If you’ve invested in LED grow lights, you’ve likely seen claims like “50,000-hour lifespan” or “LM90-rated.” But what do these numbers actually mean for your crops? In horticulture, a light that still “turns on” isn’t necessarily a light that can still grow plants effectively.

Understanding light depreciation (often called light decay) is critical. It is the silent factor that can reduce your yields year over year if you don’t adjust your strategy.

Here is the science of LED aging, how to read the lumen maintenance curve, and how to decide when to replace your fixtures.

What is “Light Depreciation”?

Unlike traditional incandescent bulbs that suddenly burn out, LEDs experience a gradual decline in light output over time. This is known as lumen depreciation.

For plant growth, we care specifically about the decline in Photosynthetically Active Radiation (PAR) and photosynthetic photon flux density (PPFD). As the diode ages, the intensity of the light decreases, and in some cases, the spectrum can even shift.

While an LED may still emit a visible glow after 10 years, its efficiency in driving photosynthesis may have dropped by 20% or more.

The Science Behind the Decay

Why do LEDs lose their power? The process is driven by a combination of electrical and thermal stress:

Heat is the enemy: The junction temperature of the LED chip directly impacts its lifespan. Even if a fixture is cool to the touch, the internal diodes experience significant heat. If cooling systems (heat sinks) fail or become clogged with dust, operating temperatures rise, accelerating decay.

Material degradation: Over time, the materials used in LEDs degrade. Research shows that while red LEDs tend to degrade gradually, blue LEDs can suddenly suffer a sharp decrease in optical power due to silicone failure after a period of stability.

Environmental factors: In a grow room, humidity is high and chemicals (like sulfur used to prevent fungal diseases) are present. Sulfur can oxidize the reflectors inside the LED, reducing light output by as much as 25% in severe cases.

How Lifespan is Measured: The L-Number System

To avoid vague claims, the industry uses a standardized metric defined by the Illuminating Engineering Society of North America (IESNA) . This is the LM rating.

When you see a claim like LM90, it tells you exactly how long it takes for the light to lose a specific percentage of its initial output.

L70 (The Standard): The time it takes for the light output to fall to 70% of its original intensity. This is often considered the “usable life” of a general lighting product.

L90 (The Grower’s Standard): The time it takes to fall to 90% of its original output (a 10% loss). For professional horticulture, L90 is a much better benchmark. Losing 10% light can equate to a 10% loss in production, which is significant for commercial operations.

For example, a premium fixture might boast an L90 > 90,000 hours. This means that after 90,000 hours of operation, the light will still produce at least 90% of the light it did on day one.

The Lumen Depreciation Curve (Visualizing the Decline)

The depreciation of an LED isn’t a straight line; it follows a decay curve, often modeled using the Weibull function for accuracy.

The “Knee” Point: Most LEDs show a sharp initial “settling” drop in the first few hundred hours, followed by a long period of linear decline, and finally a steep drop-off at the end of their life.

The Importance of Data: Manufacturers use Accelerated Life Testing (ALT) to predict this curve. They run LEDs at high temperatures to simulate years of wear in months, then extrapolate back to normal conditions.

How to Interpret Manufacturer Claims

Beware the “50,000 Hour Myth”:
You will often see claims of “50,000 to 100,000 hours” of life. However, this number is often the L70 rating—meaning the light is considered “dead” for plant growth at 70% output. This is akin to a car having a “lifetime” warranty that only covers the engine until the wheels fall off.

How to Compare Units:
When shopping for lights, look for the specific LM80 or LM90 data provided in the technical datasheet. Reputable manufacturers (like SANlight or those using Seoul Semiconductor or Osram chips) will often specify:

LM90 > 90,000h: This guarantees 90% output for 90,000 hours (approx. 10 years of 24/7 use).

End of Life: The point at which the light should be replaced is generally when it reaches 70% to 80% of its initial output, depending on your crop’s light requirements.

Practical Tips for Growers

Invest in a PAR Meter: The only way to know if your light is degrading is to measure it. Use a quantum sensor (PAR meter) to take readings at canopy level. Do this when the light is new to establish a baseline, and check it annually.

Look for Dimming or Diminishing Returns: As the lamp ages, you may find your plants are stretching more or yields are slightly down compared to previous cycles. This can be a symptom of depreciation.

Heat Sink Maintenance: Keep heat sinks and fans clean. Dust acts as an insulator, trapping heat and accelerating light decay. Regular cleaning is the easiest way to extend the L90 lifespan.

Check for Uniformity: Do some diodes look dimmer than others? This indicates inconsistency in degradation. If the light distribution becomes uneven, it’s time to consider replacement.

Conclusion

When a grow light claims a “long life,” it doesn’t mean it will grow plants well for that entire duration. The “life” of a horticultural light is tied to its lumen maintenance.

By understanding the L-rating system and the depreciation curve, you can schedule maintenance, replace your lights at the optimal economic time (before you lose a significant percentage of your crop), and ensure your plants receive the intensity they need to thrive.