Spectral Imaging In a Snapshot

1. Spectral Imaging In a Snapshot Andrew R Harvey*, David W Fletcher-Holmes, Alistair Gorman School of Engineering and Physical Sciences, Heriot Watt University, Edinburgh, UK Kirsten Altenbach, Jochen Arlt and Nick D Read COSMIC, The University of Edinburgh, Edinburgh, UK *a.r.harvey@hw.ac.uk

2. Presentation outline • Why another spectral imaging technique? • IRIS:image replication imaging spectrometry • Design issues • Example applications • Retinal imaging • Microscopy • Conclusions

3. 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…

4. Spectral retinal Imaging Diabetic Retina Normal Retina • 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

5. Requirements for a snapshot technique: retinal imaging PC15 • 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

6. Image Replication Imaging Spectrometer:IRIS F F F F F F F F F • Snapshot image • zero temporal misregistration • ‘100%’ optical efficiency • Conceptually related to Lyot filter Large format detector Spectral Demultiplexor

7. Lyot filter: principle of operation Waveplate Polariser

8. IRIS snapshot spectral imager: • 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

9. Spectral responses • 32 channel, visible-band system • 520nm 720nm • 5 Quartz retarders • 8 channel visible-band system • 520nm820m • 3 Quartz retarders • 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

10. Optical scaling laws Polariser, retarders & Wollaston prisms (index matched) Field stop Camera Bandpass filter Imaging lens Collimating lens Primary lens Hamamatsu ORCA-ER 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 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

11. Modelling and ray-tracing 50mm lenses 16mm lenses 35mm lenses 25mm lenses 15mm prisms 20mm prisms 25mm prisms 30mm prisms • 8 channel system

12. Components & Assembly • 8 channel system • 520nm to 820nm • 3 Quartz retarders • 3 Calcite Wollaston prisms

13. Measured & predicted spectral responses

14. Absolute total transmission Absolute response curves in polarised light 50 Response (%) 25 0 • Bandpass filter & polariser dominate losses • Improved system: T>80% • Theoretical throughput is 2n times higher than for spatial/spectral multiplexed techniques!

15. Blood oximetry 40 20 • Optimal spectral band for retinal oximetry • Vessel thickness ~ optical depth • 570-615 nm • Eight bands approximately equally spaced

16. Spectral Retinal Imaging Canon CR4-45NM • Difficult imaging conditions render application of traditional HSI techniques problematic • IRIS enables real-time and snapshot spectral imaging

17. Video sequence recorded with bandpass filtered inspection lamp

18. Retinal image recorded with flash illumination

19. 574 581 592 585 607 595 603 613 Coregistered and PCA images PC1 & PC2 PC2 PC1

20. 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 nm 50 Response (%) 25 0

21. 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 0.2m to 12 m • Applications • In vivo, in vitro imaging • Retinal imaging • Microscopy • Multiple fluorophors • Quantum dots • Surveillance • Remote sensing • Etc.