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.

1. Flashlight Collimating System
Can Fang
Email: mrfunder@hotmail.com
Jan, 2011
1

2. Outline
• Properties of Emitter
• Design Objective of Collimating System
• Collimating System Overview
– Reflector
– Lens
– Optics
• Proposed Directions
2

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. 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. 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. 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

7. Outline
• Properties of Emitter
• Design Objective of Collimating System
• Collimating System Overview
– Reflector
– Lens
– Optics
• Proposed Directions
7

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. 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. 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. 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

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. Outline
• Properties of Emitter
• Design Objective of Collimating System
• Collimating System Overview
– Reflector
– Lens
– Optics
• Proposed Directions
13

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. 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

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 ratio
Simulation setting:
60mm diameter paraboloid reflector, target is 10m away from the reflector
16

17. Coma: The Transition from Hotspot to Spill
spill Question: Where does the
coma coma come from?
hotspot
17

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. 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. Simulation Test
Diameter =60mm, depth =60mm Diameter =60mm, depth =30mm
20

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. 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. Outline
• Properties of Emitter
• Design Objective of Collimating System
• Collimating System Overview
– Reflector
– Lens (and reversed reflector)
– Optics
• Proposed Directions
23

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. Collimated Energy
θ θ
Hotspot
Be wasted or transformed into
spill light by incorporating with
another reflector
-θ θ
Spatial distribution (degree) 25

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 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
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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. Outline
• Properties of Emitter
• Design Objective of Collimating System
• Collimating System Overview
– Reflector
– Lens (and reversed reflector)
– Optics
• Proposed Directions
28

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. 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

31. 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

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. The Second Gen TIROS
Methodology: Reflector like, much spill
33

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. Surefire’s TIR (version A)
Methodology: Reflector like (for general use)
Protective Lens
Diffuser film attached to lens
TIR optics
35

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. Outline
• Properties of Emitter
• Design Objective of Collimating System
• Collimating System Overview
– Reflector
– Lens
– Optics
• Proposed Directions
37

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. Optics
• Optics make difference
– Appearance
– Performance
– Cost
• Start with reflector-like optics, coated PMMA
or optical glass with AR coating
39

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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