Medical Photonics Lecture 1.2 Optical Engineering

Transcription

1 Medical Photonics Lecture 1.2 Optical Engineering Lecture 9: Instruments I Michael Kempe Winter term

2 2 Contents No Subject Ref Date Detailed Content 1 Introduction Gross Materials, dispersion, ray picture, geometrical approach, paraxial approximation 2 Geometrical optics Gross Ray tracing, matrix approach, aberrations, imaging, Lagrange invariant 3 Components Kempe Lenses, micro-optics, mirrors, prisms, gratings 4 Optical systems Gross Field, aperture, pupil, magnification, infinity cases, lens makers formula, etendue, vignetting 5 Aberrations Gross Introduction, primary aberrations, miscellaneous 6 Diffraction Gross Basic phenomena, wave optics, interference, diffraction calculation, point spread function, transfer function 7 Image quality Kempe Spot, ray aberration curves, PSF and MTF, criteria 8 Instruments I Kempe Human eye, loupe, eyepieces, photographic lenses, zoom lenses, telescopes 9 Instruments II Kempe Microscopic systems, micro objectives, illumination, scanning microscopes, contrasts 10 Instruments III Kempe Medical optical systems, endoscopes, ophthalmic devices, surgical microscopes 11 Optic design Gross Aberration correction, system layouts, optimization, realization aspects 12 Photometry Gross Notations, fundamental laws, Lambert source, radiative transfer, photometry of optical systems, color theory 13 Illumination systems Gross Light sources, basic systems, quality criteria, nonsequential raytrace 14 Metrology Gross Measurement of basic parameters, quality measurements

3 The Human Eye Ref: Wikipedia

4 The Eye of Owl, Cat, Gecko, Insects Ref: Wikipedia

5 The Human Eye muscle outer skin conjunctiva choroid membrane front chamber fibres retina cornea rear chamber pupil iris lens vitreous body macula (yellow) tear liquid fibres conjunctiva nerve muscle blind spot

6 The Human Field of View < 10 : central < 30 : near-peripheral < 60 : mid-peripheral Ref: Wikipedia

7 The Human Vision

8 Optical Data of the Eye Property relaxed accomodated Refractive power dptr dptr Focal length in air 17.1 mm 14.2 mm Power of the crystalline lens 19 dptr 33 dptr Pupil diameter, smallest value for high brightness 1.5 mm Pupil diameter, largest value for night vision 8.0 mm Abbe number (approx.) Petzval radius mm Location of entrance pupil mm Field of view maximum (vertical) 108 Field of view maximum (horizontal) 200 Field of foveated seeing 5 Diameter eye ball 24 mm Distance rotation point from cornea vertex 13.5 mm Nodal point location 7.33 mm Principal plane location 1.6 mm

9 Eye Data - Overview Terms Sizes and lengths posterior chamber anterior chamber temporal vitreous humor fovea cornea crystalline lens lens capsule optical disc blind spot 7.33 mm 13.5 mm iris nasal retina 1.6 mm F PP' NN' C 1.8 mm 2.5 mm F' 4 mm 0.5 mm 3.6 mm 1.8 mm 15.7 mm 3.6 mm 24.4 mm

10 Refractive Index Distribution of the index along z Smooth index variation n cornea crystalline lens anterior chamber lens capsule vitreous humor retina of crystalline lens n z [mm] z [mm]

11 Spectral Transmission of the Eye Absorption of the eye media prevents retina damage Special truncation of UV and IR contributions 100 T [%] 80 visible after cornea before lens after lens at retina λ [nm]

12 Receptors, rods and cones Cones : Fovea, bright light, color Rods : Peripheral, dim light, no color Blind spot : no receptors Fovea : 2 field eccentric Property cones rods Location (primary) in the fovea outside the fovea Field of view small, 5 large, 108 Resolution and visual acuity large small Brightness sensitivity small, for daylight vision large, vision at night Colour sensitivity yes no Total number of elements 5 million 120 million Limiting brightness 683 lm / W 1699 lm / W Spectral maximum 555 nm 507 nm Macula lutea Periphery Papille, blind spot Range Diameter [mm] Foveola 0.35 Fovea 1.85 Prafovea 2.85 Perifovea mm off axis nasal Cones number :3500 density /mm 2 pitch 2.3 µm density : /mm 2 pitch : 3.2 µm density : 5000/mm 2 pitch : 14.0 µm Rods no rods a few rods density : /mm 2 pitch : 2.5 µm density : 50000/mm 2 pitch : 4.5 µm 1.8 no cones no rods

13 Spectral Sensitivity of the Eye Spectral sensitivity of the eye: - Depends on brightness Daylight / high brightness: cones, peak sensitivity at 550 nm At night/ low Brightness: rods, peak sensitivity at 507 nm, shifted towards blue No color distinction V(λ) night scotopic rods day photopic cones Log V(λ) cyan green λ λ

14 Accomodation Change of accomodation range due to aging 16 D [dpt] dpt 12 mean 4 0 relaxed eye 8 maximum -4 immobile point 4 minimum strongest accomodation age [years] age in years

15 Adaptation Aging effect on adaptation Aging effect on adaptation speed pupil diameter [mm] 8 6 rods Log I thresh [a.u.] 8 7 kink of Helmholtz rod monochromate night blindness 4 2 cones years age in years years 30 years years time [min]

16 Gullstrand Model Eye Six media Crystalline lens with shell Data for relaxed and accomodated eye Simple version : single crystalline lens nm 587 nm 656 nm Relaxed Accomodated Parameter Notation Value Value Focal length object sided f [mm] Focal length image sided f' [mm] Refractive power F [dpt] Location entrance pupil p [mm] Location exit pupil p' [mm] Principal point object sided P [mm] Principal point image sided P' [mm] Nodal point object sided N [mm] Nodal point image sided N' [mm] Length L [mm]

17 Resolution of the Eye Resolution of the eye depends on the shape The quantitative measure is given as angle Rough measure : 2αα cc = Least distance of (comfortable) distictinct vision: 250 mm a) letter 5' d) nonius 10'' b) grating 2' c) two points 1' e) binocular 5'' x' c = 75 µm α c x distance cones s o = 250 mm L = 22 mm

18 18 Testchart Acuity of the Eye Regular testchart E F P T O Z L P E D P E C F D E D F C F D F E L O P Z D D E F P O T E C L E F O D P C T F D P L T C E O P E Z O L C F T D

19 Visual Acuity a) b) Recognition of simple geometrical a shapes : 3a 1. Landolt ring with gap 2. Letter 'E' Blur of image on retina with distance 3a a 5a a 3a distance m image height 25 µm eye 8.9 E mm block letter original blur : a/2 blur : a blur : 2a blur : 3a blur : 4a 3a

20 Eye Diseases Four major defects original a) refraction error b) glaucom c) retina defects d) cataract

21 Spectacles Correction of refraction error by spectacle lenses 1. Myopia (short-sightedness) : negative lens eye binocular 2. Hyperopia (far-sightedness) : positive lens eye binocular

22 Loupe Magnifier Lens close to the eye Reference distance s o = 250 mm relaxed eye Magnification Angle magnification m tan w = tan w o = y f ' y s m w = o so f ' Γmag = s f 0 ' y' s = 250 mm o y lens f lens f L eye s o eye y object w o w m lens

23 Loupe Magnifier General case : Loupe distance d from the eye : Magnification y' s' y s d eye m w s s' = + 1 d + s' f ' virtual image object lens

24 Eyepiece: Basic Setup Eyepieces images a finite image of an instrument to infinity Viewing with a relaxed eye Magnification Objective exit pupil = entrance pupil of eyepiece Eyepiece exit pupil = eye pupil (size: 2-8 mm) Eye relief : distance between last lens surface and eye cornea - required : 15 mm - with eyeglasses : 20 mm Objective exit pupil Γ = 250mm f eyepiece intermediate focus Eyepiece Eye pupil tube length

25 25 Mismatch of Eyepieces Pupil mismatch Eye relief, spherical aberration, eye movement Ref: Smith, Ceragioli, Berry, Telescopes, Eyepieces, Astrographs, Willman-Bell, 2012

26 Evolution of Eyepiece Designs Huygens Loupe Monocentric Ramsden Von-Hofe Plössl Kellner Kerber Erfle Bertele König Erfle type Erfle diffractive Bertele Nagler 1 Erfle type (Zeiss) Aspheric Nagler 2 Scidmore Bertele Wild Dilworth

27 20 arcmin Huygens Eyepiece Distance f 1 + f d = 2 2 Lateral color corrected Intermediate image between lenses, not corrected Performance d Exit pupil (eye position) LONGITUDINAL SPHERICAl ABER. ASTIGMATIC FIELD CURVES DISTORTION Virtual object (image plane of objective) DIOPTER DIOPTER tan sag Distortion (%) 0

28 20 arcmin Kellner Eyepiece Corresponds to Ramsden type Intermediate image accessible Field lens moved Eye lens achromatized LONGITUDINAL SPHERICAl ABER. ASTIGMATIC FIELD CURVES DISTORTION DIOPTER DIOPTER Distortion (%) 0 tan sag

29 Collimation Collimating source radiation: Finite divergence angle is reality Geometrical part due to finite size : Diffraction part: Defocussing contribution to divergence θ G = D f λ θd 2W 2 z θ = sin u f divergence θ G /2 D source u 2W f

30 Typical Photographic Lens Types Type focal length in mm Typical f-number Field of view (full diagonal) in degrees Lens type element number Fisheye Fisheye 7-12 quasi-fisheye Fisheye 6-10 extreme wide-angle retrofocus 9-13 very large angle retrofocus 8-10 wide-angle Double Gauss, retrofocus 6-9 standard Triplet, Tessar, Sonnar, Double Gauss 3-7 short telephoto Double Gauss, telephoto 5 medium telephoto Tessar, telephoto 4-6 long telephoto telephoto 4-7 extreme telephoto telephoto 2-5

31 Requirements of Photo Objective Lenses Large field of view - correction of coma, astigmatism, distortion an field curvature Color correction Resolution - small F-number - usually the sensor is limiting, not diffraction limited correction Smart system, small and light weight - short length - plastic components Additional functionalities - zoom option - focussing - autofocus function - large field viewer

32 Photographic Lenses Tessar Distagon Double Gauss Tele system Super Angulon Wide angle Fish-eye

33 Fish-Eye-Lens Example lens fisheye y -100% 0 100% a) nm 587 nm 656 nm tan sag ideal ν 0 [mm -1 ] cyc/mm 20 cyc/mm 40 cyc/mm 60 cyc/mm b) c) solid: tan dashed: sag field angle 100

34 Photographic Lens Contax TVS Ref: H. Zügge

35 Handy Phone Objective lenses Examples US L = 4.2 mm, F'=2.8, f = 3.67 mm, 2w=2x34 US L = 6.0 mm, F'=2.8, f = 4.0 mm, 2w=2x31 EP L = 5.37 mm, F'=2.88, f = 3.32 mm, 2w=2x33.9 Olympus 2 L = 7.5 mm, F'=2.8, f = 4.57 mm, 2w=2x33 Ref: T. Steinich

36 Scan Systems 1. Sampling of the field pixel by pixel Signal digitized in the time domain Wide Field 2. Activ: Flying spot scanning Point wise scanning of illumination, often by laser beam Applications: - bar code reader - confocal microscopy - laser radar (e.g. optical coherence tomography) 3. Passiv: Remote sensing Decomposition of object signal into pixels Applications: - Night Vision - monitoring, surveying - missile tracking Source: Confocal

37 Scan Systems: Introduction Scan resolution: Number of resolvable points in the field of view corresponds to angle resolution N L D 2 DExP θ = λ = max Airy Information capacity: 1. Resolvable points 2. Speed of scanning Etendue: product of scan range and scanner area log θ angle resolution holographic scanner polygon mirror growing scan capacity D Mir θmir = DExP θ max resonant galvo scanner galvo scanner acoustic optical modulator electro optical modulator scan speed log v

38 Deflecting Components Different types of deflecting elements Non- Mechanical Deflection Electro-optic Acousto-optic EOD AOD Galvanometric Galvoscanner Scanning Oszillation Holographic Holographic Scanner Electrostatic MEMS Scanner Mechanical Rotation Polygon Rotating Prisms Polygonscanner Dove Prism Translation Lenses and lens arrays

39 Deflecting Components: Polygon Mirrors Rotating mirror with plane facets Pyramidal pyramidal polygon objective lens scan line Prismatic prismatic polygon scan line objective lens

40 Galvanometer and Electrostatic Scanner Galvo scanner MEMS-Scanner Source: scanlab.de Source: researchgate.net

41 42 Scanner Lenses Ideal scanner lens (F-θ lens): h = f θ Flat-field corrected lens: h = f tanθ nonlinear displacement: distortion correction needed Source: thorlabs.com

42 43 Example: Scanner Lens USP : Scan angle 2x30 Monochromatic diffraction limited F-θ-corrected Ref: B. Böhme