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Fundus Photography
https://t.me/med1917
AshwiniKulkarni , TimothyY.Y.Lai ,
SimonK.H.Szeto , andNirojKumarSahoo
25
25.1 Introduction
Fundus photography is used to screen, diagnose, and monitor numerous treatable and preventable causes of blindness, particularly
diabetic retinopathy, age-related macular
degeneration, retinopathy of prematurity, etc.
The quality of fundus photographs and the
simplicity of imaging have substantially
increased over time. Modern technologies can
capture a 200° view of the posterior pole in a
single acquisition, unlike older devices that
can only capture 30–60° of the ocular fundus.
The evolution of retinal imaging technology
has led to the development of clearer images,
pupil tracking, non-mydriatic cameras, ultrawide eld fundus imaging, smartphone fundus
imaging, and articial intelligence-integrated
imaging. This chapter highlights the various
fundus cameras available, fundus photography
A. Kulkarni
Kode Venkatadri Chowdary Campus, LV Prasad Eye
Institute, Vijayawada, India
T. Y. Y. Lai · S. K. H. Szeto
Department of Ophthalmology and Visual Science,
The Chinese University of Hong Kong, Faculty of
Medicine, Pok Fu Lam, Hong Kong
e-mail: tyylai@cuhk.edu.hk
N. K. Sahoo (*)
Anant Bajaj Retina Institute, Kode Venkatadri
Chowdary Campus, L V Prasad Eye Institute,
Vijayawada, India
techniques, and the use of smartphones for
fundus photography.
25.2 History
In 1886, Jackman and Webster [1] captured and
published the rst retinal images. Carl Zeiss made
the rst fundus camera in 1926, which had a 20°
eld of view (FOV) [2]; the company later produced cameras with a 30–50° FOV.Donaldson [3]
reported that Metzger released stereoscopic fundus images in 1927 that were taken using the sideto-side shifting technique. Hansell and Beeson [4]
successfully linked the electrical ash tube to the
camera in 1953, allowing light to be focused
through the pupil. The revolutionary Pomerantzeff
Equator-Plus fundus camera debuted in 1960, and
Behrendt and Wilson published the rst study on
nerve ber layer photography using the conventional Carl Zeiss camera in 1965. According to
Dobbins, in 1975, Steven Sasson of Eastman
Kodak created the rst digital camera, and the
subsequent transition from analog to digital revolutionized medical record keeping. Confocal
scanning laser ophthalmoscopy (cSLO) has
become popular in recent years; it overcomes
aberrations due to inadequate dilatation and helps
obtain detailed, high-contrast images. Sharper
images, non-mydriatic wide- eld options, pupil
monitoring, and portability have all been added to
camera systems over time [5].
© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2024
T. Das, P. Satgunam (eds.), Ophthalmic Diagnostics, https://doi.org/10.1007/978-981-97-0138-4_25
303
304
Mirror with central
photographic film/sensor
Aerial image
Objective lens
Patient’
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25.3 Technology
25.3.1 Optics ofFundus Photography
Technically, all fundus cameras are indirect ophthalmoscopes that operate on the same principles
as Gullstrand’s ophthalmoscope. Illumination
and observation pathways traverse through different regions of the patient’s pupil to avoid
reection off the patient’s cornea and crystalline
lens surfaces. Meanwhile, an inverted aerial
image is created inside the fundus camera and
projected onto the lm plane.
Optical system: The illumination system,
observation, and photography system together
form the optical system of the fundus camera.
Each part operates independently in the device,
and only the front or ophthalmoscopic lens is
shared with the other parts.
(a) Illumination system: A low-intensity incan-
descent bulb is used for the photographer to
visualize the fundus and focus the device,
and a high-intensity electronic ash tube is
used to obtain images in ash-based fundus
cameras (Fig.25.1). These two light sources
are optically merged by a semi-reecting
surface in the Zeiss fundus camera. In most
commercial fundus cameras, the incandescent lamp and the electronic ashtube are
mounted on the same base, and a transillumination technique is employed to combine the
light paths of these two sources.
(b) Observation and photographic system: The
size of the illuminated patch on the patient’s
retina is controlled by adjusting the diaphragm, which receives light from both the
viewing lamp and the electrical ash through
a common path. This light is projected onto a
circular mirror after passing through a set of
lters. After being reected by this mirror, the
light is focused by a set of lenses. The topmost
lens has a mask that imparts a doughnut shape
to the light. The doughnut-shaped light is
reected on a round mirror with a central hole,
after which it passes through the objective
lens before exiting the camera and then passes
via the cornea into the subject’s eye. If the
illumination system and the image are properly positioned and focused, the resultant retinal image exits the subject’s cornea through
the center of the doughnut, which is not illu-
aperture
s eye
Fig. 25.1 Optics of a fundus camera. (https://www.aao.org/education/image/zeiss- fundus- camera © 2023 American
Academy of Ophthalmology)
Fixation
pointer
Monochromatic
filters
Camera
Flip mirror
Eyepiece
Incandescent
lamp
Flash bulb
Beam splitter
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minated. As a result, the viewing and lighting
paths split in the plane of the patient’s pupil,
freeing the device from unwanted reexes that
will result in glare and image degradation.
The light travels through the holed mirror’s
central aperture, the astigmatic device, and the
diopter compensating lenses before returning
to the single-lens reex camera system [6].
The ophthalmoscopic lens creates a fundus
image between the holed mirror and the lens,
which can be seen through the central aperture
of the holed mirror using a compound microscope. An image of the fundus is projected by
the microscope’s objective using a ip mirror
on a ground glass screen positioned at the
focal point of the eyepiece. When the picture
is taken, the ip mirror that normally directs
the image into the eyepiece for observation
swings out of the optical path, allowing the
image to be projected onto the lm for photography. Advances in image capture have
signicantly improved the quality of captured
images.
25.3.2 Advances inImage Capture
25.3.2.1 Digital Imaging
Digital imaging is commonly used in color fundus photography (CFP), uorescein angiography
(FA), and indocyanine green angiography
(ICGA). Digital imaging offers advantages over
traditional lm-based imaging, such as higher
image resolution, ease of analysis, better storage,
and instant access to images (Table25.1). It aids
in image reproducibility and exporting images
for telemedicine. It aids in educating patients and
their caretakers about their disease condition by
allowing ophthalmologists to display and review
the images on a computer monitor. Digital imaging can shorten the learning curve for beginner
angiographers as immediate feedback on image
quality and camera alignment are available,
which improves the image-capturing technique.
A digital camera obtains a single image of the
retina using ashlight illumination. A digital
camera captures true color images of the retina.
However, ash intensity should be appropriately
controlled to get the best details.
Table 25.1 Digital fundus camera versus confocal scanning laser ophthalmoscope (cSLO)
Confocal scanning laser
Digital fundus camera
Single ash of light
used at maximum
intensity to capture
fundus image
Entire cone of light
is used
True color image Pseudo-color image
Bandwidth lters for
excitation and
emission are used
Slower image
acquisition
Poor image quality
in opaque media and
poorly dilated eyes
Uses shorter
wavelengths for FAF
(fundus autouorescence) and ICGA
ophthalmoscope (cSLO)
Continuous scanning in a
raster manner through a
pin-hole aperture
Confocal system
One excitation wavelength is
used, which acts as a laser
source
Fast image acquisition
Good image, even in
non-mydriatic pupils,
penetrates opaque media;
after-image processing
decreases motion artifacts
Longer wavelength used,
which gives better image
quality in FAF (fundus
autouorescence) and ICGA
25.3.2.2 Confocal Scanning Laser
Ophthalmoscope
The confocal scanning laser ophthalmoscope
(cSLO) can be used for several retinal imaging
modalities, such as FA, ICGA, and fundus autouorescence (FAF). It uses monochromatic laser
illumination and a confocal optical system to capture high-contrast images. Blue light and nearinfrared illumination beams are commonly used
for more efcient excitation of uorescence for
FA, FAF, and ICGA.A laser beam scans the retina
point-by-point at speeds up to 24ms in a raster
pattern. Images obtained with a cSLO are characteristically rectangular due to the raster scanning
pattern. A confocal aperture is positioned in front
of the image detector at a focal plane conjugate to
the retina, which blocks scattered light and chromatic aberration, enhancing image quality. The
light reected from each retinal point is captured
by a photomultiplier. The photomultiplier output
is recorded and displayed in a digital video format. Continuous high-speed representation of the
ow dynamics of the retinal and choroidal vessels
during angiography is also available. This is useful to document the early lling stages required
for identifying feeder vessels of macular neovas-
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cularization (MNV), retinal angiomatous proliferation (RAP) lesions, etc. It also allows the
measurement of capillary ow velocity. The ow
velocity is calculated by measuring the transit
time of blood between two points separated by a
known distance on the video monitor.
Simultaneous FA and ICGA can be performed
with a single injection of both dyes. The crystalline lens forms the principal barrier for capturing
fundus images as the lens is very uorescent in
the short-wavelength range (excitation between
400 and 600 nm results in peak emission at
520nm). The major disadvantage of the cSLO is
its inability to capture full real-color retinal
images due to monochromatic illumination.
Several frames can be captured to produce a
mean image using eye-tracking software with
high acquisition speed (e.g., automatic retinal
tracking (ART) mode in the Heidelberg system),
and this improves exposure, decreases noise, and
can overcome the effect of eye movement during
imaging; after-image processing corrects movement artifacts. Sequential cSLO scans taken at
increasing depths can create three-dimensional
topographic images of the retina or optic disc for
retinal thickness or optic nerve head topography
assessment [7]. Table25.1 highlights the differences between the digital fundus camera and
cSLO.
In 2019, a panel of experts formally dened
various terms based on the fundus FOV.According
to these experts, the posterior pole is the area of
the retina up to the outer edge of the major temporal vascular arcades and has a 50° FOV. The
mid-periphery is the area of the retina from the
vascular arcades to the posterior edge of the vortex vein ampullae, with an FOV from 60 to 100°.
Wide-eld imaging (WFI) produces a retinal
image centered on the fovea, including all four
retinal quadrants, and posterior to the vortex vein
ampullae, i.e., the mid-periphery. In ultra-wide
eld imaging (UWFI), the retinal surface anterior
to the vortex vein ampullae in each quadrant is
captured with an FOV of 110–220° [8].
25.3.2.3 Adaptive Optics SLO
Standard fundus cameras cannot focus on individual cells or structures due to aberrations in
the visual system. However, individual photoreceptors can be observed using adaptive optics,
which uses deformable mirrors to overcome the
wavefront distortions of the light reected from
the retinal surface. Recent research has demonstrated the capability of an adaptive optics-based
system to image photoreceptor mosaics, arteriolar structures, and pathological lesions with a
resolution of 2–3 μm. Its value in detecting
decreases in the number of cone photoreceptors
in retinitis pigmentosa has been demonstrated
[9] (readers are encouraged to refer to Chap. 30
for more details).
25.3.2.4 Wide-Field andUltra-wide
Field Imaging
Wide Field Imaging (WFI) has an FOV of up to
50°, while UWFI systems can image over 80% of
the retinal surface area (up to 200° FOV). The
peripheral retina can be captured even without adequate mydriasis. The UWFI can be obtained using
contact or non- contact systems. The Pomerantzeff
camera (with an FOV of 148°), RetCam (with an
FOV of 130°), and Panoret-1000™ camera (with
an FOV of 130°) are contact-type UWFI cameras.
The Optos®, Clarus® 500, and Heidelberg
Spectralis with UWFI lens are non-contact UWFI
cameras. The Heidelberg Spectralis captures
images with an FOV of 105°, and with concomitant
use of the Staurenghi lens, it can capture an image
with an FOV of 150°; however, this requires a contact procedure.
25.3.3 Examples ofAvailable Fundus
Cameras (Table25.2; Fig.25.2)
25.3.4 Artifacts inFundus
Photography andActions
toPrevent These
The fundus camera can produce a variety of
artifacts:
Illuminated iris: When the pupil is not suf-
ciently dilated, an illuminated iris causes an
orange or bright crescent at the photo’s
margin.
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(continued)
photographer required
Dynamic studies like FA are not
possible
148° Limited resolution
Difcult to set up
Can be used in non- mydriatic pupils
100°
Topical anesthesia for contact lens
placement required.
Lenticular opacities can degrade image
and lenticular opacity
Multi-color imaging
Spectral domain OCT with enhanced
depth imaging (EDI) OCT, FAF,
105°
150° with Staurenghi contact
lens
quality
Needs an experienced photographer
Pseudo-color images
infrared reectance, FA, ICGA can
be performed simultaneously
Reduced eyelash artifact
Pseudo-color images
Map distortion (when a 3D structure is
projected onto a 2D surface)
Better imaging of the superior and
inferior periphery
apertures and non-mydriatic pupils.
Faster image acquisitions
Does not provide a retinal vision
before obtaining the image
Green-light FAF, FA, ICGA, and
OCT (spectral domain [SD-OCT]
Longer image acquisition times
and swept source) possible.
Ultra-wide eld angiography
133° True-color imaging
Pseudo-color fundus images
Fewer lash and lid artifacts
OCT angiography (OCTA), FA and
ICGA, green and blue FAF,
retro-modes, and high-denition
attachment
OCT and confocal SLO (cSLO)
available
Montage with 19 images 96° Patient cooperation and a skilled
Type Description Field of view Advantages Disadvantages
Traditional Fundus
Table 25.2 Descriptions of the currently available fundus camera
Camera [10]
Contact lens-based system uses
beroptic transpupillary
illumination with scleral
transillumination
handheld digital camera; uses
Pomerantzeff
Camera [10]
Panoret [10] Transscleral illumination with a
domain OCT with cSLO
contact lens
Spectralis [9, 10] Fundus imaging with spectral
three distinct wavelength channels
(red, green, and blue)
Optos [9, 10] UWF imaging system 200° Can image through small optical
Clarus [10] Light-emitting diode (LED) and
Mirante [11] SLO-based UWF imaging system 163° with a wide-eld lens
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Lenticular opacities: Poor image
quality
Anterior segment photography is not
possible
Requires pupillary dilatation
Difculty with moderate cataracts
High cost of the machine
Lack of ICGA and FAF imaging
capabilities
Poor resolution
A. Kulkarni et al.
High-resolution digital photos.
Useful screening tool
Useful in telemedicine
80°—high- contrast pediatric
Portrait-external imaging,
130°—pediatric retina +adult
anterior chamber,
system with an external beroptic
light source
and adult,
120°—pediatric and young
adult,
Useful screening tool
30°—high magnication
120° High-resolution digital photos
Camera with integrated liquid lens
Useful in telemedicine
Color and contrast adjustments
possible
Laser photocoagulation with color
fundus photography, FA, and IR
imaging available
Eye-tracking technology available
Ability to import and store
multimodal pictures captured by
other devices viz. FAF, OCT, ICG
system and LED lighting system
solid-state laser with a wavelength
of 532nm
Convenient, easy to learn, portable
Contrast and color editing possible
Useful for screening and
angiography images
25°: with i-Examiner
Smartphone fundus photography
telemedicine
200°: with D-Eye adapter
with a 20D condensing lens
Type Description Field of view Advantages Disadvantages
RetCam [12] Contact-based digital imaging
Table 25.2 (continued)
3Nethra Neo [12] Compact and portable system
Navilas [13] Target-locked frequency-doubled
Smartphone- based
[14]
ab
P
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Action: Move the joystick in the opposite
direction. This directs the cone of light
through the pupil and avoids the iris.
Eyelashes: The eyelashes cast hazy veils over
some portions of the image.
Action: The eyelids should be kept open
to prevent the lashes from obscuring the
light.
Camera position: Depending on how close or
distant the camera is from the eye, there is a
haze and loss of detail.
Action: The eyepiece reticle should be
adjusted before focusing. To capture the
Panel 1.
deepest tone of color of the fundus, the
camera position and focusing knob should
be adjusted till the aerial image of the fundus and the crosshairs appear sharp. The
green lter can be used to improve contrast
while focusing.
A spot in the same location will appear in images
shot at various periods if there is a smudge in
the system. This typically results from the
patient touching the objective lens with their
face, nose, or eyelashes.
Action: Clean the system of all debris or
smudges [10].
a
anel 2.
Fig. 25.2 Panel 1. (a) Heidelberg Spectralis fundus
camera. (b) Multi-color imaging of a normal fundus
obtained through the fundus camera. Panel 2. (a) Wideeld Clarus fundus camera. (b) A montage of fundus
images of gyrate atrophy. Panel 3. (a) Ultra-wide eld
Optos fundus camera. (b) Fundus image of a patient with
lasered proliferative diabetic retinopathy. Panel 4. Fundus
images of a patient with retinopathy of prematurity
(ROP) obtained using a RetCam. (a) Stage 1: demarcation line shown by a yellow arrow. (b) Stage 2: demarcation ridge. (c) Stage 3: extraretinal neovascular
proliferation. (d) Stage 4A: partial retinal detachment
with fovea sparing. (e) Aggressive retinopathy of prematurity F) Post-laser photocoagulation of avascular retina.
(Picture provided by: Suman Sahu, MD)
b
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bc
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anel 3.
nel 4.
A. Kulkarni et al.
ab
a
de f
Fig. 25.2 (continued)
25.4 Types ofPhotography
Conventional ash-based fundus photography
gives a true color image of the retina. However,
several factors, like poor mydriasis, corneal scar,
cataract, vitreous opacity, and background contrast, can all affect its ability to detect certain
lesions. Several advances and modications have
been introduced to modern photography units to
make them more robust in clinical practice.
25.4.1 Monochromatic Retinal
Photography
Monochromatic photography includes capturing
retina images under colored or monochromatic
light. Vogt pioneered the usage of green light in
1925 to improve the visual contrast of anatomical
features of the fundus. Fundus photography frequently employs this method. Using contrast lters to change the subject tones in monochrome
images and limiting light scattering at shorter
wavelengths are the foundations of monochromatic retinal imaging. It is possible to improve
the visibility of specic fundus structures by
reducing the spectral range of the illuminating
source and limiting light scattering [15].
Blue light: The anterior retinal layers, typically almost transparent in white light, become
more visible under blue light. Retinal pigmentation and blood vessels absorb blue light, creating
a dark background that enhances specular reections and scattering in the front layers of the fundus. The efciency of these wavelengths can be
restricted by scattering in the ocular media.
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Short-wavelength illumination improves visualization of the semi-transparent scattering structures such as the retinal nerve ber layer, internal
limiting membrane, retinal folds, cysts, and
epiretinal membranes. Blue-green (cyan) lters
with a wavelength of 490nm are frequently used
because of the signicant scattering at very short
wavelengths. Scattering is also often seen due to
cataract or corneal edema. Blue light imaging can
be affected by simple procedures such as applanation tonometry or gonioscopy as it distorts the
corneal surface and can produce slight corneal
edema, enough to scatter blue light. After staining the cornea, residual uorescein dye can also
result in cumbersome uorescence that can hamper the image quality [16].
Red-free: Although blood absorbs green light,
retinal pigmentation reects it more than blue
light, creating the nest overall contrast and the
ideal view of the retinal features. It improves the
visibility of hemorrhages, drusen, exudates, and
retinal vasculature. Green light has less scattering
than the shorter wavelength blue light and can
offer a better view of the fundus even in media
opacities. Therefore, along with FA, green lter
“red-free” pictures are frequently taken as baseline images [16].
Red light: In red light, retinal pigmentation
gradually becomes lighter and more transparent,
displaying a choroidal pattern with greater
details. Red lters produce illumination in the
625–640nm range, signicantly reducing fundus
contrast because many retinal structures are redcolored. The optic nerve appears to be exceedingly thin and practically featureless. Red light
facilitates the imaging of pigmentary abnormalities, choroidal ruptures, choroidal nevi, and choroidal melanomas. Therefore, red light imaging
is also known as the “poor man’s ICGA.” [16].
25.4.2 Autouorescence: Blue,
Green, andNear-Infrared
Fluorescein dye is used in FA to visualize retinal
vasculature and diagnose and monitor retinal
pathologies. During the advent of this technique,
few ocular structures exhibited uorescence
without the presence of dye. Autouorescence is
the continuous emission of uorescent light without uorescein dye from ocular structures.
Imaging of these autouorescent structures aided
the diagnosis of various retinal pathologies.
The continuous deposition of lipofuscin (LF)
granules in the cytoplasm of retinal pigment epithelial (RPE) cells is a characteristic sign of
aging. The continual phagocytosis of shed photoreceptor outer-segment discs is assumed to be the
leading cause of this gradual LF accumulation.
Once generated, the RPE cell cannot degrade or
exocytose the LF material and granules into the
extracellular space. These granules are then conned in the postmitotic RPE cells. A2-E
(N-retinylidene-N-retinylethanol-amine) is the
primary component of lipofuscin along with the
precursors of A2-E, molecules created by the
combination of oxygen-containing moieties
within photo-oxidized A2-E, also known as bisretinoids [9]. The intrinsic autouorescence of
lipofuscin is utilized in FAF imaging. Lipofuscin
accumulates excessively due to a disparity
between lipofuscin synthesis and clearance
caused by RPE dysfunction. The bisretinoids
have different excitation wavelengths in the visible spectrum. The RPE cell will seem hyperautouorescent on FAF if its lipofuscin
concentration is higher. Due to the absence of
lipofuscin, atrophic or dead RPE cells exhibit
extreme hypo-autouorescence, whereas healthy
cells exhibit varied levels of hyperautouorescence and lipofuscin [9].
When exposed to light, lipofuscin absorbs
blue light of wavelength 470 nm and emits
yellow- green light of 600–610nm wavelength.
Blue auto-uorescence (BAF) or shortwave
autouorescence (SWAF) are two terms used to
describe autouorescence based on lipofuscin.
Melanin absorbs light at a wavelength of 787nm
and emits at a near-infrared wavelength. Nearinfrared autouorescence (NIRAF) is the autouorescence based on melanin. The fovea in BAF
is darker (hypo-autouorescent) because of lesser
lipofuscin content. On the other hand, the fovea
in NIRAF is brighter (hyper-autouorescent) as
melanin is more concentrated at the fovea.
Therefore, FAF is useful for the diagnosis and
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prognosis of many disorders, including central
serous chorioretinopathy (CSC), age-related
macular degeneration (AMD), retinal dystrophies, and uveitic entities such as choroiditis. A
standard fundus camera with a 50–55° FOV can
be used to capture FAF images of the retina. The
Optos, Heidelberg Spectralis, Mirante, and
Clarus all support UWF FAF [17].
Compared to confocal-based systems, which
use continuous scanning with an excitation spectrum of 488nm and acquire several images that
are averaged to produce a nal image with the
highest resolution and contrast, modied fundus
cameras use a single ash with an excitation
range of 535–585nm to acquire a single image.
FAF imaging provides details on the metabolic
changes occurring at the RPE level and aids
inlocating regions that could be particularly vulnerable to the onset of geographic atrophy or
MNV. The green light of wavelengths 514 and
532nm is used, which excites uorophores with
less absorption by macular pigments than BAF,
thus making it more useful than BAF in detecting
small atrophic patches near the fovea. Optic disc
autouorescence is higher in green reectance
(GR, 515nm) than BAF, making GR more efcient for evaluating optic nerve head diseases.
Green reectance (515nm) shows a higher autouorescent signal at the disc than the fovea and
blood vessels, unlike BAF, which shows a dark
gray optic disc [18]. NIRAF uses infrared light
for illumination. Red wavelengths are longer
than the other wavelengths in the visible spectrum; they also scatter less and are less likely to
be absorbed by blood and melanin. In contrast to
shorter wavelengths in the visible spectrum, this
results in a greater amount of reected light to
generate an image, making it a better technique
for imaging deeper structures underneath the
RPE.
25.4.3 Multi-color Imaging
Three reectance pictures are simultaneously
captured by multi-color imaging using cSLO:
blue reectance (BR; 488nm), GR (515nm), and
near-infrared reectance (IR; 820nm). The inner
retina is seen better with BR; the outer retina,
including RPE abnormalities, is seen better with
IR; and the retinal blood vessels and exudations
in the retinal layers, if present, are seen better
using GR. Given that IR is invisible and has
higher penetration, it is more advantageous than
ash-based retinal photography in imaging the
eyes of children, people with photophobia, and
people with media opacities such as dense cataracts. Additionally, IR imaging may offer a better
view of the epiretinal membranes, cystoid macular edema, and deeper structures.
25.4.4 Retro-mode Imaging
Retro-mode imaging, based on the principles of
retro illumination, is used by the F-10 confocal
scanning laser ophthalmoscope (cSLO; Nidek,
Gamagori, Japan) with an infrared laser to
observe the retina. The F-10 is a recently created
cSLO device with eight apertures (ve confocal
apertures and three apertures with a central stop)
and four distinct wavelengths (blue, 490 nm;
green, 532nm; red, 660nm; infrared, 790 nm).
Direct backscattered light and additional dispersed light make up the reected light from the
fundus in the cSLO.Only the directly backscattered light from the confocal plane can pass
through a confocal aperture. In the indirect (darkeld) mode, a centrally stopped aperture (ring
aperture) excludes direct backscattered light. As
infrared laser light can reach deeper layers, it is
used in retro-mode imaging, a modied form of
indirect imaging. The retro mode only utilizes a
portion of the annular aperture, not the entire ring
aperture used in the indirect mode. The confocal
aperture allows the passage of backscattered light
from a particular direction and prevents its passage from other directions, thereby creating a
pseudo-three-dimensional image by casting a
shadow on the side of the aberrant feature. The
scattered light that enters the deviated aperture
improves contrast and delineation by casting a
shadow on aberrant features. The laterality of the
annular aperture affects how the lesions’ shadows are visible and can be either right-deviated or
left-deviated. To identify subretinal and RPE
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