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Medical Photonics Lecture 1.2Optical Engineering
Lecture 10: Instruments III
2019-06-19
Michael Kempe
Spring term 2019
2
Contents
No Subject Ref Date Detailed Content
1 Introduction Zhong 10.04. Materials, dispersion, ray picture, geometrical approach, paraxial approximation
2 Geometrical optics Zhong 17.04. Ray tracing, matrix approach, aberrations, imaging, Lagrange invariant
3 Diffraction Zhong 24.04. Basic phenomena, wave optics, interference, diffraction calculation, point spread function, transfer function
4 Components Kempe 08.05. Lenses, micro-optics, mirrors, prisms, gratings
5 Optical systems Zhong 15.05. Field, aperture, pupil, magnification, infinity cases, lens makers formula, etendue, vignetting
6 Aberrations Zhong 22.05. Introduction, primary aberrations, miscellaneous7 Image quality Zhong 29.05. Spot, ray aberration curves, PSF and MTF, criteria8 Instruments I Kempe 05.06. Human eye, loupe, eyepieces, photographic lenses, scan lenses
9 Instruments II Kempe 12.06. Microscopic systems, micro objectives, illumination, scanning microscopes, contrasts
10 Instruments III Kempe 19.06. Medical optical systems, endoscopes, ophthalmic devices, surgical microscopes, zoom lenses
11 Photometry Zhong 26.06. Notations, fundamental laws, Lambert source, radiative transfer, photometry of optical systems, color theory
12 Illumination systems Gross 03.07. Light sources, basic systems, quality criteria, nonsequential raytrace
13 Metrology Gross 10.07. Measurement of basic parameters, quality measurements
Limitation of Optical Imaging in Medicine
𝐼𝐼𝐼𝐼0
= 𝑒𝑒𝑒𝑒𝑒𝑒 − 𝜇𝜇𝑠𝑠′ + 𝜇𝜇𝑎𝑎 𝑑𝑑
𝜇𝜇𝑠𝑠′: reduced scattering coefficient(typ. 101 − 102 𝑐𝑐𝑐𝑐−1)
𝜇𝜇𝑠𝑠: scattering coefficient(typ. 102 − 103 𝑐𝑐𝑐𝑐−1)
𝜇𝜇𝑎𝑎: absorption coefficient(e.g.10−1 𝑐𝑐𝑐𝑐−1 @ 1 µ𝑐𝑐 in water)
Penetration - Resolution:
Ballistic light (𝜇𝜇𝑠𝑠) – few mm / several µmDiffuse light (𝜇𝜇𝑠𝑠′) – depth d
Optical Imaging in Medicine
Optical Medical Imaging
Diagnostic Imaging
Ophthalmology
Dermatology
Others
Open SurgeryVisualization
Neuro/Spine
Gynaecology/Urology
ENT
Ophthalmology
Dental
Endoscopy
Gastroenterology
Cardiology
Urology
Pulmonology
Others
Endoscopes: Relay Systems
Endoscopes use various light guiding principles to relay the image overdistance
Rigid endoscopes – slab lens relay Combination of several relay subsystems GRIN lenses or doublet rod lenses and large field-angle objective lens may be used
objective 1. relay 2. relay 3. relay
Ref.: M. Rill
Rigid Endoscopes
0.5
486 nm587 nm656 nm
0.4
00 0.4
Wrms [λ]
0.8 1.2 21.6y'
[mm]
0.3
0.2
0.1diffraction limit
Example: Systems by Storz diameter 3.7 mm
Flexible Endoscopes
Helen D. Ford and Ralph P. Tatam, "Characterization of optical fiber imaging bundles for swept-source optical coherence tomography," Appl. Opt. 50, 627-640 (2011)
Use of fiber bundle array as relay
Each fiber transmits one image point Diameter: typ. 0.5-1.5 mm for 4k to 18k fibers
(data points = pixels) Pixel size: typ. 6-10 µm
Example: System by Storz
Historical Development of Surgical Microscopes
1. Head worn loupe (1876) 4. OPMI (Littmann 1953)2. Corneal loupe (von Zehender/Westien 1887) 5. Contraves Stativ (Yasargil 1972)3. Corneal loupe (Schanz/Czapski 1899)
1.
4.3.
2.
5.
Surgical Microscope
Modern surgical microscopes are stereo systemscombining ocular and digital imaging
Ref.: ZEISS
SurgicalMicroscope
ComputerData TransferPower Supply
Ref.: M. Kaschke et al. Ophthalmology
Zoom Systems
Motivation for zooming: Enlargement of image details Foveated imaging Adaptation of field of view
Basic Principle
Two thin lenses in a certain distance t:Focal length
Refractive power
Many types of zoom systemlayouts
tfffff
−+=
21
21
2121 FFtFFF ⋅−+=
c) Infinite-infinite (I-I)
b) Infinite-finite (I-F)
a) Finite-finite (F-F)
Change of Focal Length
Distance t increased First lens fixed
movedlens
changeddistance
t changed focallength f
Change of Focal Length
Distance t increased Image plane fixed
two lenses moved t f
imageplane
Mechanical Compensated Zoom Systems
Simple explanation of variator and compensator Movement of variator arbitrary Compensator movement
depends on variator:nonlinear
Perfect invariance ofimage plane possible
objectivelens
variatorlinear
compensatornonlinear
relaylens
P
P
P
imageplane
Modular F-F-Setup
Finite-finite configuration wth three parts : 1. Focusing lens2. Zoom group with movable components3. Realy lens
movable zoom lensesfocusing lens relay lensobject image
Symmetrical Three Component I-I Setup
Telescope angle magnification :
Major positions
Symmetrical layout
f1 f1f2
asymmetric 1
Γ > 1
tmax
asymmetric 2
tmin
Γ < 1
symmetric
tm tm
Γ = 1
last
first
hh
ww
=='
Γ
Magnification First distance
Second distance
|Γ| = |Γmax| > 1 tmax 0 |Γ| = 1 tm tm
|Γ| = 1/|Γmax| < 1 0 tmin
Example: Three Groups I-I Optical Compensated
Γmax / Γmin = 6
Γ = 0.41
Γ = 0.92
Γ = 1.41
Γ = 1.92
Γ = 2.44
Mechanical compensation
Variable distances:d3 and d4
Zoom factor 6
18
Optical Ophthalmic Diagnosis
Imaging
Anterior Segment
Slit lamp
OCT
Posterior Segment
Slit lamp
Ophthalmoscope
Fundus Camera
OCT
Measuring
Refractive Power
Objective Refraction:
Autorefractor
Subjective Refraction:Phoropter
Wavefront
Aberrometer
Visual Field
Perimeter
Cornea Topography
Topographer (Placido)
Keratometer
Eye lengths
Biometer (OCT)
Retinal layers
OCT
SL Polarimeter
19
Slit Lamp
Ref.: ZEISS
Köhler (slit) illumination (“slit lamp”)a) from below (Zeiss type)b) from above (Haag Streit type)
Stereo microscope
CMO type Greenough type
http://media.labcompare.com/
20
Slit Lamp
Projection of a slit onto thecornea with small NA
Scattering in the eye Scanning in the anterior of
the eye to detect inhomo-geneities
With the use of (neg.) contact lens or (pos.) auxilliary lens imaging of thefundus is possible
Ref.: M. Kaschke et al. Ophthalmology
Diffuse illumination Slit illumination
ParfocalSwivel
21
Direct Ophthalmoscope
Inspection of an illumination pathreflected on the retina withoutmicroscope
Selection of different aperturesby a rotatable wheel
Compensation lens enforces acoincidence with the observation
Ref.: M. Kaschke et al. Ophthalmology
22
Indirect Ophthalmoscope
Pupil mismatch between patient and observer reduces field of view in directophthalmoscope
Indirect ophthalmoscope: additional ophthalmoscopy lens close to the eye creates an enlarged image of the patients pupil
Ref.: M. Kaschke et al. Ophthalmology
23
Fundus Camera
Observation and photographic inspection of the retina Inspection of the fundus structural analysis to detect morphological deceases Separation of illumination and observation beam path to avoid disturbing reflections Typically ring-shaped illumination
Ref.: M. Kaschke et al. Ophthalmology
24
Fundus Camera Modalities
Ref.: M. Kaschke et al. Ophthalmology
25
Confocal Laser Scanning Ophthalmoscope
Confocal imaging of a fundus spot by scanning (CSLO)
Pinhole mirror separates illumination and detection
Confocal pinhole suppresses straylight
Ref.: M. Kaschke et al. Ophthalmology
Problem: axial optical resolution
∆z ≈ 2λ/NA² ≈ 1.7 µm (1.33 ⋅ 1/24)-2
≈ 555 µm
26
Optical Coherence Tomography (OCT)Using of a low-coherence source enables 3D imaging
Time-domain OCT
Ref.: M. Kaschke et al. Ophthalmology
OCT combines unique resolution with high sensitivity• axial resolution is independent of the numerical aperture (pupil) • by heterodyne detection a photon-noise limited sensitivity is
achieved (R ≥ 10-10) ∆z = lc ≈ 0.44 λ/∆λ≈ 5 µm
27
Optical Coherence Tomography (OCT)
Ref.: M. Kaschke et al. Ophthalmology
Spectral-domain OCT
• Better sensitivity by simultaneous detection of spectral components
• Depth information obtained by Fourier transform
Ref.: ZEISS
28
Optical Coherence Tomography (OCT)
Ref.: Zeiss
OCT-Scan
OCT-Scan
For Glaucoma diagnostics: Either measurement of topology of the blind spotor the thickness of the RNFL
Thickness of RNFL
Measurement against normative database:
Topology of the nerve head
RNFL = Retina Nerve Fiber LayerThe yellow band represents healthy persons
is measured by a circular OCT scan
Depth information enables measurement of layer thickness for diagnosis
29
Refractometer
Autorefraction measurement of the eye power
Test pattern projected onto the retina (mire = target pattern)
Fundus reflected light is observed (Ophthalmoscope)
z-differences correspond to focal power errors (optometer method)
Ref.: M. Kaschke et al. Ophthalmology
30
Aberrometer
Measurement of the human eye wavefront with a Hartmann-Shack wavefront sensor
Illumination spot on the fundus reflected
Ref.: M. Kaschke et al. Ophthalmology
31
Keratometer
Measuring the refractivepower of the cornea
Main contribution: curvature,only R measured
Principle:Determination of image size y‘
To correct for motion a double image is used as reference
Ref.: M. Kaschke et al. Ophthalmology
1𝑠𝑠′
=1𝑠𝑠
+1𝑓𝑓′
𝑦𝑦′𝑦𝑦
=𝑠𝑠′𝑠𝑠
𝑟𝑟𝑐𝑐 = 2𝑓𝑓′ = 2𝑠𝑠 � 𝑦𝑦′𝑦𝑦
y’∆y
32
Keratometer
Helmholtz-type keratometer
Littmann keratometer
Achieved accuracy: ∆rc = 0.05...0.1 mm
Ref.: M. Kaschke et al. Ophthalmology
33
Cornea Topography by Placido Disk
Projection of a ring mask onto the cornea (Placido mask) Imaging the rings onto a camera Evaluation of the imaged ring widths Reconstruction of the topology of the cornea
Ref.: M. Kaschke et al. Ophthalmology
projected patternimage
reconstructedtopology
real deformedimage
34
Corneal Topographer
Realization of the Placido-projectionand imaging of the reflected light
Ring-by-ring reconstruction of thecornea surface
Ref.: M. Kaschke et al. Ophthalmology
35
Biometer (OCT)
Ref.: ZEISS
Special OCT technique to determine:
Measurement of full eye along optical axis possible with time-domain OCT (double beam technique) or swept source OCT
Low lateral resolution ensures large depth of focus
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