating, the LED chips may only draw 3 W each.

Horticultural LEDs are designed this way to reduce the heat at the LED junction, which increases light energy efficiency and extends the longevity of the entire unit.

Before purchasing an LED lighting system, check both the full-capacity wattage and the actual wattage. The closer these two wattages are to each other, the faster the system’s light energy will degrade over time.

If you opt to use a high-quality LED lighting system, you can expect to use it for 5-10 years before needing to replace it.

Read More: The Modern Farmer’s Guide to Artificial Lighting, LED Edition

Light Measurements Growers Should Use

There are many light measurement devices available to hobby growers. Unfortunately, most of the affordable light meters will not give an accurate reading when it comes to PAR or usable plant energy. However, that does not mean you can’t use a light meter to help determine when a bulb needs replacing.

Most light meters sold at hydro stores can be used to compare two or more of the same bulbs as long as the light meter is placed at the exact same distance from the lighting source. Just be sure the bulbs are the exact same wattage and type, and are from the same manufacturer.

The best way to use light meters is to take readings when the bulb is new, then at the six-month and one-year markers, and record the results. Using that data as a baseline, the light meter should help you determine when a bulb of the exact same kind is diminishing in its light output.

As for comparing different brands, there is just too much variance between different bulb types and manufacturers to make an accurate comparison of different technologies.

Read More: A Beginner's Guide to Calculating Lighting Needs

When I was managing a hydro shop, many new gardeners would ask me what simple thing they could do to make a noticeable difference in the garden. Can you guess what I told them?

That’s right, I taught them how to maximize the use of their light energy. The better you use your light energy, the better your yields are going to be.

When other customers would come into my store complaining of reduced yields, the first thing I asked them was, “When was the last time you changed your bulbs?”

Grow lamps replaced in a timely manner will keep the light energy of an indoor garden consistent and will help you maintain consistent yields harvest after harvest.