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LED optics in Flashlight

The document summarizes the design of a flashlight collimating system. It discusses properties of LED emitters and the objective to collimate light into a bright hotspot to increase throw. A collimating system typically uses a reflector, lens, or optics combination. Reflectors can vary the depth to diameter ratio to control hotspot size and spill light. Lenses can form a sharp image of the emitter but require addressing chromatic aberration. Optics provide more flexibility and allow total internal reflection for high efficiency. The document proposes improving reflective coatings and developing optimized reflector-like optics.

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Flashlight Collimating System Can Fang Email: mrfunder@hotmail.com Jan, 2011 1
Outline • Properties of Emitter • Design Objective of Collimating System • Collimating System Overview – Reflector – Lens – Optics • Proposed Directions 2
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
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
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
Some Photometry Fact of Cree Emitters LED Name XP-E XP-E Hew XP-G XM-L Size 1×1 mm 1×1 mm 1.4×1.4 mm 2×2 mm Luminous Flux 250 @1A 330@1A 500@1.5A 1000@3A (lumen) Max Luminous Intensity 80 105 159 318 (candela) Luminance (cd/m2) 8.0 e7 1.05 e8 8.0 e7 8.0 e7 Note: • Data for best Bin available • cd/m2 also called “nits” • Observed Luminance from outside of the emitter ≈ luminance/n2, where n ≈ 1.5 6
Outline • Properties of Emitter • Design Objective of Collimating System • Collimating System Overview – Reflector – Lens – Optics • Proposed Directions 7
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
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
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
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 limit Osram and Luminus offering the emitter without hemisphere lens: 11 CBT-90-W Golden dragon
Other Concerns • Efficiency: minimize the loss of the light • Spill light, transition between the spill and hotspot • Smoothness of the hotspot • Manufacturability, cost 12
Outline • Properties of Emitter • Design Objective of Collimating System • Collimating System Overview – Reflector – Lens – Optics • Proposed Directions 13
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
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 0.2 Example: when θ=45 degree, we will 0 have 90 degree of spill light, hotspot will 0 0.5 1 1.5 2 has about 50% energy and spill light has Depth/diameter ratio about 50% energy 15
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 ratio Simulation setting: 60mm diameter paraboloid reflector, target is 10m away from the reflector 16
Coma: The Transition from Hotspot to Spill spill Question: Where does the coma coma come from? hotspot 17
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
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
Simulation Test Diameter =60mm, depth =60mm Diameter =60mm, depth =30mm 20
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
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
Outline • Properties of Emitter • Design Objective of Collimating System • Collimating System Overview – Reflector – Lens (and reversed reflector) – Optics • Proposed Directions 23
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
Collimated Energy θ θ Hotspot Be wasted or transformed into spill light by incorporating with another reflector -θ θ Spatial distribution (degree) 25
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 length observed 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
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
Outline • Properties of Emitter • Design Objective of Collimating System • Collimating System Overview – Reflector – Lens (and reversed reflector) – Optics • Proposed Directions 28
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
Total Internal Reflection Air: 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
The “Standard Optics” Methodology: All light will be Square spot formed by convex lens collimated (no spill) Example: 1st SF Gen KL1, KL3 ARC LSHP, Longbow Round spot formed by reflector 31
INOVA’s TIROS (1st Gen) Comment: A weird design, some narrow spill, large length, replaced by reflectors in second gen T series 32
The Second Gen TIROS Methodology: Reflector like, much spill 33
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
Surefire’s TIR (version A) Methodology: Reflector like (for general use) Protective Lens Diffuser film attached to lens TIR optics 35
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
Outline • Properties of Emitter • Design Objective of Collimating System • Collimating System Overview – Reflector – Lens – Optics • Proposed Directions 37
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
Optics • Optics make difference – Appearance – Performance – Cost • Start with reflector-like optics, coated PMMA or optical glass with AR coating 39
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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