10.2.5   Imaging Light Source

General optical requirements for the imaging light source are low spatial
coherence, a narrow spectral band, uniform illumination, and high optical
energy deliverable during a short exposure. Low spatial coherence helps to
reduce image speckle noise, which can mask retinal information in the
recorded image. High-resolution retinal imaging is particularly sensitive to
speckle as microscopic features, such as single cells, approach the size of
individual speckles (which is inversely related to the numerical aperture of
the eye). A narrow spectral band prevents blurring caused by the large amount
of chromatic aberrations in the human eye. Note that too narrow a bandwidth,
however, will lead to significant temporal coherence and increased speckle
noise. High optical energy overcomes the large loss of light in the eye (~10^-4)
[9], and short exposures arrest blur induced by retinal motion even for a fixating
eye. In addition to these requirements, the Lagrange invariant fundamentally
limits the efficiency of the illumination channel as dictated by the size
and divergence of the light source, the NA of the eye, and the desired illumination
patch (field of view). There are few commercial sources that meet all
of these requirements.

As an example, the RAOI and RAOII use a broadband krypton fl ash lamp
that emits 200 J of optical energy in 4π steradians per 4-ms pulse. This corresponds
to 50,000 W (during a pulse)! A very small fraction of this is collected
by the illumination channel and is further reduced by an interference filter
located conjugate to the pupil (IF placed at p2 in Fig. 10.1). This location
minimizes beam nonuniformities (at the retina) that result from the filter. The
krypton flash lamp emits over a broad range of wavelengths and provides very
high beam uniformity (at the retina), but the fl ashes can only be generated
once every 5 to 10 s (which is the time required to recharge the source capacitors).
As a compliment to this, the IAO uses a 10-mW SLD passed through
a multimode fiber. In the fiber, the light is distributed among the fiber modes
with modal dispersion causing each to propagate at different axial velocities.
The 25 m was of sufficient length to cause the time delay between exiting
modes to be larger than the temporal coherence length of the SLD that effectively
mitigated the source’s spatial coherence (removes speckle). The SLD
is highly directional and quasi-monochromatic, making it substantially more
efficient than the krypton flash lamp. For example, for the same exposure
time (4 ms) and retinal illumination size (1°), the SLD is 5 million times more
efficient (50,000 W versus 10 mW). The SLD source also occupies a much
smaller physical space. Furthermore, the SLD can be modulated by a computer
(with > kHz frequencies), which enables high-speed imaging (see also
Chapter 17). Disadvantages of the SLD approach are that SLDs emit over a
narrow spectral range (typically <50 nm) compared to a flash lamp and are
available at only limited wavelengths with the shortest being 0.675 μm. To
expand to other wavelengths without sacrificing camera performance, the
Indiana group recently demonstrated in the human eye their high-speed fiber
technique with a laser diode (see also Chapter 17), which offers a wider range
of possible wavelengths.

To obtain a sufficiently bright image of a 1° patch of retina, the RAOI and
RAOII require about 1 μJ per fl ash at the cornea for a single frame of 550-nm
light. The IAO requires about 4.5 μJ at the more reflective wavelength of
679 nm. Power levels depend heavily on the field size and must be calculated
carefully for each system.

The flash or exposure duration has to be short enough to arrest the motion
of the retina, otherwise the image will be blurred. Exposure times of 4 ms
have been empirically found to yield a high incidence of frames without visual
evidence of motion blur. The IAO ophthalmoscope has been used to explore
other exposure durations ( 1/3, 1, 4, 10, 20, 33, 66, and 100 ms). Although the
results are preliminary, they may be helpful as a rough guide: 1/3- and 1-ms
exposures yielded the highest incidence of sharp frames and were necessary
to freeze cellular motion in capillaries. Many of the images were visually
acceptable, but 10 ms produced some blur. Exposure durations above 10 ms
were largely unacceptable with 33 ms and longer producing substantial blur
that destroyed most microscopic detail. Interestingly, even at 100 ms, microscopic
structure could be observed in the sporadic image that happened to
coincide with the endpoint of a retinal movement at which time the retina
momentarily came to rest.

If an interference filter is used to control the wavelength of the flash (as
with the krypton flash lamp), the filter bandwidth must be sufficiently narrow
to prevent chromatic blur induced by the chromatic aberrations of the eye.
Typical filter bandwidths are less than 25 nm. Figure 10.2 shows a plot of the
chromatic change in refractive power of the eye as a function of wavelength.
The right scale bar converts the change in diopters to a corresponding change
in RMS wavefront error for a 7-mm pupil.

The illumination path can also be controlled to regulate the reflected
intensity of different features in the retina. This is done by controlling the
size and location of the entrance pupil diameter (EP at p1 in Fig. 10.1) in the
illumination path. For example, the cone photoreceptors act as waveguides,
and they will reflect maximally if they are illuminated along their optical
axis. If illuminated obliquely, the cones will reflect less light and the uncoupled
light will leak into the interstitial cone media contributing not only to
a loss in cone reflectivity but to an added loss in cone contrast. To increase
the visibility of cones, therefore, it is advised to use a small entrance beam

for illumination, centered on the pointing direction of the cones, which corresponds
to the peak of the Stiles–Crawford effect [15]. This point can be
found by moving the entrance beam location to maximize reflected intensity.
This effect is demonstrated well in Roorda and Williams where they measured
the optical fiber properties of cones using the RAOI instrument [16].
Conversely, the illumination angle can be made intentionally oblique to
reduce the contribution of the cones and increase the contrast of other
features.