Evaluation of illumination uniformity metrics in design and optimization of light guides DGaO 2012
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Transcript of Evaluation of illumination uniformity metrics in design and optimization of light guides DGaO 2012
Evaluation of illumination uniformity metrics in design and
optimization of light guides
Milan MaksimoviċFocal Optical Systems, Oldenzaal, The Netherlands
113th Annual Meeting DGaO,
29 May ‐ 2 June 2012 , Eindhoven, The Netherlands
Summary and Motivation
• Non‐imaging/ Illumination optics: – Analytical tools are scarce and application specific– Heuristics/experience is driving a force in design– Randomness in computational model (Monte‐Carlo ray tracing )
• Optimization merit function construction and landscape – Intrinsic roughness in the landscape (computational paradigm)– Merit function is not smooth w.r.t. parameters (intrinsic or accidental ?)– Uniformity metrics: local vs. global sampling – Small subset of traditional local and global algorithms available in commercial software
• Examples of uniformity metrics impact on merit function landscape– Rectangular tapered light guide– Circular tapered light guide– Freeform light guide
• Toward the ( new) optimization algorithm requirements ?
Transfer Efficiency and Uniformity metrics
• Transfer Efficiency– Output‐to‐input flux ratio T= (Total Flux In )/ (Total Flux Out)
• Uniformity in spatial or angular domain– Illuminance [lm/m2] or Luminance [lm/m2 sr]
• Point‐to‐point (local sampling)– Min‐Max Ratio: U1=Emin/Emax– Contrast: U2=(Emax‐Emin)/(Emax+Emin)
• Point‐background (global sampling)– Min‐Average Ratio: U3 =Emin/Eavg– Relative Standard Deviation: U=σE/Eavg
• Complex pattern structure is not captured– Constraint using contrast sensitivity function and relative standard deviation
sets minimum perceptible non‐uniformity
Rectangular and circular tapered light guides
• Source– Lambertian radiation pattern (ideal)– Square source 2x2mm (or 1x1 mm)
• Light guide– Rectangular tapered geometry– PMMA– Input size 1mm (set to source size for
rectangular lightguide)– Output size 5x 5mm ( small angle conversion)
• Uniformity metrics computed for – 1D dependence :
• Change length L• Input and output size fixed
– 2D dependence• Change length and output size• Input size fixed
• Detector– 28x28 bins– Position at the light guide output – Size equal to output size of the light guide
Estimate transfer efficiency within NA=0.2!
Estimate spatial uniformity !
Estimate angle uniformity !
Uniformity metricsrectangular light guide
Uniformity Metrics in Position Space
20 40 60 80 100 1200.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
L [mm]
Rel
ativ
e S
tand
ard
Dev
iatio
n
20 40 60 80 100 1200.75
0.8
0.85
0.9
0.95
1
L [mm]
Min
-Mea
n R
atio
20 40 60 80 100 1200
0.05
0.1
0.15
0.2
0.25
L [mm]
Con
trast
ratio
20 40 60 80 100 1200.65
0.7
0.75
0.8
0.85
0.9
0.95
1
L [mm]
Min
-Max
Rat
io
# of minima= 28 # of minima= 28
# of minima= 24 # of minima= 23
20 30 40 50 60 70 80 90 100 110 1200.09
0.095
0.1
0.105
0.11
0.115
0.12
L [mm]M
erit
func
tion
20 30 40 50 60 70 80 90 100 110 1200.2
0.4
0.6
0.8
1
L [mm]
Effi
cien
cy
20 30 40 50 60 70 80 90 100 110 1200
0.02
0.04
0.06
0.08
L [mm]
Rel
ativ
e S
tand
ard
Dev
iatio
n
Merit functionrectangular light guide
N
ii
N
iiii VT
ΜF
1
1
2
Shape of merit function landscape dominated by uniformity measure!
20 30 40 50 60 70 80 90 100 110 1200.09
0.095
0.1
0.105
0.11
0.115
0.12
L [mm]
Mer
it fu
nctio
n L=48.18mmL=45.17mm
L=32.2mm L=35.1mmL=32.2mm L=35.1mmL=32.2mm L=35.1mmL=32.2mm L=35.1mmL=32.2mm L=35.1mmL=32.2mm L=35.1mm
L=45.17mm
L=32.2mm L=35.1mm
L=48.18mmL=45.17mm
L=32.2mm L=35.1mm
20 30 40 50 60 70 80 90 100 110 1200.09
0.095
0.1
0.105
0.11
0.115
0.12
L [mm]
Mer
it fu
nctio
n L=48.18mmL=45.17mm
L=32.2mm L=35.1mm
20 30 40 50 60 70 80 90 100 110 1200.09
0.095
0.1
0.105
0.11
0.115
0.12
L [mm]
Mer
it fu
nctio
n L=48.18mmL=45.17mm
L=32.2mm L=35.1mm
Merit functionrectangular light guide
Transfer efficiency rectangular light guide
# of minima= 32
Transfer efficiency is smooth with small number of shallow local minima!
Uniformity measure rectangular light guide
# of minima= 795
# of minima= 650
# of minima= 812
# of minima= 796
Uniformity measure is rough with large number of shallow local minima!
Transfer efficiency circular light guide
Transfer efficiency is smooth with small number of shallow local minima!
Uniformity metricscircular light guide
Number of local minima
38
Min-MeanRatio
22263119
Relative Standard Deviation
ContrastMin-Max Ratio
Transfer Efficiency
Freeform light guide
• Source– Lambertian radiation pattern (ideal)– Square source or 1x1 mm
• Light guide– Free-form (radial) geometry– PMMA– Input size fixed at 1 mm radius– Output size fixed at 5mm radius– Length Fixed
• Uniformity metrics computed for – 3D dependence– Parameters: equispaced points along the length– Vertical displacements L1,L2,L3– L1: from 2 to 6mm – L2: from 1 to 5mm– L3: from 1 to 5mm
• Detector– 28x28 bins– Position at the light guide output
L2L1 L3
• Convex and concave shape accessible!• Perturbation of tapered circular light guide !• Search space : > 1200 shapes!
Example freeform light guide
High Transfer Efficiency (98 % ), bad uniformity!
Low Transfer Efficiency (38 % ), improved uniformity?
Uniformity metricsfreeform light guide
4
Min-MeanRatio
4711Number of local minima
Relative Standard Deviation
ContrastMin-Max Ratio
• Local minima estimated excluding boundaries of the domain• Multi‐directional search in 3D parameter space
L3=1mm L3=1.4mm L3=1.8mm L3=2.2mm L3=2.6mm
L3=3mm L3=3.4mm L3=3.8mm L3=4.2mm L3=4.6mm
L1
L2
L1
L2
L1
L2
• Cross‐sections: relative standard deviation
L1=from 2 to 5.6 mmL2=from 1 to 5 mm(11x11 points)!
Concluding remarks• Merit function landscape and uniformity measure
– Global and local sampling uniformity measures induce large number of local minima at all resolutions and for all geometries!
– Relative standard deviation measure has smallest number of local minima of all measures, but still large number !
– Landscape is dominated by shallow regions modulated by local minima/maxima roughness !
– Roughness in the landscape dominated by uniformity measure !
– Random search algorithms may produce sub‐optimal solution: theory necessary for starting point (especially for freeform light guides)!
• Toward the reliable optimization method– Use flux measure and global uniformity sampling in the merit function and local
sampling measure as a constraint (application specific)!
– Robust (global) optimization techniques needed: asymmetric escape function for escaping trap of local minima?
– Nature of local minima: how many true local minima vs. saddle points?
– Is there a structure in the merit function landscape?