•There is increasing interest in the application of spectral
imaging for biochemical functional mapping of the retina. The solution to
recording the required three-dimensional data cube using a two-dimensional
detector array is normally to record images in time sequence in a way that
scans one of the cube dimensions; typically either a sequence of narrow-band
images are recorded and subsequently co-registered or a hyperspectral line
image is scanned across the retina. In both cases the time sequential nature
is undesirable: the increased time required to record the data combined with
the infirmity that is common of patients with eye disease is problematic -
both for the patient and in the impact on image quality; spectral calibration
and image co-registration can be highly problematic and it is not possible to
record en face time-resolved spectral images. Unfortunately a putative
two-dimensional spectral camera; the spectral imaging equivalent of
conventional RGB colour camera has been notable by its absence .
•We report here on the development of a novel
two-dimensionalsnapshot retinal camera
that records a spectral data cube directly onto a conventional detector array
and requires no complicated inversion algorithm to retrieve the spectral
information.The snapshot capability
removes the fundamental problems associated with time-sequential techniques
which affect accurate blood oximetry [2, 3, 4].
•The key component of this unique retinal camera is a novel image replicating
imaging spectrometer (IRIS) that employs polarising interferometry and
Wollaston prism polarising beam splitters to simultaneously replicate images
of the retina in multiple spectral bands onto a single detector array [5, 6],
See Fig. 1. N Wollaston prisms and N retarders produce 2Nreplicated spectral
images. In principle the technique is 100% optically efficient enabling the
intensity of light at the retina to be minimised.An unusual aspect of IRIS is that the
spectral transmission bands are not quite orthogonal in spectral space, but
this issue is not significant relative to the high signal-to-noise ratio and
•A proof-of-concept demonstration using a pre-existing IRIS
system is shown in Fig. 4, in which eight replicated narrow-band images were
recorded in a single snapshot. The variations of the grey levels are due to
the spectral filtering functions of the IRIS system. Notice the clear
discrimination between veins and arteries although better discrimination is
expected as a result of the optimisation process described above.
•We have described a new retinal imaging instrument for
research and clinical application that provides multiple spectral images in a
single snapshot. The shape and number of IRIS spectral bands can be optimised
for specific applications such as snapshot oximetry. The spectral retinal data
required for this optimisation can be obtained by a time-sequential random
access technique such as we describe in [3,4]. The resulting optimised
snapshot spectral retinal imager will enable enhanced biochemical measurements
in the retina by eradicating calibration and misregistration problems
associated with time-sequential techniques and the snapshot function is
particularly valuable in a clinical setting.
1.A notable exception is W.
R. Johnson et al. “Snapshot hyperspectral imaging in ophthalmology”, J.
Biomed. Opt. Vol. 12 (2007)
2.A. R. Harvey et al.,
“Spectral imaging of the rat”, ARVO 3844-B585 (2007).
3.D. J. Mordant et al.,
“Hyperspectral imaging of the human retina - Oximetric studies”, ARVO 148-B257
4.I. Alabboud et. al,
“Quantitative spectral imaging of the retina”, ARVO 2581-B658 (2007).
5.A. R. Harvey et al.,
“Spectral imaging of the retina”, SPIE Vol. 6047 (2006).
6.A. R. Harvey et al.,
“Spectral imaging in a snapshot” SPIE Vol. 5694 (2005).
disc camera from Marcher Enterprises Ltd. (UK).
•An assembled, 8-band IRIS system integrated into a Discam fundus camera  is
shown in Fig. 2. The integration of IRIS system into any fundus camera is a
straightforward process: the input object plane of IRIS is located at the
output image plane of the retinal imager; thus the output image is replicated
onto the CCD detector array.
•Fig. 1. Functioning principles of an 8-band image
•IRIS transmission bands may be optimised for operation in
specific spectral regions; we report here on optimisation for the spectral
bands appropriate to blood oximetry. Previous research [3-6] has shown that an
appropriate spectral window for blood oximetry occurs from 560nm to 600nm.
Also, the blood’s extinction coefficient experiences the maximum variations
with oxygenation and two isobestic points (desirable in the physical model
that yields a value for oxygen concentration).
•For comparison, two IRIS systems have been assembled to
optimally measure blood oxygenation; operating in the rannges 560nm to 600nm
and 577nm to 600nm. The transmission bands modulated by spectral
blood-transmission are shown in Fig.3. Note that some bands are optimised to
maximise the spectral separation between oxygenated and deoxygenated blood,
others produce retinal images practically insensitive to oxygen saturation
•Fig. 2. 8-band IRIS
system integrated into Discam retinal camera.
•Fig. 3. IRIS passbands as a function of oxygen saturation (red, 100%;
and blue, 40%) for a 100μm vessel and modulated
by Lambert-Beer’s law. Note the isobestic point at 586nm and its
•Fig. 4. Replicated
spectral images of the retina at the detector plane of IRIS.