Understanding LEDs

If we look at the Rebel ES specification in detail you will find it’s 700mA for 220 lumen output at 3.0V forward voltage. Which means the total LED consumption is 0.7A x 3.0V = 2.1W, delivers 220 lumen. So the LED efficiency should be 104.8 lm/W.

Note: In 2013, the Rebel ES LED was updated to V-Bin, which delivers 250 lumen at 700mA. So the LED efficiency is 250 lm / (0.7A x 3V) = 119 lm/W. Regarding Luxeon M it’s 905 lumen by 700mA, 11.2V forward voltage. So it’s 905 lm / (0.7A x 11.2V) = 115.4 lm/W.

Alternatively using a Cree XPG chip package which means the total LED consumption is 0.35A x 2.9V = 1.015W, delivers 130 lumens. So the LED efficiency should be 128.07 lm/W.

Using the Luxeon M information and based on 115.4 lm/W LED efficiency which we calculated earlier, we then go to consider the LED module efficiency and luminaire efficiency. After the LED efficiency, we have driver efficiency (E/E), thermal efficiency, and lens efficiency and power supply efficiency to consider.

For example say 115.4 lm/W x 95% (E/E efficiency) x 91% (thermal decay) x 90% (lens decay) x 93% (power supply efficiency). So for system side (luminaire side) it is 83 lm/W for our SJ3 and SJ6 luminaire series and for SM series, it’s 95 lm/W.

When choosing a LED solution we must also consider the expected life which is measured under LM80 TM-21 tests.
For example:
Philips Lumileds are continuously testing the Luxeon M LED to TM-21 (LM70 was replaced with LM80 & TM-21). Currently tests have been done for over 6,000 hours so they can release the midway report to claim for 6 times the LED lifespan, which is 36,000 hours.

At this stage it doesn’t mean that this LED cannot meet more than 50,000 hours lifespan.

Once the test is over 8,000 hours, Lumileds will update the midway TM-21 report to claim for 8,000 x 6 = 48,000 hours LED lifespan legally. When the test is over 10,000 hours, Lumileds will update the midway TM-21 report to claim for 10,000 x 6 = 60,000 hours LED lifespan legally etc…