·
Minimal angle of resolution
of an eye with a 6/6 acuity: One degree of
arc
·
An eye with 6/6 visual acuity read the 6/24 lines from 24 m away.
·
Snell's law of refraction: the
incident and refracted rays and the normal to the surface at the point of
incidence lie in the same plane and the ratio of the sine of the angle of
incidence i to the sine of
the angle of refraction r is a constant for any two media. This constant is
called the relative index of refraction ie. sine i / sin r = refractive index.
·
Apex angle in degrees of a 10 dioptres prism made
of glass: 10 degrees
·
The nodal point of a thin lens is at the
intersection between the principle axis and the principle plane.
·
The
refracting power of a cylindrical lens is at 90 degrees to the axis.
·
The image
produced by a negative lens is virtual, erect & diminished.
·
A focimeter
measures the BACK VERTEX POWER of a lens.
·
A decentration of 10 mm from the optical centre of a +5 dioptre lens produces a prismatic effect of 5 prism dioptres.
·
A X8 loupe has
an equivalent power of 32 dioptres.
·
A lens of +10
dipotres fully correct an hyperopia
and now the lens is moved forward 10mm, what is the new lens power needed to
correct the hyperopia: +9D
Dn = Do/1-dDo
where Dn = the new power
Do = the old lens power
d = difference in the location (in meters), -ve
if it is moved forward and +ve if it is moved
backward.
·
In trifocals
the intermediate lens usually has half power over the distance correction.
·
The LTF
stands for LUMINANCE TRANSMISSION when considering tints.
·
Relative spectacle magnification = Actual size corrected with spectacle / size seen by the emmetropic eye
·
Visible light
contains wavelengths between 400 and 780nm. Ultraviolet A has wavelength of
315-400nm whereas ultraviolet C 200- 280nm. Thus
in order of increasing wavelengths: ultraviolet C, ultraviolet B,
ultraviolet A; visible light; infrared A, infrared B and infrared C. The lens
is very efficient at absorbing ultraviolet than infrared light. The thermal
burn in eclipse burn is caused by infrared light.
·
Red-green defect is seen in acquired optic nerve disease, cone dystrophy and Stargardt's
disease.
Blue-yellow defects are seen in most retinal dystrophy,
glaucoma and autosomal
dominant optic neuropathy.
·
Farnsworth-Munsell hue 100 test contains 84 colour
discs to be arranged in order of closest colour match
with the reference colours at each end. The colours only differ in hue with same brightness and
saturation. Ishihara test plates are
mainly for congenital red-green colour defect. Wavy
lines are used in Ishihara plates for illiterates or children. Lanthony New Colour Test can
be used in children.
·
Illuminance and irradiance are terms used to refer to the amount of light arriving at a given point. Radiance refers to the amount of light leaving
a certain point.
·
The refractive index of a human lens increases with age due to the
development of cataract. In
addition, the human lens does not have uniform refractive index being higher in
the nucleus (1.400) than the cortex (1.380) in a non-cataractous
lens.
·
The image formed by a prism is erect, virtual and deviated towards the
apex.
·
Fresnel prisms reduce the
weight of conventional prism and used widely in treating patients with strabismus. They are made up of a series of small
prisms. The most common type are made up of polyvinyl
chloride. They reduce the visual acuity mainly through chromatic aberrations.
They are usually applied to the the back of patients'
glasses.
·
Prentice rule: Prism dioptre = distance from the optic centre (cm) X dioptre power of the lens.
The induced prismatic effect is 1D.
·
The vergence power of a lens is affected by the vergence
power of each surface, thickness of the lens and the medium on either side of
the lens. The later explain why cornea has a
stronger refractive power than lens in human eye although its refractive index
is less. The wavelength of the light also affects the vergence power of the lens.
·
The first focal length has the same length as the second focal length only
if the media on either side of the lens are the same. The first
focal length of a convex lens is to the left of the lens whereas that of the
concave lens is to the right of the lens.
By convention, the second focal length has a positive sign for convex lens
and a negative sign for concave lens. As lenses are designated by their second
focal lengths, a convex lens is also called a plus lens and a concave lens a
negative lens.
·
The magnifying power is calculated to be M/4 where
M is the power of the lens in dioptres.
·
The increasing prismatic effect of the more peripheral parts of a spherical
lens is responsible for:
· spherical aberration
· ring scotoma
· jack-in-the-box effect
· image distortion so that a thick plus lens gives a pin-cushion
effect and a thick minus lens
gives a barrel effect.
·
A cylindrical lens has one plane surface and the
other with curve surface. It has no power along its axis. Its power is 90
degrees to the axis and the lens forms a focal line parallel to its axis. It is
used in Maddox rod for the measurement of phoria.
Maddox rod is used to measure distant phoria
and Maddox wing for near phoria. Double
Maddox rod can be used to measure cyclotorsion.
·
The circle of least confusion of the conoid
of Sturm is located at the focal point of the spherical equivalent
lens.
·
Duochrome test is a
subjective test using the principle of chromatic aberration. It uses letters or
numbers of the same colour usually black against
different backgrounds: red and green. It is sensitive changes in refraction of
0.25D or less. To an emmetropic
patient, letters of both the colours look equally
sharp; while to a slightly myopic patient the red letters appear sharper and to
a slightly hypermetropic patient the green letters
look sharper.
·
High-order aberrations are ones that cannot be corrected by simple spherocylindrical systems, such as spectacles or contact
lenses. They are caused by minute
misalignments of the eye's optical components and include, in order of visual
significance, spherical aberration, coma, higher-order astigmatism, and others.
They can be detected with wavefront analysis
performed with an instrument called aberometer.
Theoretically, an ablation that removes aberrations increases visual contrast
and the spatial detail of images seen by the eye.
·
Purkinje-Sanson images: The images are formed at 4 surfaces: the
anterior (image 1) and posterior (image 2) corneal surfaces and the anterior
(image 3) and posterior (image 4) lenticular
surfaces. The first three images are erect and virtual whereas the last one is
inverted and real. The first image is used
for keratometry and images 3 and 4 are used for
accommodation.
Hirschberg's test made use of the first captoptric image which is
located on the anterior corneal surface for measuring ocular deviation. This
image is also used for keratometry.
·
An image
known as Sturm's conoid
is produced by an astigmatic eye; such image can only be corrected with a spherocylindrical lens.
·
Unilateral nucleosclerosis and not posterior subcapsular cataract can lead to index myopia. Central
serious retinopathy can cause hypermetropic shift.
·
SRK formula is used for IOL
calculation. However, it is not accurate for eye less than 22mm or longer than
24.5mm. SRK II and SRK-T are more accurate than SRK. The SRK II uses adjusted A
constant depending on the length of the eye. For eye with an axial length
shorter than 21mm, Hoffer Q is preferable.
·
The Franklin design is a split bifocals in which a
distance lens is mounted on a near lens. The problem of prismatic jump is
related to the power of the lenses as well as the distance between the distance of the interface from its optical centre. Image
jump can be reduced by moving the optical centres
towards the junction of the two portions as in the executive glasses.
Alternatively, a base up prism can be incorporated into the reading section.
Varifocal glasses has three sections: distance, intermediate (for VDU or working at arm length)
and near (for reading). Varifocal or progressive
lenses have no visible interface between the distance and near portions unlike
bifocal or trifocal lenses. There is a power progression corridor (intermediate
portion) which reduces the image jump seen in bifocal glasses; however on
either side of the corridor aberration or astigmatism induced can become
intolerable.
·
Soft contact lens with toric surface can be used to correct higher astigmatism.
Piggyback contact lens involves the use of two lenses, the soft contact
lens provides the fitting surface for the rigid gas permeable lens.
·
Because of
the prismatic effect of glasses with reading (base in in
myopes and base out in hypermetropes),
the myopes need less convergence and accommodation than when using
the contact lenses, the converse is true for hypermetropes.
Because of the additional accommodation and convergence required, contact lenses can cause eyestrain in presbyopic
at an earlier age.
·
Corneal warpage refers to change in the corneal curvature
associated with contact lens wear. Corneal oedema
is absent. It is commoner with rigid gas permeable contact lens than soft contact
lens. The result is reversible.
·
Giant papillary conjunctivitis is more common with soft than
RGP lens. It is thought to be caused by deposits of denatured proteins. Ptosis is more
common with RGP lens. It is thought
to be related to the way the RGP lens is removed ie. temporal stretching of the lids combined with forceful lid
closure.
·
Diabetic maculopathy is better detected with
indirect ophthalmoscope than direct one.
·
The slit-lamp can be used to detect the presence of macular hole using the Watzke's
sign.
·
The Hruby lens is a powerful plano-concave lens which gives a
virtual, erect and diminished image. It is difficult to master and the image is
too small for reliable photocoagulation. Coupling solution is not required as
the lens does not come into contact with the eye. It is held with the concave
side towards the patient. The image is formed within the eye.
·
The International Safety Classification of Lasers
divides the lasers into 4 groups. Group 3 is subdivided into 3a and 3b. Class
3b and above is damaging to the eye and their powers are 5MW and above. All lasers used in ophthalmology are classed as 3b and
above. Safety goggles should always be worn by people in the vicinity.
·
Karl Himly (1806) was the first to employ the technique
of oblique illumination examination.
·
Gullstrand
invented the slit-lamp.
·
Babbage in
1848 invented the direct ophthalmoscope.
·
von Helmholtz in 1850 reinvented and popularised the ophthalmoscope.
·
Nagel in
1864 invented the indirect ophthalmoscopy.
·
·
Two types of keratometers used in practice are Javal-Schiotz model and Bausch & Lomb model.
·
LASIK is laser-assisted
in-situ keratomileusis. It is performed using ALK (automated lamellar keratectomy) machine and
the excimer laser. This procedure is good for myopia
of more than – 8D.
·
types of retinoscopes:
1. Mirror
retinoscopes, which may consist of a simple plane
mirror or a combination of a plane mirror (on one end) and a concave mirror (on
the other end). e.g., Pristley-Smith’s mirror.
2. Self-illuminated
streak retinoscope.
·
uses of red and
green glasses or filters:
1.
Diplopia charting
2.
Worth’s four-dot test
3.
Malingering test
While testing, the red glass is
kept in front of the right eye and the green glass is kept in front of the left
eye.
·
Crown glass with refractive index 1.5223 is most commonly
used for making spectacles.
·
+20 D is
the power of the convex lens most commonly used in
indirect ophthalmoscopy
