Why Do Grow Lights Dim Over Time? LED Heat, Thermal Derating, and L70

Grow lights lose output for two distinct reasons: temporary performance drops during daily operation and permanent hardware aging over years of use. When an LED grow light powers on, heat accumulates within the semiconductor diodes. This heat temporarily reduces conversion efficiency and can trigger the power driver to automatically reduce current to protect internal electronics. Once the light turns off and cools down, this lost output typically recovers as the fixture returns toward its normal operating temperature.

Over thousands of operating hours, however, sustained operating heat causes irreversible material breakdown in the diodes, phosphor coatings, and primary lenses. Permanent degradation reduces the fixture’s total photon output (PPF), while the resulting PPFD measured at the canopy depends on mounting height, secondary optics, reflective tent walls, and coverage.

Short Term Operating Heat vs. Long Term Hardware Wear

Diagnosing why a grow light appears dimmer starts with distinguishing between short term heat accumulation and permanent diode wear.

• Temporary Output Loss (Thermal Droop and Active Derating): Often becomes noticeable during the first several minutes to an hour of operation. As the diodes and power supply warm up, photon emission drops typically by a small percentage under normal conditions, or more significantly if airflow is blocked. This output typically recovers as the fixture returns toward its normal operating temperature.
• Permanent Output Loss (L70 Depreciation): Develops gradually over months and years of continuous photoperiods. Prolonged exposure to elevated junction temperatures degrades phosphor coatings, darkens protective silicone encapsulants, and increases electrical contact resistance. This loss cannot be reversed.

How to Tell Whether Your Grow Light Is Losing Output

If you notice a drop in canopy light levels during daily operation, run this simple field benchmark to evaluate how heat affects your fixture:

1. Take a Cold Baseline Reading: Turn the light on after it has been off for several hours. Place a dedicated PAR meter or calibrated light sensor at a fixed distance directly beneath the center of the fixture. Record the initial PPFD reading immediately.
2. Run the Fixture for 60 Minutes: Allow the light to operate undisturbed while maintaining stable room temperature and ventilation.
3. Take a Warm Measurement: Record the PPFD at the exact same distance, position, and sensor angle after 60 minutes of operation.
4. Evaluate the Difference (Practical Field Guidelines):
   • Minor Drop: Normal thermal stabilization may be occurring. Diodes naturally experience a slight efficiency drop as they reach thermal equilibrium.
   • Moderate Drop: Elevated heat buildup. Investigate heatsink temperature, surrounding airflow, and mounting clearance to improve heat dissipation.
   • Significant Drop (Accompanied by Falling Wall Power): Active driver derating becomes more likely. The power supply is actively throttling current to prevent thermal damage.

Note: Sensitivity thresholds vary significantly based on diode package construction, heatsink mass, and driver programming. These observations are useful for practical troubleshooting, not for determining whether a fixture strictly meets a manufacturer’s exact performance specification.

A Note on Measurement Tools: For accurate diagnostic testing, always use a dedicated quantum PAR meter. While smartphone light apps paired with paper diffusers can provide a rough relative estimate of light distribution, uncalibrated camera sensors lack precise cosine correction and accurate spectral responses across the PAR spectrum (400–700 nm).

Practical Steps to Prevent Excessive Heat Buildup

Preventing driver derating and extending overall fixture lifespan requires managing the heat dissipation pathway around your setup:

• Maintain Overhead Air Clearance: Follow the manufacturer’s recommended clearance guidelines, ensuring unobstructed airflow around the heatsink. As a general rule, leaving several inches of open vertical space above top mounted heatsinks prevents rising hot air from becoming trapped over the cooling fins.
• Maintain Gentle Canopy Air Movement: Position a small circulating fan to move air across the top of the heatsink fins. Moving air disrupts stagnant boundary layers, significantly lowering operating temperatures.
• Remote Mount the Driver: If your fixture design allows, unbolt the driver power supply and mount it outside your grow tent or shelf rack. This removes a primary heat source from the plant canopy area.
• Keep Cooling Fins Clean: Dust acts as an insulating blanket on aluminum heatsinks. Gently wipe down cooling fins every few months to maintain efficient convection.
• Control Ambient Room Temperature: Passive heatsinks rely on the temperature differential (Delta T) between the fixture and the surrounding room. As ambient temperature rises, the temperature difference (Delta T) between the fixture and surrounding air becomes smaller, reducing the effectiveness of passive convection and making thermal management more difficult.

How Operating Temperature Reduces LED Photon Output

Inside every LED diode is a microscopic semiconductor layer known as the p-n junction. Light is produced when electrical current passes through this junction, causing electrons to release energy as photons. However, LEDs are not perfectly efficient a substantial portion of supplied electrical energy is converted directly into heat at this diode junction.

Internal Quantum Efficiency and Non Radiative Recombination

The temperature at the semiconductor die itself is known as the junction temperature (Tj). As Tj increases, atomic lattice vibrations within the semiconductor crystal intensify. These vibrations create non radiative recombination pathways (such as Shockley Read Hall and Auger processes, as detailed in E. F. Schubert’s Light Emitting Diodes), where electrons release energy as heat (phonons) rather than usable photosynthetic light (photons).

This reduction in Internal Quantum Efficiency (IQE) means that as operating temperature climbs, less of the incoming current yields photosynthetic light. While the exact loss depends on diode quality and drive current density, elevated junction temperatures consistently cause an immediate drop in photon emission.

Bandgap Narrowing and Spectral Dynamics

Heat causes the crystal lattice of the semiconductor material to expand slightly, which narrows its energy bandgap (Eg). Because photon energy is inversely proportional to wavelength, a narrower bandgap shifts emitted light toward longer, lower energy wavelengths.

Different diode chemistries handle heat differently:

• Red Diodes (AlInGaP): Highly sensitive to operating heat. As junction temperatures climb, red photon output drops faster than other wavebands, and peak emission shifts slightly deeper into the red spectrum.
• Blue and White Diodes (InGaN): Exhibiting higher thermal stability, blue chips experience smaller efficiency losses at elevated temperatures.

Thermal boundaries and output characteristics for high power LED components can be referenced across manufacturer documentation, such as the Cree XLamp Thermal Management Guide, the ams OSRAM Thermal Management Guide, and official Samsung LED High Power Datasheets. In broad spectrum fixtures, high operating temperatures alter the red to blue photon ratio delivered to the plant canopy.

The Heat Dissipation Pathway (Thermal Resistance)

For an LED fixture to maintain stable output, heat generated at the microscopic diode junction must continuously move into the surrounding air. Heat travels through a chain of materials according to standardized measurement frameworks (such as the JEDEC JESD51-50 Series for Thermal Measurement of LEDs), with each material presenting its own thermal resistance (Rth):

Junction (Tj) → Diode Package → Circuit Board (MCPCB) → Thermal Interface (TIM) → Heatsink → Ambient Air (Ta)

If any link in this chain presents high thermal impedance such as an uneven layer of thermal grease or restricted air movement around the fins heat backs up at the die. An aluminum heatsink may feel only moderately warm to the touch (45°C), while the sealed junction inside the diode exceeds 85°C.

In addition, as diodes warm up, their forward voltage (Vf) decreases slightly under constant current drive. Because total wattage equals current multiplied by voltage (P = I × Vf), this voltage drop reduces overall power draw, contributing to a slight drop in light output.

How LED Drivers Protect Circuits (Thermal Derating)

While diode efficiency drops naturally as heat increases, a sharp drop in light output after 15 to 30 minutes of operation is often caused by active thermal derating built into the power driver.

Modern LED drivers integrate negative temperature coefficient (NTC) thermistors that monitor circuit temperatures. When poor ventilation or tight mounting clearances cause internal temperatures to cross programmed protection thresholds configured by manufacturers based on specific driver engineering the driver automatically throttles output current (I_out). The electrical principles of negative temperature coefficient driver protection are detailed in the Mouser Technical Lighting Derating Guide and the RECOM Power Thermal Derating Technical Application Paper.

This automatic power reduction prevents component failure or circuit damage. However, it also means that, for example, a fixture designed to draw 100 W may substantially reduce its input power draw when thermal protection engages, without throwing an error code or turning off entirely.

High Temperatures and Permanent L70 Lifespan Loss

Temporary photon loss resolves when the light cools down. Long term exposure to high operating temperatures, however, causes permanent physical wear over time.

Sustained internal heat causes several structural changes:

• Phosphor Degradation: Broad spectrum white LEDs use phosphor coatings over blue chips. Excessive heat accelerates phosphor degradation, lowering photon output and causing gradual color shifts.
• Silicone Lens Discoloration: The clear silicone lens encapsulating the diode chip darkens over thousands of thermal cycles, trapping light inside the package.
• Thermal Interface Breakdown: Thermal pads and paste between the printed circuit board and aluminum heatsink can dry and crack over time, increasing thermal resistance and causing the light to run hotter in subsequent years.

To evaluate long term reliability, diode performance is tested under IES LM-80 standards (Approved Method for Measuring Lumen Maintenance of LED Light Sources). Manufacturers then use ANSI/IES TM-21 extrapolation algorithms (Projecting Long Term Lumen Maintenance of LED Light Sources) to project the fixture’s L70 lifespan the total operating hours required for photon output to permanently decay to 70% of its initial rated output. Further empirical summaries on package maintenance behavior are published by the U.S. Department of Energy SSL Program and the DOE SSL Lumen Maintenance Study.

Note: Because L70 projections depend heavily on diode package architecture (e.g., mid-power 2835 vs. high-power ceramic 3535), drive current density, and heatsink design, universal hours cannot be assigned to a single temperature value. Always consult verified manufacturer LM-80/TM-21 test reports for specific fixture projections.

Illustrative Scenario: Observing Output Changes During Operation

Disclaimer: The numerical values in the table below represent an illustrative mathematical model designed to demonstrate thermal stabilization trends. They do not represent empirical measurements from a specific commercial fixture.

To illustrate how thermal stabilization affects operating performance, consider this benchmark model of a passively cooled 100W quantum board operating in a 22°C room with static air clearance:

In this model, the initial drop represents standard thermal droop as the diodes warm up to normal operating temperatures. If heat accumulates above the heatsink, driver thermal protection engages, reducing power draw and canopy PPFD further.

Frequently Asked Questions

Is a small drop in light output normal after turning on a grow light?

Yes. A minor drop in PPFD during the first 30 to 45 minutes of operation is expected. Diodes naturally experience a slight efficiency drop as they transition from ambient room temperature to steady operating temperatures.

Will blowing a fan directly on an LED grow light increase its brightness?

If your fixture is actively experiencing thermal derating, improving air movement will restore lost output by cooling the driver and lowering diode junction temperatures. If the light is already operating within normal temperature parameters, additional airflow will not increase power beyond its rated specification, but it will help extend hardware lifespan.

Does dimming a grow light reduce heat stress on internal components?

Yes. Running an LED fixture below its maximum rated output generally reduces heat generation at the LED board and can improve photon efficacy (µmol/J), while also reducing thermal stress on internal components over time.