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[4]. This eventually paved the way for Allvar
Gullstrand to use the concept of a focused slit to
illuminate the eye and observe its structures [5].
The source of light was a Nernst lamp with an
adjustable slit at the other end. In 1915–16, this
illumination system was combined with
Czapski’s binocular device by Otto Henker, and
thus, the slit lamp was invented [3]. In the following years, further improvements were made to
the slit lamp, including a swivel design to allow
movement of the illuminating arm and different
light sources. Hans Goldmann coupled the focal
planes of these two arms and engineered a joystick that could control the movement of the
entire unit [6]. The introduction of the rotatory
magnifying system by Hans Littman enabled a
comprehensive examination of the ocular structures [7]. Alfred Vogt played a pivotal role in
advancing our understanding of slit lamp examinations through his innovative and meticulous
utilization of this tool [8]. His pioneering work,
documented in his atlas, helped establish the slit
lamp as a crucial instrument in ophthalmology.
Over the next few decades, various modications
and advancements were made, including adding
lters to enhance contrast and visualization of
specic structures within the eye. Additionally,
introducing different types of lenses allowed for
better magnication and focused examination of
different eye regions. With the rapid advancements in technology, modern slit lamps have
become even more sophisticated and now feature
the ability to capture high-resolution images and
videos of the eye.
14.3 Parts ofaSlit Lamp
The slit lamp consists of three parts: the illumination system, the observer system, and the
mechanical unit coupling these two. Figure14.1
depicts the different parts of the slit lamp.
14.3.1 The Illumination Unit
The illumination unit produces a precise slit
beam of light of variable length, breadth, and
A. Kate and S. Basu
Fig. 14.1 Image of a slit lamp with marked parts. (1)
patient frame, (2) forehead band, (3) chin rest, (4) slit
width control, (5) slit height control, (6) lters, (7) slit
angle indicator, (8) illumination tower, (9) decoupling
control, (10) magnication, (11) eyepiece, (12) joystick,
(13) illumination knob, (14) base lock, (15) applanation
tonometer, (16) power switch, (17) screw lock, (18)
reecting mirror, (19) xation target
brightness based on the examination area. The
light source can be situated above or below the
observer unit, as seen in the Haag-Streit (Koniz,
Switzerland) and Zeiss (Jena, Germany) slit
lamps, respectively. Additionally, prisms and
mirrors are used to manipulate the beam of light
within the small connes of the illumination unit.
The sources of light include halogen lamps, lowvoltage incandescent lamps, and light-emitting
diodes (LED). The choice of the light source is
balanced between its durability and the color
temperature. With respect to the latter, light with
a greater degree of blue within it is preferred
since it scatters easily, thus making detecting
pathologies easier [3]. Köhler’s principle of illumination is used to produce a homogeneous
beam of light wherein the light from the source is
transmitted through a collector system of planoconvex lenses and then passed through a slit
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aperture [9]. This aperture acts as a new light
source, emitting rays that pass through the objective lens to generate an image of the slit onto the
area under examination. The height and width of
the slit beam can be altered by opening the vertical and horizontal shutters of the light aperture. It
can also be swiveled across the horizontal plane
to pass this light beam through different eye
structures. The slit beam can also be rotated
through different angles and made completely
horizontal. This is particularly useful when measuring different structures and anomalies. It is
typically carried out by reducing the slit beam
width to 0.5–1mm and altering the height until it
matches the structure being evaluated.
Different lters add perspective to the various
pathologies viewed through a slit lamp biomicroscope. These lters are located between the slit
aperture and the objective lens. The cobalt blue
lter (Wratten 47/47A) is employed with the uorescein dye and aids in identifying areas where
the corneal or conjunctival epithelial layer has
been compromised [10]. It also helps evaluate the
health of the tear lm, perform Seidel’s test, and
assess the t of contact lenses. Optionally, a yellow barrier lter that blocks the incoming blue
light and improves contrast can also be used.
Although the Zeiss slit lamps have an inbuilt lter, models that do not have this feature can have
the lter added as an attachment [10]. The redfree lter enhances the evaluation of any area of
inammation, and this is particularly useful
inlocalizing the depth of inammation in episcleritis and scleritis. A neutral density lter decreases
the intensity of the light and enables visualization
in symptomatic/photophobic patients. A diffuser
is also present external to the illumination arm
which can be manually brought into place to illuminate the examined area evenly.
14.3.2 Observer System
This consists of a collation of lenses that help
assess the observed area with sufcient magnication to allow the required examination details.
This includes two telescopic systems, one at each
end of the observer system. A Galilean telescope
is present at the objective end of the slit lamp and
is formed by a combination of a convex and a
concave lens with a total power of +22D [3, 11].
Similarly, a Keplerian/astronomical lens is situated within the eyepiece and is fashioned with
two convex lenses with a combined power of 10D
[3, 11]. Since the latter produces an inverted
image, a Porro prism is placed between the two
telescopic systems to correct the same [3, 11].
These telescopes function on the principle that
the image formed by one lens falls on the principal focus of the second lens.
Additionally, a graded magnication change
is possible either by changing the eyepieces or
using an incorporated changer. The latter comprises two Galilean telescopes, which can be
rotated to accommodate different orders of magnication. The degree of magnication varies
among the slit lamps and can range from 6.3× to
40×. Each eye of the examiner has an independent path between the source and the image
formed, thus allowing good stereopsis. The eyepieces converge at an angle of 10–15° to facilitate
the same. The eyepieces also feature lenses that
can be set to plano or adjusted according to the
examiner's refractive error, eliminating the need
for them to wear glasses.
14.3.3 The Mechanical System
The illumination and observer systems are connected through a base mechanical unit which not
only facilitates the coupling of the two systems
but also allows a three-dimensional movement of
the entire complex. This is enabled with a joystick that can move forward, backward, and along
an up-down axis as well. This is usually accompanied by a lock system that can x the unit in a
desired location. The control of illumination with
an intensity rheostat and a power switch is also
located within the base of the mechanical unit.
The slit lamp has a motorized system that enables
vertical movement of the entire unit with a
control.
The patient frame consists of a forehead band
against which the patient rests the head and an
adjustable chin rest beneath this. This frame also
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has a canthus height marker that indicates the
level to which the lateral canthus should be
aligned. Ensuring proper alignment of the patient
to these three landmarks will ensure that the
patient's eyeball falls within the focal plane of the
slit lamp system. Two handles extend laterally
from this frame over which the patients can rest
their hands during examination. A mobile blinking light is also incorporated into the slit lamp to
serve as a xation target for patients in various
gaze positions to ensure consistent eye
alignment.
14.4 Machine Specications
The growth and development of the slit lamp
have been greatly facilitated by two prominent
companies, namely Zeiss and Haag-Streit.
These companies have played a pivotal role in
advancing the eld of ophthalmology by continuously innovating and producing high-quality slit lamps that have become integral tools
for eye care professionals worldwide. Today
several ophthalmological diagnostics companies manufacture slit lamps, and within each
company, the specications vary based on different models. As describing all these modications is beyond the scope of this chapter, the
salient points of some of these devices are summarized in Table14.1.
14.5 Examination Techniques
The various methods of examination employed in
slit lamp examination are derived from different
variations of brightness, height, breadth, and position of the illumination arm. Broadly this can be
divided into direct and indirect forms of illumination. In the former, the area of interest is directly
illuminated by the light source. In indirect illumination, the examined area is illuminated by the
light reected from a structure posterior to it.
These techniques are discussed in detail below.
14.5.1 Direct Illumination
This includes diffuse illumination, focal illumination, and specular reection.
(i) Diffuse illumination: In diffuse illumination,
the width of the slit is broad, or a diffuser is
used, and a low magnication is preferred.
This is typically employed when an even
illumination is desired while the initial gross
examination is done. It is also used for slit
lamp photographs for documentation, in conjunction with a slit beam, to provide context
to the structures within the slit (Fig.14.2).
(ii) Focal illumination: This uses the scattering of
light within the area of interest to highlight
abnormalities (Fig.14.2). The dimly lit sur-
Table 14.1 Comparison of the commonly used slit lamp platforms
Slit lamp company Design
Haag-Streit Slit lamps
(Haag-Streit, AG Koniz,
Switzerland)
Zeiss slit lamps (Carl Zeiss AG,
Jena, Germany)
Appasamy (Appasamy
Associates, Chennai, India)
Nidek (Nidek, Gamagori,
Aichi, Japan)
Topcon (Topcon Medical
Systems, Tokyo, Japan)
LED Light emitting diode
a
Tower: light source from above, integrated: light source from below
a
Tower 6.3/10/16/25/40×
Tower/
integrated
Tower 6/10/16/25/40×
Integrated 5/8/12.5/20/32× LED Gray, red-free, blue
Tower/
integrated
Magnication Light source Filters
LED Gray, red-free, blue
10/16/25×
10/16×
6/10/16/25/40×
5/8/12/20/32×
10/16/25×
10/16/24×
10/16×
10/16/25×
6/10/16/25/40×
LED/Halogen Blue, red-free, yellow
(optional)
LED Heat-absorbing,
red-free, blue
LED/Halogen Blue, red-free, gray
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rounding area offers a high degree of contrast
and makes it easy to detect subtle changes. It
provides three-dimensional information,
which allows us to understand the prole and
thickness of the structure being examined and
its position with respect to the source of the
light. Within the section of light, structures
that are present closer to the source are anterior and vice versa. This is perhaps the most
commonly used mode of slit lamp examination and yields a plethora of information
depending on the height, width, and angle of
the slit. Based on these parameters, focal illumination can further be divided into a parallelopiped and an optical section.
(a) Parallelopiped: Here, the width of the slit is
between 1 and 2mm and is placed at an angle
of 35°–45°. This provides a threedimensional block view of different ocular
structures and enables localization of the
same. Since the eld of view is greater than
a b
the optical section, it is more sensitive at
picking up ndings.
(b) Optical section: Here, the term “section”
is used because the beam of light provides a slit image of a semi-transparent
structure which is analogous to a histopathological section of tissue. In an optical section, the width of the slit is
extremely narrow and ranges between
0.1 and 0.3mm, and the angle of illumination can be varied between 30°–50°.
The magnication depends on whether
the lesion location or thickness is measured. For the former, a higher magnication of 16–25x is preferred, while a
magnication of 10–16x can be used for
the latter since a decent eld of view is
also required. The light intensity is
another key factor that can improve the
sensitivity of slit lamp examination and
has to be titrated based on the opacity of
c d
Fig. 14.2 (a) Schematic representation of diffuse illumi-
nation. A diffusor is placed in front of the illumination,
which provides an evenly illuminated eld. (b) Slit lamp
image of the right eye under diffuse illumination. (c)
Schematic representation of focal illumination with a
moderate slit. (d) Slit lamp image of a moderate slit with
focal illumination depicting brin strands in the pupillary
plane (yellow arrows)
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a
Fig. 14.3 (a) Schematic representation of an optical sec-
tion. (b) Slit lamp image of corneal granular dystrophy
examined with an optical section which reveals the exact
depth of different deposits. (c) Diffuse illumination image
of an eye with a white media opacity. (d) An optical sec-
b
tion shows a relatively clear and compact cornea with dispersed loose cortical matter in the anterior chamber and a
traumatic cataract. Thus, the optical section helps us
understand the structures involved
the structure being examined. Very high
or low intensities may mask important
ndings and should be avoided.
Increasing the angle of illumination will
increase the width of the optical section
and thus enhance the details of the image
thelium will lie the second Purkinje image,
which is usually dimmer. The corneal endothelial cells can be examined within this area
(Fig.14.4). This examination technique can
also be used on the conjunctival surface and
for the tear lm.
(Fig.14.3).
(iii) Specular reection: It uses the principle of
specular reection where the angle of incident light is equal to the angle of the reected
14.5.2 Indirect Illumination
light [12]. To achieve this with a slit lamp,
the observer and illuminating arm are placed
at an angle of approximately 60° to each
other. To begin with, the slit height is reduced
to 2–3 mm, and the width is kept around
1mm with a low-moderate intensity of light.
Once the slit is brought into focus, the illuminating arm is moved until the reection of
light is sighted, which is the rst Purkinje
image. Posterior to this on the corneal endo-
This includes proximal illumination, sclerotic
scatter, and retro-illumination. The properties of
refraction and absorption are used to emphasize
subtle changes.
(i) Proximal illumination: In this type of illumi-
nation, the two arms of the slit lamp are
placed in such a way that the examined tissue
acts as a light source, and the structures
within it are backlit. To achieve this, the illu-
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a b
173
Fig. 14.4 (a) Schematic representation of specular
reection, with the incident light angle being equal to that
of the reected light. (b) Slit lamp image, which has captured the specular light reection (blue arrows). The vertical slit must coincide with this image. In this image, the
mination beam is placed tangential to the
area of interest such that the light is directed
toward its base and gets reected from it.
This is particularly useful to highlight details
of semi-transparent structures based on how
the light gets refracted or absorbed within
them (Fig.14.5; top row).
(ii) Sclerotic scatter: This employs the principle
of total internal reection of light and creates
this phenomenon within the cornea. Any
structural abnormalities will produce a scattering of light, thus highlighting these areas.
However, this technique will not give information on the depth of these irregularities.
The illumination arm is rst decoupled from
the observer arm to perform a sclerotic scatter. A 4–5 mm slit beam is focused on the
limbus with a high intensity of light, and the
magnication is set so that the entire cornea
falls within the eld of view. The position of
the observer arm is adjusted until a brilliant
circle of light is visible, encircling the cornea. This is the point where a total internal
reection of light occurs within the stromal
layers. A normal cornea appears dark, which
allows the examiner to detect very subtle
changes. A limitation of this technique is that
it may be challenging to perform on a photophobic patient since it involves a very bright
beam of light (Fig.14.5; bottom row).
anterior slit image is from the corneal epithelium, and this
is the rst Purkinje image (red arrows). The second
Purkinje image behind it is from the corneal endothelium
(yellow arrows)
(iii) Retro-illumination: Here, a posterior struc-
ture is used to reect the light from the
source. This can further be divided into direct
and indirect retro-illumination based on the
position of the area of interest.
(a) Direct retro-illumination: In direct retro-
illumination, the area of interest is directly
illuminated by the light reected off the
iris, lens, or fundus. The angle of the illuminating arm can vary based on the structure that the light has to reect. In the case
of retro- illumination from the iris or the
lens, the angle of illumination is kept
between 30° and 50°, with a moderate to
high light intensity and a slit width of
1.5–2mm. When retro- illumination from
the fundus is desired, the illuminating arm
is kept coaxially with a low to moderate
light intensity (Fig.14.6).
(b) Indirect Retro-illumination involves plac-
ing the area of interest between the slits
formed on the cornea and the posterior
structure, be it the iris or the lens. The
observed area is examined against a dark
background which offers a contrast.
Increasing the illumination angle creates
greater space between the two slits and
allows better viewing of the in- between
structures (Fig.14.7). Minimal decentration of the observer arm may be required
to visualize the pathologies.
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a
a
Fig. 14.5 Top Row: (a) Schematic representation of
proximal illumination wherein light tangential to a mass
lesion strikes its bases and acts as a new source of light
from within the lesion. (b) Slit lamp photograph of a cyst
within the head of a pterygium under diffuse illumination.
(c) The proximal illumination depicts the cystic nature of
a b
b
the lesion. Bottom Row: (a) Schematic representation of
sclerotic scatter. (b) Slit lamp image of sclerotic scatter
depicting keratic precipitates in the inferior half of the
cornea with surrounding corneal haze. The superior half
of the cornea, which is relatively uninvolved, appears dark
Fig. 14.6 (a) Schematic representation of retro-
illumination from the fundus. Slit lamp photographs of
fundus retro-illumination illustrating. (b) A subluxated
lens in the inferior half of the pupillary area. (c) A lamellar
cataract. (d) Markings in a toric intraocular lens along
with the margins of the capsular bag
cd
a
cd
b
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Fig. 14.6 (continued)
175
Fig. 14.7 (a) Schematic representation of direct retro-
illumination wherein the reected light from the iris falls on
the corneal endothelial pigments. (b) Schematic representation of indirect retro-illumination wherein the corneal endothelial pigments are visualized between the corneal and the
iris slits. (c, d) Slit lamp image of corneal endothelial pigments visualized under direct (against a well-lit background,
pink arrows) and indirect illumination (against a dark background, blue arrows), (e) Diffuse slit lamp photograph depicting temporal corneal edema. Further details are difcult to
discern. (f
folded Descemet’s membrane is visualized under direct retroillumination (against a lit background, black arrow) and indirect retro-illumination (against a dark background, red arrow)
) However, on retro- illumination off the iris, the
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e f
Fig. 14.7 (continued)
A. Kate and S. Basu
14.6 Stepwise Procedure
ofExamination
1. Setting up the slit lamp: A methodical
approach to slit lamp examination can ensure
that all structures are observed appropriately.
Firstly, it is essential to ensure the comfort of
the examiner and the patient. It is necessary to
sanitize the patient frame and handles with
70% ethanol disinfectant before each examination [13]. The patient’s chair is adjusted to
the appropriate height, and the chin rest is
adjusted so that the lateral canthi of both eyes
align with the canthal marker on the patient
frame. The eyepiece of the slit lamp is adjusted
to the examiner's interpupillary distance and
typically set to plano or corrected to accommodate their refractive error. The examiner
chair is also adjusted for comfortable sitting
with both arms on the table frame. The illumination is then switched on at the lowest setting
and gradually increased until the ocular structures are visible. The joystick is moved forward until the structures are clear and focused.
Once the area of interest is in the eld of view,
the slit lamp can be locked into place, and a
detailed examination can be carried out.
Focusing on the structures such as the cornea,
lens, and tear lm can be challenging because
of their semi-transparent nature. The use of
adjacent opaque structures can aid in this process, where the examiner rst focuses on them
and then slowly shifts both the eld and the
focus to the desired area of examination. For
example, the iris can rst be focused on using
a moderate slit beam which is then shifted and
then moved slightly posteriorly to bring the
lens into focus.
2. Ocular examination: Before utilizing the slit
lamp to examine each eye, it is crucial to
assess both eyes under natural light without
any aids. This preliminary step is essential for
evaluating symmetry, detecting potential
issues, and obtaining an overview of any
inammation or mass lesion. The slit lamp
examination of each eye typically begins at a
low magnication and diffuse illumination.
Due to the convex nature of the external surface of the eyeball, the slit lamp unit has to be
moved back and forth to bring different structures into focus. A sequential approach for
ocular examination will ensure all structures
from outwards in are examined meticulously,
as described below. Detailed examination
techniques of the different structures of the
eye have been addressed in the other chapters.
This chapter will focus on the different slit
lamp techniques and their modications
required based on the structure being
examined.
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(a) Lids: Examination of the lids should
include their apposition, lid margin
abnormalities or inammation, and lash
abnormalities. Eversion of the lids should
be routinely carried out in all cases to
detect any foreign bodies, follicles, papillae, etc. This technique can also detect
subtle cicatricial changes. Concurrent
assessment of puncta will help identify
abnormalities of the same.
(b) Tear lm, sclera, and conjunctiva: The
next structure to be examined is the tear
lm. It is assessed for the presence of any
debris, its height, and distribution. This is
a dynamic assessment that occurs with
each blink. The movement of the debris
can be used as a marker to focus on the
tear lm, typically examined under direct
illumination with a moderately wide slit
beam. The specular reection is also used
to assess the movement of the lipid layer
and to measure the height of the tear lm
meniscus. This is followed by concurrent
examination of the sclera and the conjunctiva-the bulbar and the palpebral. In
addition to any mass lesions or signs of
inammation, it is also important to
assess any abnormality in the folds, such
as excessive laxity or irregularity. The
conjunctival examination can be carried
out with a combination of an optical section and a moderately wide slit beam to
assess the prole of the surface. It is
advisable to refrain from using high light
intensities as they have the potential to
obscure ndings due to overexposure.
Proximal illumination can reveal the cystic nature of a mass lesion, while specular
reection can be used to detect surface
irregularities. The red-free lters are particularly useful while discerning scleral
inammation and should be routinely
employed to differentiate between episcleral and scleral inammation. Scleral
nodules or thinning can be best detected
using an optical section.
(c) Limbus and cornea: The next examina-
tion is of the limbal palisades and the cor-
nea. The limbus can be viewed with direct
diffuse and focal examination techniques.
The use of focal illumination can help
understand the prole of the palisades,
especially in eyes with lesser degrees of
pigmentation. Examination of the cornea
uses a combination of different examination techniques. It is important to meticulously examine every layer individually.
A lower magnication is required to
assess changes such as ectasia, corneal
thinning, or edema. Determining the relative depths of different pathologies will
require a higher magnication. The illumination angle can be increased to widen
the width of the optical section and better
understand the exact depth of scars, inltrates, or foreign bodies. It is recommended to perform the parallelopiped
examination before the optical section to
detect subtle changes which can be easily
missed because of the smaller eld of
view in the latter. Both direct and indirect
retro- illumination can be used to assess
the presence of vascularization within the
cornea. Similarly, sclerotic scatter is a
sensitive technique for evaluating irregularities within the corneal layers. The
specular reection assesses the endothelial layer and detects any guttae or dropout areas within the cells.
(d) Anterior chamber: Since a normal ante-
rior chamber is typically optically clear,
subtle changes can get missed even in
minor defocus. It is also necessary to use
a higher magnication of 16×–25×. For
the same reason, it is important to
decrease the slit beam height within the
pupillary margin to prevent obscuration
of the eld of view from the backscattering of light from the opaque iris. This
conical light beam allows the optics to
pick up the scattered light from oating
particles within the aqueous humor and
elicit Tyndall effect [14]. Reducing the
illumination of the examination room can
facilitate this process by providing higher
contrast. Grading the depth of the anterior
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