Optician’s Guide (A Mannual for Opticians) Ajay Kumar Bhootra
Page number followed by f refer to figure
Accommodation 88
Aspheric lens 77
Binocular single vision 102
Cataract 88
Choroid 85
Ciliary body 85
Compliance 77
Contact lenses
advantages 79
gas permeable lens 81
hard lens 81
instruments for contact lens verification 83
manufacturing techniques 82
materials 81
patient selection 82
oxygen permeability 80
sagittal depth 81
thickness 81
water content 80
Curvature of lenses 51
Diopter 6
Dispensing spectacles to children 74
lens tips for children 76
problems associated with dispensing spectacles to kids 75
proper frame selection 74
pupillary distance measurement 75
solving problems while fitting spectacles to children 76
structure of human eye 84
Frames 42
High index lens 76
Laws of reflection 3f
Lens grinding 55
bifocal 60
cylindrical 58
Lens power to meridian values 15
Lenses 5
classification 5f
concave 6f
convex 6f
cylindrical 8
spherical 7
Lensometer 14
Light 1
reflection of 2
refraction of 3
velocity of 4
Mechanism of vision 86
Meridian values to lens power 17
Meridians 10
Near vision notation 103
Ophthalmic lens materials
crown glass 10
flint glass 10
high-index glass 11
plastic lens 11
polycarbonate 11
Optical center 7
Optical medium 2
PENTAX OLH-1 lensometer 14
Polycarbonate lenses 77
Presbyopia 88
Prescription 23
Prism 12
application of prism in optics 13
detection of prism
in a lens using lensometer 14f
in an optical lens 14
prism and lens decentration 13
Progressive addition lens
designs 32
fitting 52
Quality control 70
Soft lens materials 81
Special types of lenses 25
access lens 30f
antireflection coated 28
aspheric 26
blended invisible bifocal 30
fused bifocal 29
lenticular 28
photochromatic 25
polaroid 27
progressive addition lens 31
tinted lenses 33
Spectacle frame fittings 44
base of the bridge 45
front fitting 47
length of the sides 46
other measurements 49
temple width 48
Structure of the human eye
coats of the eyeball 84
refractive media of the eyeball 86
Transposition 15
rules 18
Troubleshooting 90
Some useful terms
aberration 98
adaptation 98
amblyopia 98
anisometropia 98
aphakia 98
atrophy 98
binocular single vision 102
blink 99
bubble 99
caliper 99
Canada balsam 99
color blindness 102
cycloplegia 99
datum line 99
decentration 99
diplopia 99
dispersion 100
distortion 100
dominant eye 100
dry eyes 100
floaters 100
haloes 100
lap 101
night blindness 100
parallax 101
proptosis 101
ptosis 101
scotoma 101
UV rays 100
visual acuity 101
Verification of
progressive lens 68
spectacles before the patient's visit 62
Geneva lens measure 63
lensometer 63
neutralization 63
Vertex distance calculation chart for contact lens fitting 107
Visual acuity scales for distance vision 103
Chapter Notes

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LightChapter 1

Light is a form of radiant energy, arising out of a source and always travels in a straight line. A straight line represents the path of a ray of light. A few such rays taken together is called the beam of light. It is not possible to produce a single ray of light. However the small source may be, it always produces a beam of light, consisting of several rays.
A beam of light may be of three different types (Figs 1.1 to 1.3):
  1. Parallel
  2. Divergent
  3. Convergent.
In a parallel beam of light the rays are parallel to one another (Fig. 1.1). Rays of light coming from a very distant source, are regarded as parallel. When the rays of light are produced from a point and are spread out is called the divergent rays (Fig. 1.3) and when it travels towards a point is called convergent rays (Fig. 1.2).
When the light falls upon the retina, it stimulates certain highly specialized cells, which produces nerve impulses.
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Figure 1.1: Parallel rays
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Figure 1.2: Convergent rays
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Figure 1.3: Divergent rays
These produce perception of light as color, form, and size. It has been found that light travels 1, 86, 000 miles per second in the air and takes about 8.3 minutes to reach the earth from the sun.
A substance or any portion of space through which light can pass is called optical medium. All the objects or substances around us are either self-luminous or nonluminous. A body is called self-luminous when it emits light by itself. It may be used as a source of light, such as sun, burning candles, etc. Bodies which are not selfluminous are not visible unless light falls upon them and is scattered in all directions. Each point then behaves like a luminous source. Most of the objects around us are nonluminous, such as table, chair, etc.
Nonluminous bodies may be transparent, translucent, and opaque. A transparent body is one which transmits the light according to the straight line law whereas a body is opaque when it does not transmit the light at all. There are substances which transmit the light but not in straight line. They are called translucent bodies. Light in passing through them is scattered. Translucent bodies may be used as screen on which optical images are projected.
When the rays of light passes from one medium to another, some of the light is reflected or bent back to the original medium. The phenomenon is known as reflection of light.
If the ray of light falls on a very smooth and polished surface, all the reflected rays will travel in a definite direction, keeping a relation with the incident rays like in the Figure. 1.4.
The reflection occurs in accordance with the following two laws:
  1. The incident ray, the reflected ray and the normal ray or the ray perpendicular to the reflecting surface— All lie in the one plane.
  2. The angle of incidence is equal to the angle of reflection.
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Figure 1.4: Reflection of light
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Figure 1.5: Laws of reflection
The Figure 1.5 shows the reflection of a single ray of light. Here AO is the incident ray which is reflected along OB by a plane mirror MN. The ray AO is the direction in which the light falls on the surface. O is the point of incidence and OB is the reflected ray. PO is the normal ray which is perpendicular to the surface MN at the point of incidence. The angle made by the incident ray with the normal ∠AOP is called the angle of incidence which is equal to the ∠BOP which is the angle of reflection.
If the surface is not plane, the reflected rays will have various directions and the light will be scattered. But at each point the law of reflection is obeyed. When the surface is rough or matt, the reflected light is scattered or diffused.
When a ray of light travels from one medium to another, it changes its path. The change in direction of light is known as refraction.
Suppose a ray of light traveling along AB in the air falls obliquely on the block of glass. The ray will now enter into the glass block. But the direction of the AB now takes the path BC (Fig. 1.6).
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Figure 1.6: Refraction of light
When a ray of light passes from rare medium (i.e. air) to a dense medium (i.e. glass) it bends towards the normal and when it passes from a dense medium to rare medium, it bends away from the normal. But when a ray of light from one medium falls normally on another, it passes straight into the second medium without any change of its direction.
The velocity of light is different in different media. It is greater in optically rare medium and less in optically dense medium. So when a light passes from a rare medium to a dense medium its velocity reduces and vice versa. For this reason the ratio of velocity of light in air to that in a given medium is taken as the refractive index {(µ) pronounced as mew} of the medium (air being taken as standard).
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Example 1: To find the refractive index of crown glass, (if the velocity of light through it is 1,22,000 miles per second):
µ= 1.52
Refractive index of crown glass = 1.52.
Example 2: To find the refractive index of flint glass (if the velocity of light through it is 1,13,000 miles per second):
µ= 1.65
Refractive index of flint glass is 1.65.