LED optics in Flashlight


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This slides includes theoretic analysis, design principles and some existing designs in LED optics used in portable LED lights (such as flashlight).

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LED optics in Flashlight

  1. 1. Flashlight Collimating System Can Fang Email: mrfunder@hotmail.com Jan, 2011 1
  2. 2. Outline• Properties of Emitter• Design Objective of Collimating System• Collimating System Overview – Reflector – Lens – Optics• Proposed Directions 2
  3. 3. Emitter Analysis• Mainstream LEDs: a square emitter located in the center of a hemisphere lens: Include: Cree XP-E, XP-G, XM-L; SSC P4, P7; Lumileds K2, Rebel; Luminus SST series. Exclude: Cree XR-E (has a reflector ring), Luminus CBT-90, Osram golden dragon (no lens), diamond dragon• This Type of LEDs can be approximately formulated as Lambertian sources 3
  4. 4. Spatial Distribution of Flux Energy• The spatial distribution of flux energy can be deducted from the intensity distribution diagram given by the LED manual θ Observation: emitter flux light in 180 (hemisphere) degree, although the intensity peak is θ=0 degree, the energy peak is θ=45 degree 4
  5. 5. The Effect of Hemisphere Lens• The hemisphere lens, which is known to be the “first optics”, has the “magnification effect”• The size of emitter under the lens is magnified to be about n times of its real size, where n is the refractive index of the lens Left: Photo of real emitter(size under lens); Right: Rending model (shows actual size) As an example, when n=1.5, the 2×2mm emitter of XM-L looks like a 3×3mm emitter under the lens. This, however, will decrease the observed luminance of the emitter 5
  6. 6. Some Photometry Fact of Cree EmittersLED Name XP-E XP-E Hew XP-G XM-LSize 1×1 mm 1×1 mm 1.4×1.4 mm 2×2 mmLuminous Flux 250 @1A 330@1A 500@1.5A 1000@3A(lumen) MaxLuminous Intensity 80 105 159 318(candela)Luminance (cd/m2) 8.0 e7 1.05 e8 8.0 e7 8.0 e7Note:• Data for best Bin available• cd/m2 also called “nits”• Observed Luminance from outside of the emitter ≈ luminance/n2, where n ≈ 1.5 6
  7. 7. Outline• Properties of Emitter• Design Objective of Collimating System• Collimating System Overview – Reflector – Lens – Optics• Proposed Directions 7
  8. 8. The Function of Collimating System• Reform the light into desired pattern• What is the “best” pattern? Answers depend on the applications• In typical flashlight, it should has a bright hotspot• This indicates we need to collimate the light from LED, which is distributed in 180 degree, into a small angle (usually several degree)• In the language of flashaholic, increase the “throw” 8
  9. 9. The Calculation of “Throw”• In ANSI standard, the distance of throw is defined as the distance which the flashlight produces a illuminance of 0.25 lux• Or: throw = Luminous Intensity 0.25 Example: Fenix TK35, claimed has luminous intensity of 27739 cd, its throw can be calculated as: 27739  333 (metres) 0.25 Conclusion: Throw is only determined by luminous intensity of the flashlight (when the target is faraway, hotspot size is much larger than the diameter of the light) 9
  10. 10. Theoretical Limit of Throw• It can be deducted from optical laws (process omitted): 2  nreceiver  I max  Lemitter  Aoptic     nemitter  Where Imax is the maximum luminance intensity, Lemitter is the Luminance of the emitter, Aoptic is the projective area (to the target direction) of the collimating system, nreceiver and nemitter is the refractive index of the media in which target and emitter located, respectively.Example: An XM-L powered light, the diameter of the collimating system is 50mm,nreceiver = 1 and nemitter = 1.5, the maximum Luminous Intensity we can achieve is: 2  1 8.0 e8    0.025   2   70000 (candela)  1.5  10
  11. 11. Ways to Increase The Throw From the formula, to increase the limit of throw, we can: 1. Choose emitter with higher Luminance (such as XP-E Hew and XR-E); 2. Use larger diameter of collimating system; 3. Remove the hemisphere lens of the emitter (is it possible? ) In the engineering side: • Adopt better design to approach the theoretical limitOsram and Luminusoffering the emitterwithout hemisphere lens: 11 CBT-90-W Golden dragon
  12. 12. Other Concerns• Efficiency: minimize the loss of the light• Spill light, transition between the spill and hotspot• Smoothness of the hotspot• Manufacturability, cost 12
  13. 13. Outline• Properties of Emitter• Design Objective of Collimating System• Collimating System Overview – Reflector – Lens – Optics• Proposed Directions 13
  14. 14. Overview• Most widely used in flashlight manufacturers• Simple and effective• With good hotspot shape and significant of spill light• Will still be the mainstream in foreseeable future 14
  15. 15. Energy Distribution: Collimated vs. Spill Spill light angle = 2θ Spill Hotspot -θ θ Spatial distribution (degree) 1 Portion of collimated θ 0.8 0.6 engergy 0.4 0.2Example: when θ=45 degree, we will 0have 90 degree of spill light, hotspot will 0 0.5 1 1.5 2has about 50% energy and spill light has Depth/diameter ratioabout 50% energy 15
  16. 16. The Effect of Depth/Diameter Ratio Peak illuminance (Lux) Spill Angle (degree) 1200 180 160 1000 140 800 120 600 100 80 400 60 200 40 20 0 0 0 0.5 1 1.5 2 0 0.5 1 1.5 2 Depth/diameter ratio Depth/diameter ratioSimulation setting:60mm diameter paraboloid reflector, target is 10m away from the reflector 16
  17. 17. Coma: The Transition from Hotspot to Spill spill Question: Where does the coma coma come from? hotspot 17
  18. 18. The Cause of “Coma” • The emitter is not a pinpoint, thus we can not get real parallel beam φ2 • The diverge angle is smaller (tighter∠φ1 >∠φ2 beam) when the reflector is larger and/or the emitter is smaller • At each point of the reflector, the diverge angle is different, thus we φ1 cannot get a sharp hotspot A • The diverge angle is the maximum when Diverge angle θ= 60 degree. θ B θ (degree) 18
  19. 19. Deep Reflector vs. Shallow Reflector φ2 β1 β2 φ1 ∠φ1 >∠β1 >∠β2>∠φ2 A Deep reflector has a smaller hotspot and a larger coma A’ B’ B 19
  20. 20. Simulation TestDiameter =60mm, depth =60mm Diameter =60mm, depth =30mm 20
  21. 21. Efficiency of Reflector• Light loss mainly caused by the imperfect mirror reflection, the reflectivity <100%• Current technologies: – Aluminum coating 70~89%, mainstream (OP is lower) – Silver coating 90~95% (smooth) – Dielectric coating, up to 99+%• Usually a protection lens in the front, AR coating can reduce the loss 21
  22. 22. Summary of Reflector• The depth/diameter ratio will affect: – The size of hotspot – The size of the coma – The proportion of collimated energy – The angle of spill light• The intensity of spill light can not be controlled by the reflector• The efficiency of the reflector is mainly determined by the reflection coating 22
  23. 23. Outline• Properties of Emitter• Design Objective of Collimating System• Collimating System Overview – Reflector – Lens (and reversed reflector) – Optics• Proposed Directions 23
  24. 24. Overview• Used by some “throwers”• Strong and sharp hotspot• The hotspot is a “image” of emitter Aspheric lens bezel Reversed reflector (also known as “recoil LED”) 24
  25. 25. Collimated Energy θ θ Hotspot Be wasted or transformed into spill light by incorporating with another reflector -θ θSpatial distribution (degree) 25
  26. 26. Hotspot Size Simulation test Since it is imaging system, hotspot size is only determined by focal length: spot size target distance  observed emitter size focal lengthobserved emitter size = real emitter size  refractive index of first optics Example: focal length = 60mm, target is 10m away, XM-L led emitter size is 2mm, the refractive index of first optics (hemisphere lens) is 1.5. The spot size = 10000x2x1.5/60=500mm 26
  27. 27. Other Concerns• For reversed reflector, thermal control is more difficult• Lens system has chromatic aberration (false color) issues• Since the numerical aperture of lens is usually large, aspheric surface should be adopted to remove spherical aberration• Fresnel lens can be used to reduce the thickness and weight 27
  28. 28. Outline• Properties of Emitter• Design Objective of Collimating System• Collimating System Overview – Reflector – Lens (and reversed reflector) – Optics• Proposed Directions 28
  29. 29. Overview• May use reflection and/or refraction to collimate light. In most cases, it combines reflection and refraction.• More freedom, more variety in the design• In proper design, both spill light and hotspot can be better controlled• Total Internal Reflection(TIR) instead of reflection coating 29
  30. 30. Total Internal ReflectionAir: nair ≈1  nair  • When:   sin 1    nmedia  Mostly refracted (pass through), some reflected  nair  • When:   sin 1    nmedia  θ 100% reflected, no pass through θ Glass or other media nmedia>1 It is the most efficient way to redirection light! 30
  31. 31. The “Standard Optics”Methodology: All light will be Square spot formed by convex lenscollimated (no spill)Example: 1st SF Gen KL1, KL3 ARCLSHP, Longbow Round spot formed by reflector 31
  32. 32. INOVA’s TIROS (1st Gen) Comment: A weird design, some narrow spill, large length, replaced by reflectors in second gen T series 32
  33. 33. The Second Gen TIROS Methodology: Reflector like, much spill 33
  34. 34. LED lenser’s “Zoom Optics” Methodology: Zoom Capable Nearly no spill in “spot” state The shape of emitter can be noticed in “spot” state 34
  35. 35. Surefire’s TIR (version A) Methodology: Reflector like (for general use) Protective Lens Diffuser film attached to lens TIR optics 35
  36. 36. Surefire’s TIR (version B) Methodology: A large, strong spot, very light spill (for tactical use) Protective lens with diffuse film attached Lens are AR-coated 36
  37. 37. Outline• Properties of Emitter• Design Objective of Collimating System• Collimating System Overview – Reflector – Lens – Optics• Proposed Directions 37
  38. 38. For Reflectors• Properly choose depth/diameter ratio to balance several performances issues• Seek for better reflective coating to minimize the difference between bulb lumens and OTF lumens 38
  39. 39. Optics• Optics make difference – Appearance – Performance – Cost• Start with reflector-like optics, coated PMMA or optical glass with AR coating 39
  40. 40. An Example It is not only reflector-like, it is better: • Higher efficiency: TIR reflectivity ratio is 100%, when multi layer AR coated, reflection loss can be below 1%, absorption loss around 1%, 95% total transmission is easy to achieve; • Wider spill, more than 90 degree is easy to achieve, even when the “TIR reflector” is deep; • Appearance stands out of lame brands use reflectors, AR coating makes it looks even better • One-peace design, reduce the cost in mass- production 40
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