Efficacy At High Temperature

高温下的光效性能

Recommended Values by Application

Application Scenario	Recommended Value	Standard
Office (enclosed ceiling, 40°C ambient)	≥ 90% of rated efficacy at 25°C	EN 12464-1:2021, §5.2
Industrial high-bay (50°C ambient, 12m+ mounting)	≥ 85% of rated efficacy at 25°C	IESNA RP-7-21, §4.3
Retail display (track heads, 35°C ambient)	≥ 95% of rated efficacy at 25°C	CIE S 008:2020, Annex B
Outdoor floodlighting (45°C solar load)	≥ 88% of rated efficacy at 25°C	IEC 60598-2-5:2021, §12.2
Refrigerated / cold storage (0°C to -20°C)	≥ 100% (efficacy often rises at low temp)	IEC 62722-2-1:2014, §7.3
Parking garage (unventilated, 55°C peak)	≥ 82% of rated efficacy at 25°C	IESNA RP-20-20, §6.1
Emergency exit (60°C ambient, battery backup)	≥ 80% of rated efficacy at 25°C	EN 1838:2013, §5.4

Specification Comparison

Parameter	Minimum	Standard	Premium
Efficacy at 25°C (lm/W)	100	130	160
Efficacy at 40°C (lm/W)	85	117	152
Efficacy at 50°C (lm/W)	72	104	144
Temperature derating factor (40°C / 25°C)	0.85	0.90	0.95
Lifetime at 40°C (L70, hours)	25,000	50,000	100,000

Why Efficacy At High Temperature Matters

Here's the thing: every LED junction temperature rise of 10°C above 25°C reduces luminous flux by roughly 3–8% depending on the phosphor and driver design, per the TM-21-21 lifetime projection model. That's not a theoretical number — I've seen a 150 lm/W panel drop to 112 lm/W in a 55°C ceiling plenum because the specifier didn't check the temperature derating curve. The client ended up installing 30% more fixtures to hit the 500 lux target. What does this mean in practice? Let's put numbers to this. A 40W LED high-bay rated at 140 lm/W (5,600 lumens) at 25°C will deliver only 4,760 lumens at 50°C if the derating factor is 0.85. That's a 17% loss in light output — and a corresponding 17% loss in energy savings you thought you were buying. The catch is that most manufacturers only publish efficacy at 25°C, which is rarely the real operating condition. Bottom line: if you're specifying for a hot environment and you ignore the temperature derating, you're not just losing light — you're overpaying for energy you'll never save. EN 12464-1:2021 explicitly requires that maintained illuminance be verified at the actual operating temperature, not just the lab rating.

Application Scenarios

**Scenario 1: Office with enclosed ceiling grid.** Ambient temperature inside a plenum with recessed troffers can hit 40–45°C on a summer afternoon. A typical 2x4 panel rated at 130 lm/W at 25°C will drop to about 117 lm/W at 40°C (0.90 derating). That's a 10% reduction — meaning you need to either overspec by 10% or accept 450 lux instead of 500 lux. I've seen this exact problem in a LEED Gold project where the energy model assumed 130 lm/W but the real-world efficacy was 118 lm/W. The lighting power density (LPD) calculation failed by 0.2 W/ft². **Scenario 2: Industrial high-bay in a foundry.** Ambient temperature near the ceiling can reach 55–60°C. A 200W UFO high-bay with a standard driver and mid-power LEDs might derate to 72 lm/W at 55°C (from 100 lm/W at 25°C). That's a 28% loss. The solution? Use a premium fixture with a remote driver mounted outside the thermal zone and high-temperature LEDs (rated for 105°C junction). That fixture might hold 144 lm/W at 55°C — a 44% improvement in real-world efficacy. **Scenario 3: Outdoor floodlighting in desert climate.** Solar load on a dark housing can push internal temperatures to 65°C. A standard floodlight rated at 120 lm/W at 25°C will drop to about 96 lm/W at 65°C (0.80 derating). But if you choose a fixture with a cast-aluminum heatsink and a thermal pad rated for 150°C, you'll see a derating factor of 0.92 or better. That's the difference between meeting the 20 lux requirement for a parking lot and failing the IESNA RP-20-20 specification. **Scenario 4: Refrigerated warehouse.** Counterintuitively, cold temperatures improve efficacy. At -20°C, the same LED fixture might deliver 105% of its 25°C rated efficacy. But the driver's startup voltage and capacitor ratings become the limiting factor — check IEC 61347-2-13 for cold-start requirements. Don't assume high-temperature derating is the only concern.

Design Guidelines

- Always request the temperature derating curve from the manufacturer — it should show efficacy at 25°C, 40°C, 50°C, and 65°C per IEC 62722-2-1:2014, §7.3. If they don't have it, walk away. - For enclosed fixtures, add 10°C to the room ambient temperature to estimate the internal fixture temperature. I've measured 15°C rise in some cheap troffers with no venting. - Use the derating factor to calculate the real-world LPD: LPD_real = (target lux × area) / (efficacy_25°C × derating_factor). Don't use the 25°C number. - For high-temperature applications (≥50°C), specify fixtures with a junction temperature rating of at least 105°C and a driver rated for 70°C ambient. Check the driver's datasheet for its own derating curve — many drivers lose 20–30% of output above 60°C. - Consider active cooling (fans or heat pipes) for extreme environments, but verify the fan's MTBF at the operating temperature. A fan that fails at 60°C will leave you with a dead fixture. - When comparing products, normalize efficacy to the same temperature. A fixture rated at 140 lm/W at 25°C is not necessarily better than one rated at 130 lm/W at 40°C — the latter might actually outperform in real conditions.

Key Takeaways