INFLUENCE OF VARIABLE APERTURE STOP

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1 INFLUENCE OF VARIABLE APERTURE STOP IN TELECENTRIC IMAGING LENSES SILL OPTICS GMBH CO. KG TECHNOLOGY FORUM MACHINE VISION 2015/16 Konrad Hentschel, Dipl-Phys. Andreas Platz, M.Sc. Project Management

2 CONTENT Aperture Stop definition parameters Influence on Properties of Telecentric Lenses illumination resolution & contrast depth of field vignetting telecentricity distortion Boundaries of Variable Stops Fixed Stop or Variable Stop? 2

3 DEFINITION APERTURE STOP stop - limitation of optical path aperture stop - limitation of aperture angle and amount of light inside the lens field stop - limitation of field angle and maximum object and image size 3

4 DEFINITION APERTURE STOP stop - limitation of optical path aperture stop - limitation of aperture angle and amount of light inside the lens field stop - limitation of field angle and maximum object and image size 4

5 DEFINITION APERTURE STOP stop - limitation of optical path aperture stop - limitation of aperture angle and amount of light inside the lens field stop - limitation of field angle and maximum object and image size 5

$DEFINITION APERTURE STOP parameters describing aperture stop numerical aperture NA NA = n sinσ n = refractive$

6 DEFINITION APERTURE STOP parameters describing aperture stop numerical aperture NA NA = n sinσ n = refractive index of the medium σ = ½ aperture angle parameter for constant object distance larger NA describes larger aperture 6

7 DEFINITION APERTURE STOP parameters describing aperture stop F - number k, F# k = F# = f 2 h EP f = focal length of the lens Ø EP = diameter of entrance pupil parameter for variable object distance and imaging from infinity lower F# describes larger aperture 7

8 INFLUENCE OF APERTURE STOP Illuminance (luminous flux per surface) for fixed object distance (telecentric lens) E ~ NA² illuminance is directly proportional to NA squared large aperture large illuminance short exposure time high refresh rate 8

$diffraction) diffraction limit = 2.44 λ f D EP = 2.$

9 INFLUENCE OF APERTURE STOP Resolution & Contrast each optical system is a low pass filter (because of diffraction) diffraction limit = 2.44 λ f D EP = 2.44 λ F# large aperture improved contrast level higher resolution 9

10 INFLUENCE OF APERTURE STOP Resolution & Contrast demonstration in Modulation Transfer Function (MTF) Correctal T85/0.10, NA=0.01, F#=4.9 10

11 INFLUENCE OF APERTURE STOP Resolution & Contrast MTF compared for different NA NA=0.013, F#=3.8 NA=0.010, F#=4.9 NA=0.005, F#=9.8 11

12 INFLUENCE OF APERTURE STOP Resolution & Contrast verification of importance for each specification which resolution is required? which contrast is required? depends on (telecentric) lens target illumination sensor software 12

13 INFLUENCE OF APERTURE STOP Depth of Field defined by resolution, focal length and magnification variable parameter: aperture 13

14 Contrast (MTF) INFLUENCE OF APERTURE STOP Depth of Field Example: telecentric lens β = 0.58, WD = mm, pixel size 5.6 µm, NA = 0.08 Depth of Field = 0.7 mm Working Distance 14

15 Contrast (MTF) INFLUENCE OF APERTURE STOP Depth of Field example: telecentric lens β = 0.58, WD = mm, pixel size 5.6 µm, NA = small aperture large depth of field Depth of Field = 1.6 mm Working Distance 15

16 INFLUENCE OF APERTURE STOP Depth of Field Rules of Thumb for pixel size < 6µm DOF = 1/ β [mm] for pixel size >> 6µm DOF = ± 1/ β [mm] not in general depends on stop size! 16

17 INFLUENCE OF APERTURE STOP Vignetting lens size depends on beam size and aperture angle small aperture less vignetting 17

18 INFLUENCE OF APERTURE STOP Telecentricity telecentric imaging demands that chief rays are parallel to optical axis chief rays always intersect optical axis in aperture stop aperture stop is imaged to infinity position of stop is determining telecentricity small / large aperture no impact on telecentricity 18

19 INFLUENCE OF APERTURE STOP Distortion refraction of ray chief is too large or too small depending on field size position of stop decides if there is b) positive (pin cushion) or c) negative (barrel) distortion stop size doesn t affect chief ray path 19

20 INFLUENCE OF APERTURE STOP Distortion small / large aperture no impact on distortion 20

21 BOUNDARIES large depth of field vs. high resolution manufacturing tolerances for variable stops variable mechanics vs. glued mechanics for moving parts defined stop positions mechanical steps scale locking screws 21

22 WHICH STOP TYPE? fixed stop advantages: defined parameters smaller tolerances in stop size (reproducibility) lower cost Correctal TCL Series suitable for industrial purposes, even for moving parts and vibrations variable stop advantages: enhancement of illuminance enhancement of depth of field enhancement of resolution reduction of vignetting 22

23 WHICH STOP TYPE? one lens two applications S5LPJ3025 telecentric lens β = 0.25, WD = mm, max. object size Ø mm Case 1 depth of field min. 6.0 mm, minimum contrast 30% for pixel size 5.6 µm Case 2 maximum resolution, minimum contrast 50% for pixel size 3.45 µm NA = NA = Depth of Field = 12 mm Depth of Field = 2 mm 23

24 WHICH STOP TYPE? Sill Optics - Catalog: nearly every lens with variable stop fixed stop: blue dot tests? loan with purchase option fixed stop modification for each lens possible data sheets: 24

25 THANKS FOR YOUR ATTENTION. Do you have questions? Please contact us: (0) 9129/ Thank you very much for your interest and your attention! 25

26 REFERENCES Thöniß, Thomas: Objektive in der industriellen Bildverarbeitung, LINOS Photonics GmbH & Co. KG, 2006 Pedrotti et al.: Optik für Ingenieure, 3.Auflage, 2002, Springer Verlag Schröder, Treiber: Technische Optik, 10. Auflage 2007, Vogel Fachbuch Weissler, Gerhard A.: Einführung in die industrielle Bildverarbeitung, 2007, Franzis Verlag Groß, Herbert: Optical Design with ZEMAX Imaging, 2013, Institut of Applied Physics, Friedrich-Schiller-Universität Jena Naumann et al.: Handbuch Bauelemente der Optik, 7.Auflage 2014, Hanser Verlag 26

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