led optics in flashlight

Flashlight Collimating System

Can Fang

Email: [email protected]

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:

• This Type of LEDs can be approximately formulated as Lambertian sources

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

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(lumen) Max

250 @1A 330@1A [email protected] 1000@3A

Luminous Intensity (candela)

80 105 159 318

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




– Reflector

– Lens

– Optics


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:

27739333 (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

maxreceiver

emitter optic

emitter

nI L A

n

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

2 18.0 e8 0.025 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

11

Osram and Luminusoffering the emitter without hemisphere lens:

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




– Reflector

– Lens

– Optics


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

θ

Spill light angle = 2θ

Spatial distribution (degree)θ-θ

Spill

Hotspot

Example: when θ=45 degree, we will have 90 degree of spill light, hotspot will has about 50% energy and spill light has about 50% energy 15

0

0.2

0.4

0.6

0.8

1

0 0.5 1 1.5 2

Po

rtio

n o

f co

llim

ate

d

en

gerg

y

Depth/diameter ratio

The Effect of Depth/Diameter Ratio

Simulation setting:60mm diameter paraboloid reflector, target is 10m away from the reflector

0

200

400

600

800

1000

1200

0 0.5 1 1.5 2Depth/diameter ratio

Peak illuminance (Lux)

16

0

20

40

60

80

100

120

140

160

180

0 0.5 1 1.5 2Depth/diameter ratio

Spill Angle (degree)

Coma: The Transition from Hotspot to Spill

spill

hotspot

coma

Question: Where does the coma come from?

17

The Cause of “Coma”

φ1

φ2

A

B

• The emitter is not a pinpoint, thus we can not get real parallel beam

• The diverge angle is smaller (tighter 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 cannot get a sharp hotspot

• The diverge angle is the maximum when θ= 60 degree.

θ

Diverge an

gle

θ (degree)

∠φ1 >∠φ2

18

Deep Reflector vs. Shallow Reflector

A

B

A’

B’

φ1

φ2β1 β2

∠φ1 >∠β1 >∠β2>∠φ2

Deep reflector has a smaller hotspot and a larger coma

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




– Reflector

– Lens (and reversed reflector)

– Optics


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

Spatial distribution (degree)θ-θ

θ θ

Hotspot

Be wasted or transformed into spill light by incorporating with another reflector

25

Hotspot Size

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

Since it is imaging system, hotspot size is only determined by focal length:

Simulation test

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




– Reflector

– Lens (and reversed reflector)

– Optics


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

Glass or other medianmedia>1

Air: nair ≈1

θ

θ

• When:

Mostly refracted (pass through), some reflected

• When:

100% reflected, no pass through

1sin air

media

n

n

1sin air

media

n

n

It is the most efficient way to redirection light!

30

The “Standard Optics”

Square spot formed by convex lens

Round spot formed by reflector

Methodology: All light will be collimated (no spill)Example: 1st SF Gen KL1, KL3 ARC LSHP, Longbow

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” stateThe shape of emitter can be noticed in “spot” state

34

Surefire’s TIR (version A)

35

Methodology: Reflector like (for general use)

Protective Lens

TIR optics

Diffuser film attached to lens

Surefire’s TIR (version B)

36

Methodology: A large, strong spot, very light spill (for tactical use)

Protective lens with diffuse film attached

Lens are AR-coated

Outline




– Reflector

– Lens

– Optics


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

40

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

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