1Photonics West, 2005 Andy Harvey: [email protected]
Spectral Imaging In a Snapshot
Andrew R Harvey*, David W Fletcher-Holmes, Alistair GormanSchool of Engineering and Physical Sciences,
Heriot Watt University, Edinburgh, UK
Kirsten Altenbach, Jochen Arlt and Nick D ReadCOSMIC, The University of Edinburgh, Edinburgh, UK
2Photonics West, 2005 Andy Harvey: [email protected]
Presentation outline
• Why another spectral imaging technique?• IRIS:image replication imaging spectrometry• Design issues• Example applications
• Retinal imaging• Microscopy
• Conclusions
3Photonics West, 2005 Andy Harvey: [email protected]
Why another spectral imaging technique?
• Traditional approaches• Time sequential spectral multiplex
• Monochromatic two-dimensional image in snapshot• Time sequential spatial multiplex
• One-dimensional spectral image in a snapshot• (and Fourier-transform equivalents)
• Problems• Cannot record two-dimensional spectral images of time-varying
scenes• Optically inefficient
• Time-resolved (snapshot) spectral imaging is required for• Dynamic scenes
• In vitro, in vivo imaging and microsocopy• Combustion dynamics, surveillance…
• Irregular motion between scene and imager• In vivo imaging• Ophthalmology• Remote sensing, airborne surveillance, industrial inspection…
4Photonics West, 2005 Andy Harvey: [email protected]
Spectral retinal Imaging• By 2020 there will be 200 million visually-
impaired people world wide• Glaucoma, diabetic retinopathy, ARMD• 80% of those cases are preventable or
treatable • Screening and early detection are
crucial • Spectral imaging provides a non-invasive
route to monitoring retinal biochemistry• Blood oximetry, lipofuscin accumulation
800nm
Diabetic Retina
Normal Retina
5Photonics West, 2005 Andy Harvey: [email protected]
Requirements for a snapshot technique: retinal imaging
• Improved calibration
• Patient patience
• Remove misregistration artefacts; imperfect coregistration arises due to
• Distortion of eye ball with pulse
• Variations in imaging distortion between images
• Similar issues with other in vivo applications
• Imaging epithelial cancers
PC15
6Photonics West, 2005 Andy Harvey: [email protected]
Image Replication Imaging Spectrometer: IRIS
• Snapshot image• zero temporal misregistration
• ‘100%’ optical efficiency• Conceptually related to Lyot filter
Large formatdetector
SpectralDemultiplexor
7Photonics West, 2005 Andy Harvey: [email protected]
Lyot filter: principle of operation
n=1 � l Cos2@pîDDCos2@pîDDCos2@2pîDDCos2@pîDDCos2@2pîDDCos2@4pîDDCos2@pîDDCos2@2pîDDCos2@4pîDDCos2@8pîDD
PolariserWaveplate
8Photonics West, 2005 Andy Harvey: [email protected]
• Wollaston prism polarisers replicate images• Each Wollaston prism-waveplate pair provides both cos2 and sin2 responses
• All possible products of spectral responses are formed at detector
Exploded view of N Wollaston prisms N wave plates
2N spectral images at detector Field
stop
Input polarizer
)(sin
)(cos2
2
)2(sin
)2(cos2
2
)4(sin
)4(cos2
2
IRIS snapshot spectral imager:
9Photonics West, 2005 Andy Harvey: [email protected]
Spectral responses
• Bands are overlapping bell shapes• Choose cost function to minimise sidelobes
• Small (~5%) reduction in spectral separation• Cut-off filters used to define spectral range
Theoretical system response
0
20
40
60
80
100
450 500 550 600 650 700 750 800 850
Wavelength (nm)
Res
po
nse
(%
)
•8 channel visible-band system
•520nm820m
•3 Quartz retarders
•32 channel, visible-band system
•520nm 720nm
•5 Quartz retarders
10Photonics West, 2005 Andy Harvey: [email protected]
Optical scaling laws
Hamamatsu
ORCA-ER
Inputs:
FoV
Sub image size on CCD
CCD pixel size
Primary lens magnification & F#
Collimating lens back focal distance, focal length & front element diameter
Prism birefringence
Outputs:
Field stop size
Collimating lens rear element diameter
Splitting angles, apertures & depths of prisms
Apertures of retarders, polarisers and filters
Imaging lens focal length & front element diameter
Field stopCollimating
lens
Bandpass
filter
Imaging
lens
Camera
Polariser, retarders & Wollaston prisms
(index matched)Primary lens
11Photonics West, 2005 Andy Harvey: [email protected]
Modelling and ray-tracing
Trade off
2
4
6
0.5 1.0 1.5 2.0 2.5 3.0
FoV half angle (°)
F#
15mm prisms
20mm prisms
25mm prisms
30mm prisms
16m
m le
nses
25m
m le
nses
35m
m
lens
es
50m
m le
nses
•8 channel system
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Components & Assembly
• 8 channel system• 520nm to 820nm• 3 Quartz retarders• 3 Calcite Wollaston prisms
13Photonics West, 2005 Andy Harvey: [email protected]
Measured & predicted spectral responses
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Absolute total transmission
• Bandpass filter & polariser dominate losses
• Improved system: T>80%
• Theoretical throughput is 2n times higher than for spatial/spectral multiplexed techniques!
0
25
50
Re
sp
on
se
(%
)
Absolute response curves in polarised light
15Photonics West, 2005 Andy Harvey: [email protected]
Blood oximetry
• Optimal spectral band for retinal oximetry• Vessel thickness ~ optical depth• 570-615 nm• Eight bands approximately equally spaced
0
2
4
6
8
10
12
14
16
18
20
565 575 585 595 605 615 625
Wavelength (nm)
Tra
nsm
issi
on (
%)
40
20
16Photonics West, 2005 Andy Harvey: [email protected]
Spectral Retinal Imaging • Difficult imaging conditions render application of traditional HSI
techniques problematic• IRIS enables real-time and snapshot spectral imaging
Canon CR4-45NMCR4-45NM
17Photonics West, 2005 Andy Harvey: [email protected]
Video sequence recorded with bandpass filtered inspection lamp
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Retinal image recorded with flash illumination
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574581585592595603607613
Coregistered and PCA images
PC1PC2PC1 & PC2
20Photonics West, 2005 Andy Harvey: [email protected]
Application to microscopy:Imaging of multiple fluorophors
• IRIS fitted to conventional epi-fluorescence microscope
• Germinating spores of Neurospora crassa stained with• GFP – nucleii fluoresce at 510 nm• FM4-64 – membranes fluoresce at >580 nm0
25
50
Re
sp
on
se
(%
)
21Photonics West, 2005 Andy Harvey: [email protected]
Conclusions
• IRIS is a new spectral imaging technique that enables snapshot spectral imaging in 2D• No rejection of light• No data inversion
• Highest-possible signal-to-noise ratios• Simple logistics
• Inherently compact and robust• Simply fitted to conventional imaging systems
• Birefringent materials exist for applications from 0.2m to 12 m
• Applications• In vivo, in vitro imaging
• Retinal imaging• Microscopy
• Multiple fluorophors• Quantum dots
• Surveillance• Remote sensing• Etc.
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