Design Considerations of LED based Visible Notification Appliance SUPDET 2015
LED Applications Indoor/Outdoor Video Wall TV/Monitor backlighting Automatic Lighting General Lighting Can LED use in Visible Notification appliance?
LED Strobe Design Considerations 1. Is LED bright enough? 2. How to do pulse current? 3. How to keep LED Junction Temperature low? 4. What kind of optical lens required? 5. Verification Experiments
Xenon vs LED for Visible Notification Ken Savage research (2011/2012) 1. LED has much lower peak intensity than Xenon; LED can only lengthening its pulse width to be comparable to mainstream candela rating. 2. Pulse duration > 100 ms is in-detectable, essentially invisible
Xenon/LED for Visible Notification John D. Bullough research (2013) 1. Indirect view as a method to evaluate the light source performance 2. Indirect effectiveness quantity (IEQ) based on formula for Effective Intensity with a=0.01s I EQ = t1 t2 I t dt a + t2 t1 3. IEQ value for room illuminance of 500 lux is 750cd with detection rate 90%, for a light source at a distance of 20 feet.
IEQ and LED flux IEQ definition I EQ = t2 I t dt t1 a + t2 t1 IEQ for LED with flat top intensity I EQ = I p t p a + t p LED peak intensity I p = I EQ(a + t p ) t p For a lens with K i cd/lm, LED Flux = I EQ(a + t p ) K i t p with a = t p = 0.01s and K i = 0.25 cd/lm IEQ value of 750 cd, equivalent to LED Flux = 2 x 750 0.25 lm = 6000lm
LED Driving Scheme 27.35mm 3.45mm 3.45mm 27.35mm Lens too big Feasible Not bright enough
LED Pulse Current LED can be overdriven to 300% of its maximum rated continuous current, without reliability issue if the pulse is less than 10% duty cycle. The efficacy (Lumen/Watt) drops with overdriven current. It takes three times rated LED power to double the luminous flux.
LED Junction Temperature In DC operation, the LED junction temperature T j can be found by: T j = T A + P D R th,ja = T A + P D (R th,js + R th,sa ) In periodic pulse operation, the LED junction temperature T j can be found by: T j = T A + 1 D P D Z th (t p ) + DP D R th,ja Pulsed operation Transient Temperature Continuous operation Average Temperature Z th t p : dynamic thermal impedance D = t p T.
LED Junction Temperature Pulsed operation Transient Temperature Continuous operation Average Temperature
LED Junction Temperature ~20ms If LED junction temperature is higher than the desired value, the thermal management system needs to be improved. Otherwise, there is room for operating the LED at a higher current or ambient temperature.
Optical Design : Cross light pattern
Optical Design : Ideal light pattern
Experimental Prototype LED : 3250lm @ 12V and 2.4A 7000lm @ 7.7A 40 ⁰ Cone lens : Peak intensity : 20150 cd IEQ = 10075 cd. LED current (red trace) / Forward voltage (purple trace)
Room Setup
LED Strobe light effect LED Strobe Light Prototype Camera : shutter speed 1s, aperture F/8 and ISO 200
Xenon strobe light effect Xenon Strobe Light 185 cd Camera : shutter speed 1s, aperture F/8 and ISO 200
LED vs Xenon Strobe Light Light pattern of LED strobe light (10ms) IEQ = 10075 Light pattern of xenon strobe light at 185 cd IEQ = 3380
LED vs Xenon Strobe Light LED strobe light (10ms) / IEQ = 10075 Xenon strobe light 185 cd / IEQ = 3380
Indirect Viewing Experiment LED : 7000 lm with lens 0.27 cd/lm IEQ 945 cd for 635 lux ambient. Xenon : Effective Intensity : 135 cd IEQ 2466 cd Subject 1. Working on 5 mins questionnaire 2. Unaware of purpose of experiment 3. Sitting 10 feet away facing the same wall as the strobe light 4. Average horizontal illumination on table top : 635 lux. 20 feet
Indirect Viewing Experiment LED : 7000 lm with lens 0.27 cd/lm IEQ 945 cd for 635 lux ambient. Xenon : Effective Intensity : 135 cd IEQ 2466 cd #1 Flashing 10 times Lamp type IEQ / cd Duration / s #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 Detection rate / % LED 945 10 Yes No No No Yes No No No Yes No 30 Xenon 2466 10 No No No Yes No Yes No No No No 20
Indirect Viewing Experiment LED : 7000 lm with lens 0.27 cd/lm IEQ 945 cd for 635 lux ambient. Xenon : Effective Intensity : 135 cd IEQ 2466 cd #2 Flashing Xenon only until noticed Lamp type IEQ / cd Duration / s #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 Detection rate / % Xenon 2466 > 60 Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes 100
Indirect Viewing Experiment LED : 7000 lm with lens 0.27 cd/lm IEQ 945 cd for 635 lux ambient. Xenon : Effective Intensity : 135 cd IEQ 2466 cd #3 Flashing 60 times Lamp type IEQ / cd Duration / s #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 Detection rate / % LED 945 60 Yes Yes No Yes Yes Yes Yes Yes Yes Yes 90 Xenon 2466 60 No Yes Yes - Yes Yes Yes Yes Yes Yes 89 1. LED prototype strobe lamp with uniform light pattern lens shows similar detection performance as a 135 cd xenon strobe lamp. 2. LED experiment obtained similar result (90% detection rate with 945 cd @ 635 lx) as the baseline luminous intensity requirement of 750 cd @ 500 lux.
LED Technology for Visible Notification LED can provide a reasonably high luminous intensity with short pulse length for strobe light applications. A formula for estimating the lumen requirement of LED for strobe lights is derived. Short LED pulse current can ease the design of thermal system. Light energy could be reduced with lens of shaped light patterns. Experimental verification has shown that the design of LED based visible notification appliances is feasible.