1Photonics West, 2005 Andy Harvey: a.r.harvey@hw.ac.uk

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

*a.r.harvey@hw.ac.uk

2Photonics West, 2005 Andy Harvey: a.r.harvey@hw.ac.uk

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: a.r.harvey@hw.ac.uk

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: a.r.harvey@hw.ac.uk

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

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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

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Image Replication Imaging Spectrometer: IRIS

• Snapshot image• zero temporal misregistration

• ‘100%’ optical efficiency• Conceptually related to Lyot filter

Large formatdetector

SpectralDemultiplexor

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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: a.r.harvey@hw.ac.uk

• 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: a.r.harvey@hw.ac.uk

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: a.r.harvey@hw.ac.uk

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: a.r.harvey@hw.ac.uk

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

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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: a.r.harvey@hw.ac.uk

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: a.r.harvey@hw.ac.uk

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

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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

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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: a.r.harvey@hw.ac.uk

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.