11.

APPLANATION TONOMETER
BY RENAT0

Until the introduction of the Maklakov tonometer in the late nineteenth century,
intraocular pressure was estimated by digital palpation of the globe.
Over the past century, many additional tonometers have been developed. With the
development of the Schiotz tonometer in 1905, tonometry became relatively accurate
and widely available. Today, a variety of mechanical and electronic tonometric devices
are available, but all exploit one of two principles to measure the intraocular pressure,
indentation, or applanation. Indentation tonometry measures the amount of deformation
or indentation of the front surface of the globe produced by a fixed amount of force.
Applanation tonometers apply a variable amount of force to the corneal surface, to
produce a fixed amount of deformation or flattening.
The Goldmann-type applanation tonometers are the current standard for
tonometry throughout the world.
As previously mentioned, applanation tonometry uses a variable amount of force
to produce a fixed amount of flattening of the corneal surface. Applanation tonometry is
based on the Imbert-Fick principle, which states that the external force (F) exerted to
a sphere, equals the pressure inside this sphere (P) times the area (A) which is
flattened or "applanated" by the external force (F =P × A). This principle is valid
only for an ideal, dry, thin-walled sphere.
The Imbert-Fick principle has been modified for use in applanation tonometry to
account for the corneal thickness and the moisture on the surface of the cornea. The
tips on all Goldmann-type tonometers are standardized to applanate a 3.06 mm diameter
of the cornea, which standardizes the area of applanation. This standardized area of
applanation has been calculated so that resistance of flattening of the cornea created by
the thickness of the cornea is essentially negated by the capillary attraction of the tear
film between the tonometer tip and the cornea.
To measure the intraocular pressure with a Goldmann tonometer, a topical
anesthetic and fluorescein are instilled in the tear film to enhance visualization of
contact between the tonometer and the corneal surface. A cobalt blue light on the slit

1
lamp is used to illuminate the fluorescein and the biprism of the applanator. The
applanation tip is brought into gentle contact with the front surface of the cornea. The
tear meniscus at the margin between the applanation tip and the cornea is visualized
through the slit lamp.
The tip of the applanation device contains two beam-splitting prisms which
optically convert the circular point of contact between the tonometer and the cornea into
two semicircles. A variable amount of force can be applied to the tonometer tip to create
the desired amount of flattening of the corneal surface. The force of applanation is
adjusted so that the inner edge of the semicircles meet and the intraocular pressure is read
from a scale on the applanation instrument. By adjusting the overlap of the semicircles,
the desired area of applanated cornea is achieved. The scale on the tonometer is in grams
of force and must be multiplied by 10 to convert to millimeters of mercury.

Applanation tonometry displaces much less aqueous humor from the anterior
chamber than indentation tonometry and is largely unaffected by variations of
scleral rigidity. However, like indentation tonometry, applanation tonometry also has
sources of potential error. Excessive or inadequate amounts of sodium fluorescein in the

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tear film cause inaccurate tonometric measurements. Incorrect alignment of the
semicircles and corneal scarring or edema may result in inaccurate measurements.
Regular (especially greater than four diopters) or irregular corneal astigmatism may lead
to inaccurate readings with applanation tonometry. With regular astigmatism, errors
can be minimized by rotating the dividing line of the biprisms to 45° from the major
axis of the astigmatism.

OTHER TYPES OF TONOMETRY

Other types of applanation tonometers include the Perkins and Draeger
tonometers. These are handheld instruments which use the same principles and
techniques as conventional Goldmann tonometry and are often useful for
examinations in the operative suite or for individuals who cannot sit at a slit lamp.
Another applanation tonometer, the MacKay-Marg, applanates a smaller area
of the cornea and may be useful in eyes with extensive corneal scarring.
A recently developed tonometer, the Tonopen, incorporates a small, strain
gauge in the tip which touches the cornea and calculates the intraocular pressure
electronically. This computer-based, pen-shaped tonometer collects and averages
multiple readings which are presented on a digital display.
Another tomometer, the pneumotonometer uses an air pressurized tip to sense
the force required to distort the surface of the cornea.
Because of the small size of the pneumotonometer tip, it is also useful in eyes
with corneal scarring.
The pneumotonometer, MacKay-Marg, and Tonopen accurately measure the
intraocular pressure in patients wearing soft contact lenses. Goldmann applanation
tonometry is not useful in this setting because contact between the applanation device and
the contact lens distorts the readings.

Applanation tometer Prasad

Tonometers are instruments used to measure intraocular pressure [IOP] and consists of
two main types: Applanation and indentation Tonometers. Appalanation tonometer
flattens a portion of the cornea during the estimation of IOP and rely on the Imbert-Fick
principle for their accuracy. This law states that the force required to flatten a spherical

3
surface is equal to the pressure within the sphere multiplied by the area of flattening. For
a non-perfect spherical surface such as the eye, forces related to the surface tension of the
tear film and corneal elasticity have to be considered. When the diameter of the portion of
the cornea that is flattened is 3.06 m [the size of the double prism used], the two forces
are equal and opposite magnitude enabling IOP to be accurately measured. Although
aqueous is displaced during tonometry, the volume is so small [= 500 nanoliters] that the
rise induced in IOP during Goldmann appalanation tonometry is negligible, making
accurate pressure readings possible. The Perkins tonometer in immobile patients but
requires a degree of skill to obtain accurate measurements.
The Schiotz tomometer an example of an indentation consists of a foot-plate, plunger and
handle to which additional weights can be added. With the patient supine, the footplate is
placed on the anaesthetized cornea before the IOP is read off a scale. Since significant
amounts of aqueous are displaced, it tends to less accurate than Applanation Tonometers.
The Schitoz tonometer also tends to overestimate the IOP in eyes with abnormal scleral
rigidity [myopia, previous scleral-buckling, etc.]. This can be confirmed by
demonstrating a discrepancy in IOP readings when different weights are applied to the
plunger. Shitoz tonometer can be difficult to perform, especially in apprehensive patients,
and is now rarely employed in develop countries.
Non-contact Tonometers measure the time taken for the cornea to deform when a puff of
air is directed at it. It has become popular amongst high street opticians because it is easy
to use, caries no risk of cross-infection and avoids the use of a topical agents. The
measurements correlate reasonably with Goldmann tonometry, although it tends to over-
estimate higher pressures.
 The Goldmann Applanation tometer uses a spilt-field plastic prism arrangement to indicate
precisely when a corneal area 3.06-mm is diameter has been applanated.

 The Applanation tometer consists of head mounted on a lever, the tension of which can be
adjusted with knob. The head contains two prisms, with the apex and base in the opposite
direction.

 Underestimates occur in with-the- rule astigmatism and overestimates in against-the-rule.

 Errors can occur if the semicircular bands are not equal size.

 In Applanation Goldmann tonometer 1-g weight of force is then equal to the pressure of 10
mmHg.

 The prism p1 should be visualized as lying above the plane of the diagram and prism p2 below it.
The adjacent plane edges being in contact.

 Fluorescein stain is used so that the prismatic ring of tear liquid surrounding the applanated area
becomes luminous under blue light.
Goldmann tonometer is conveniently used in conjunction with a slit lamp.

12. SPECULAR MICROSCOPE

BY RENATO

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 Specular reflection is a technique which sets the slit beam at 60 to 80 degrees
from the viewing arm with a narrow slit.
The angle of observation is equal to the angle of illumination and this allows
structures on the front or back surface of the eye to be assessed.
Specular microscopes that applanate the cornea have a pachometer attached to the
focusing apparatus so that the corneal thickness can be measured simultaneously.
A combination of these slit lamp techniques allows observation of lesions in the cornea
and accentuates the abnormalities. The slit can be used to approximate corneal thickness,
anterior chamber depth, and a short beam is used to detect anterior chamber reaction.
Specular reflection outlines the endothelial cells. Direct, slit, and retroillumination
technique can identify abnormalities of the iris or lens. The anterior vitreous can be
viewed with a slit lamp biomicroscope, but special lenses are needed to view the
posterior vitreous and retina.
Specular photomicroscopy uses the principal of specular reflection to permit visualization
and photography of the corneal endothelial mosaic on the back of the cornea. Initial
specular microscopic techniques involved the use of a photomicroscope attached to an
applanating cone. Wide-field specular photomicroscopes are now available and regional
corneal anatomy can be compared. The cornea has to be reasonably clear to allow
visualization of the specular reflex. The normal corneal endothelial cell density
decreases with age but is normally approximately 2400 cells per square millimeter
in middle and older age. The coefficient of variation in mean cell area is defined as the
mean cell area divided by the standard deviation of the mean cell area; an increased
coefficient of variation is a sensitive indicator of endothelial instability. Pleomorphism is
a measure of the increased variability in cell shape, usually recorded as the percentage of
cells deviating from the normal hexagonal shape. Hexagonal cell percentage greater than
50% generally reflects a healthy endothelium. Polymegathism is a variation in cell size
and has been correlated with endothelial dysfunction and with the risk of developing
future corneal decompensation. Endothelial cell counts and endothelial cell
morphology reflect endothelial physiologic function, although not necessarily in a
linear relation.

5
 The main value of the technique is in screening patients with suspected
endothelial disease or in following patients with serial progressive changes in the
endothelium as a result of some insults such as increased intraocular pressure, uveitis,
intraocular lenses or other form of surgery, contact lenses, or monitoring the effects of
pharmaceutical agents.

SPECULAR MICROSCOPY by Asok

The term specular microscopy refers specifically to the method of examining endothelial
cells utilizing specular reflection from the interface between the endothelial cells and the
acqueous humour. The technique can be performed either contact or non-contact. In both
cases the instruments are designed to separate the illumination and viewing light paths, so
that reflection from the anterior corneal surface does not obscure the very weak reflection
from the endothelial cell surface.

Endothelial cells can also be seen using slit-lamp when the illumination axis and viewing
axis are symmetrically displaced on either side of the normal to the cornea. A narrow
illumination slit must be used; hence the field of view is narrow.

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Photographic recording has been made possible by adding a long-working distance
microscope system on the viewing axis, and a flash capability to the illumination system.
Pt eye motion is the chief problem with this technique.

In contact specular microscopy, the illumination and viewing paths are through opposite
halves of a special microscope objective, the front element of which contacts the cornea.
This tends to reduce lateral motion of the pts eye, and it effectively eliminates the
longitudinal motion that produces defocus. Higher magnifications are possible by this
technique than with the slit lamp, so that cellular detail and endothelial abnormalities are
more readily examined.

Wide field specular microscopes employ special techniques to assure that the reflection
from the corneal contact element interface does not overlap the image of the endothelial
cell layer. Since scattered light from edema in the epithelium and stroma can also
degrade the endothelial cell image, variable slit widths are sometimes provided to
minimize this problem.

Analysis of specular micrographs may consists simply of assessment of cell appearance

together with notation of abnormalities such as guttata or keratic precipitates.
Cell counts are done by superimposing a transparent grid of known dimensions on the
endothelial cell image and simply counting cells in a known area.
The normal cell density in young individuals exceeds 3000 cell/mm2.
The average density in the cataract age group is 2,250 cell/mm2 which suggest a gradual
decrease with age.

Specular microscopy has been important in studying the morphology of the endothelium
and in quantifying the damage to the endothelium produced by various surgical
procedures and even certain types of IOL. This in turn has led to refined surgical
procedures and new IOL designs.

Specular microscope by Prasad

Corneal specular microscopy is a technique for examining corneal structures at high
magnification and is used mainly for viewing the corneal endothelium. It may be
performed either with a contact or a non-contact instrument. The former offers higher
magnification and resolution but may be difficult to see with apprehensive or
postoperative patients.
The optics of specular microscopy are based upon the laws of reflection. Light incident
upon a surface of different or refractive index [epithelium-air interface or endothelium-
aqueous interface] may be reflected, transmitted. Similarly light passing through the
various corneal constituents may be reflected, transmitted or absorbed. In practice only a
small amount of light is reflected, and this may either be specular [where the angle of
incidence is equal to the angle of reflection] or diffuse [where the angles differ]. During
clinical specular microscopy, a beam of light is projected onto the cornea and an image
formed by collecting the reflected light with an objective lens. This light consists of rays
reflected from both the corneal epithelium and endothelium as well as scattering from the

7
corneal stroma. The image contains several zones, which correspond to different regions
of the cornea. Definition of the endothelial cell layer is improved by use of a narrow
beam of light that reduces interference from stromal light scattering. Similarly,
interference from epithelial reflections can be minimized by increasing the angle of
incidence of the illumination beam, resulting in the interfering reflection moving to one
side. Increasing the angle of incidence also permits use of a wider beam, which increases
the area of the endothelium in view. Most non-contact microscopes employ these features
with a wide beam, a large angle of incidence, and an objective with a small depth of field
to ensure that only one layer is sharply focused at a time.
 Photographic recording has been made possible by adding a long-working distance microscope
system on the viewing axis and a flash capability to the illumination system.

 Wide-field specular microscopes employ special techniques to assure that the reflection from the
corneal contact element interface doesn’t overlap the image of the endothelial cell layer.

 Analysis of specular micrographs may consist simply of assessment of cell appearance together
with notation of abnormalities such as gutta or keratic precipitates.
Specular microscopy has been important in studying the morphology of the endothelium and in quantifying
the damage to the endothelium produced by various surgical procedures and even certain types of
intraocular lenses.

13. OPERATING MICROSCOPE

BY RENATO

The operating microscope forces the surgeon to assume a particular posture that often
must be maintained for several hours, and this immobility sometimes hinders certain
manipulations. The surgeon must understand this immobility and learn to work around it.
The visual field is restricted, as is the space available for manipulation between the
microscope and the operative field. The surgeon must become familiar with the space
limitation and learn to manipulate instruments while visualizing them only through the
operating microscope objectives. Mastering the principles and basic techniques of ocular
microsurgery is gratifying. The resulting surgical precision is elegant in both its technical
and visual components.
The operating microscope consists of the following elements: oculars, beam splitter,
magnification system, and objective (Fig. 1).1 The microscope may be mounted to the
ceiling or on a mobile floor stand. Familiarity with the microscope is essential. To
thoroughly understand the alignment of the movable parts of the microscope, the surgeon
must take the time to disassemble and reassemble them. In addition, the beginning

8
surgeon must become familiar with every rheostat and bulb on the operating microscope
to compensate if an unexplained change in the illumination occurs during the surgical
procedure. Both focus and magnification should be adjustable with a remote foot control
(Fig. 2). The optical axis of the microscope on the operating field also may be adjusted
with a foot pedal. When the same operating theater is used repeatedly, a ceiling-mounted
microscope requires little preparation on the part of the operator and permits rapid
exchange of patients between operations under antiseptic conditions. It is beyond the
scope of this chapter to describe the various types of microscopes available; however,
surgeons selecting an operating microscope should be familiar with at least the major
principles of function.
Fig. 1. The operating microscope has four essential elements:the oculars (a), the
beam splitter (b), the magnification system (c), and the objective (d).
Fig. 2. View of the remote foot control. The focus (a), magnification (b), and
optical axis of the microscope (c) may be adjusted. The joy stick is used to adjust the
optical axis of the microscope. The operating microscope lights and room lights also may
be adjusted with the use of the foot pedal (d).

OPERATING MICROSCOPE by Asok

The basic operating microscope is composed of an ordinary pair of prism

binoculars looking at the pts eye through an additional single objective lens.
Interposed between the prism binoculars and the objective lens is a wheel of
selectable Galilean or astronomic telescopes comprising a " magnification
changer"

The working distance of the microscope ( the distance from the objective lens to
the pts eye) is equal to the focal length of the objective lens. Commonly used
objective focal lengths in opthalmic surgery are 150, 175 and 200 mm.

The total magnification of the operating microscope is equal to the product of the
magnifications of the various components.
Various illumination systems are available, but most important for opthalmic
surgery is " coaxial" illumination especially for visualization of the posterior
capsule and for vitreous surgery.

Operating microscopes are designed to be parfocal ( remain in focus) with

change of magnification, but the microscope must be focused properly to begin
with if this feature is to be useful. Up and down focusing of the microscope itself

9
should first be performed under highest magnification. This accurately places the
object viewed in the focal plane of the objective lens. Then without changing the
up and down focus of the microscope, the microscope should be changed to
lowest power, and each eyepiece should be focused in turn by screwing outward
to fog, and turning inward until best focus is just attained.

Zoom lens principle by Prasad

A zoom lens consists of a series of movable lenses which allow magnification to be
varied whilst the object in view remains in focus. The most basic zoom lens has two
convex lenses with a movable concave lens between them. The system length of the first
convex lens, defined as the sum of the object and image distances, varies from positive to
negative infinity in a parabolic fashion if all object positions are considered. The smallest
system length, four times the focal length, occurs when the object is placed two focal
lengths in front of the lens. Since magnification equals the ratio of image and object
distances, changing the system length will change magnification. Large changes in image
position would occur as the lens position and this can be reduced by introducing two
additional lenses or elements. The middle concave lens moves from front to back, thereby
increasing the level of magnification, and such as a zoom system where the image
positions still varies called 'uncompensated'.
‘Compensated’ zoom systems overcome image shift, either mechanically or optically. In
mechanical systems, the rear element moves in a non-linear fashion to neutralize the
effect of the moving middle lens while the final element remains fixed. The optical form
introduces further movable or active elements, which make image shift arbitrarily small.
Such a system is more expensive but has the advantage that aberrations are reduced. In
essence, there is no difference in principle between the mechanically and optically
compensated zoom systems, only in the number of active elements. As they have to cope
with changes in object position, photographic cameras incorporate further focusing
mechanisms, however in ophthalmic instruments this is usually not a problem since the
position of the object is usually fixed.
Since the moving elements of a zoom lens can be easily mechanized, when they are used
in the slit lamp or operating microscope they have clear advantage over their fixed focal
length counterparts.

CORNEAL PACHYMETER
BY RENATO

In order to understand the optical principles of pachymetry, some of the optical

behavior of a plano plate of glass of a given thickness must be understood. We as
student often feels that such a device is an optical neuter, neither adding to, subtracting

10
from, nor modifying the image in any measurable way. This is not so, for introducing a
plano plate of glass is capable of both changing the focal distance and producing a
lateral shift of rays of light passing through it.
To understand this phenomenon, one must remember that an oblique ray
of light striking the plano surface of a material with a higher index of refraction is
bent toward the normal of the surface. This has the effect of making the light beam
pass through the plate of glass by a more direct route. When it exits the glass, it is
rebent to parallel its original course. If converging rays of a pencil of light are
considered, each of the rays converging toward a point on the opposite side of the pencil
are bent slightly outward at the air-glass interface, continue at the new course through the
thickness of the glass, and are again bent inward at the glass-air interface. The effective
focal length of such a system and the size of the image are unchanged by the insertion of
the plate, but the physical focal length of the system is lengthened. A minor lengthening
depends only on the thickness of the plate and the difference of the index of refraction of
the glass and that of air. Following the same line of logic, it may be appreciated that the
lateral displacement of a ray striking a plate of glass obliquely is dependent only on
its thickness, its index of refraction, and the angle of incidence. Although an exact
mathematical formulation is somewhat tedious, it is approximately true that this
displacement is equal to d = (N-1 ) T q/N. For a given glass plate with a fixed index of
refraction and thickness, this displacement is directly proportional to the angle of
incidence, which could be varied by rotating the glass plate.
These two principles find application in a modern pachymeter such as is
illustrated in Figure below. An aperture is placed before the illuminating beam in order
to increase the depth of focus of the slit, and the beam is positioned so that there is a
40° angle between the illumination and observation paths. The instrument is placed
so that the slit beam perpendicularly strikes the center of the cornea. Two piano
glass plates, one of which is fixed and the other movable, are mounted, one above the
other, in front of the observation beam. The movable plate can be rotated from side to
side and serves to displace the image laterally. The fixed plate is of a different thickness
and compensates for differences in focus of different parts of the eye. A scale connected
to the rotary plate permits the amount of image displacement to be determined, since for

11
small angles there is a linear relationship (between plate rotation and image
displacement). When an image-dividing eyepiece is used and the device is set up to
measure corneal thickness, a view such as is seen in the Figure is observed through the
eyepiece. Some distortion of the apparent thickness occurs, due to the refractive power of
the cornea itself. The figures obtained from the pachymeter may be corrected if extremely
accurate work is required; however, for most purposes, the uncorrected readings are
adequate. Other pachymeters have been described but are not offered commercially.

The slit
projector (S) projects its beam through an aperture (A) along the optic axis. The aperture
serves to increase depth of field. The eye (E) is viewed by the biomicroscope (M) half
through a glass plate (O) rotated through an angle 0 and half through an unrotated plate
(not shown). The beam path through the plate is displaced laterally a distance d. In
anterior chamber depth measurements, this results in the view illustrated. Note that the
corneal thickness must be subtracted..

PHACOMETER by Asok

The thickness of the cornea and the depth of the anterior chamber may be measured by means of a
pachometer

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The pachometer uses the Purkinje-Sanson images formed by the anterior and posterior corneal surfaces
( image I and II) to measure corneal thickness and the Purkinje-Sanson images formed by the posterior
corneal surface and anterior lens surface ( images II and III) to measure anterior chamber depth.

The pachometers in current use depend on the principle of image doubling. The doubled image is aligned
by the operator so that the surfaces in question ( anterior and posterior corneal surfaces, or posterior corneal
and anterior lens surface) coincide. The corneal thickness or anterior chamber depth can then be directly
read off a scale.
The image doubling can be achieved either by splitting the incident beam of light, or by means of a
specially adapted eye-piece which splits the observer's view of the eye

A perspex plate covered by coloured celluloid and having a cut-out area is placed in the slit lamp beam.
The beam is thus split, some light proceeding undeviated via the cut out zone and some being laterally
deviated by passage through the perspex plate. The images formed by the 2 beams at the surfaces of the
cornea are viewed through the slit lamp and the plate rotated until the images in question are superimposed.

The pachometer in current use with the Haag Streit 900 slit lamp. Image doubling is achieved by a specially
adapted eye piece which is substituted for the normal slit lamp eye piece. Measurement is made by rotating
a transparent plate so that the 2 images are aligned in such a way that the surfaces in question, eg anterior
and posterior corneal surfaces are juxtaposed.

Corneal Pachymeter by Prasad

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The pachymeter is a device used primarily to measure corneal thickness. A variety of
instruments are available that rely on optical doubling, optical focusing or
Ultrasonography.
Optical doubling.
An attachment for the slit lamp gives a split image of the corneal section. When the
amount of doubling is adjusted so that the anterior surface in one-image lines up with the
posterior surface in the other image, the corneal thickness is read directly from a dial.
Optical focusing.
Certain specular microscopes are calibrated so that when the endothelium is in focus, the
thickness of the cornea is automatically displayed. The zero is established by focusing on
the interface between the contact element and the epithelial layer.
Ultrasonogrphy.
Ultrasonography techniques have been refined to the point that they can used to measure
corneal thickness. The technique is similar to that used for axial length measurement of
the globe. A modified B-scan instrument.
Strictly speaking, an accurate measurement of corneal thickness depends on the precise
knowledge of ultrasound velocity in the cornea [Ultrasonography] and velocity of light or
index of refraction [in optical doubling and optical focusing]. This becomes important
when comparison is made of results by different techniques, less important when
measurements are always made by the same method.
The Maurice-Giardine instrument consists of a rectangular Perspex plate with a central
aperture that acts as a beam splitter. Part of the slit lamp beam is refracted by the plate
while the rest passes through the aperture unhindered. The plate is rotated in measured
manner until the reflections from the anterior [catoptric image 1] and posterior [catoptric
image 2] corneal surface are superimposed. The thickness of the cornea is then read from
a scale. Some distortion of the apparent thickness occurs due to the refracting properties
of the cornea but in practice these are not significant. Anterior chamber depth is
measured in a similar manner using the reflex from the anterior part lens capsule
[catoptric image 3].
 
The Haag-Streit pachymeter consists two plano glass plates mounted infront of the
observation beam, a narrow aperture in front of the illuminating beam and an image-
doubling eyepiece. The fixed plate serves to compensate for difference in focus between
different parts of the eye whilst the moveable plate is used to displace the image laterally.
The position of the images viewed thorough the image-doubling eyepiece are aligned by
rotating the moveable plate, enabling the corneal thickness to be read from a scale.
 From images 1 and 2 the corneal thickness, and from images of 2 and 3 the anterior chamber depth
can be measured.

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