Theory And Practice Of Optics And Refraction [5 ed.] 8131263711

Table of contents :
Front Cover
Theory and Practice of Optics and Refraction, 5th edition
Copyright
Dedication
Preface
Acknowledgements
Table of Contents
1 Elementary and physiological optics
Elementary optics
Light
Measurement of light
Physical optics
Geometrical optics
Rectilinear propagation of light
Reflection of light
Specular light reflection
Laws of reflection
Mirrors: reflection at regular surfaces
Types of mirrors
Cardinal data of a spherical mirror
Images formed by mirrors
Images formed by a plane mirror
Images formed by a concave mirror
Images formed by a convex mirror
Retroreflection
Applications of retroreflection
Diffuse reflection (reflection at an irregular surface)
Calculation of the position and magnification of the image formed after reflection
Refraction of light
Refraction through various surfaces
Refraction through plane media
Refraction through prisms
Uses of prisms in ophthalmology
1 Diagnostic uses
2 Therapeutic uses of prisms
3 Use of prisms in optical instruments
Refraction at a curved surface
The lenses
Cardinal data of a lens
Refraction through spherical lenses
1 Convex lens
2 Concave lens
Refraction through a cylindrical lens
Sturm’s conoid
Combination of lenses and gauss’ theorem
Calculation of the position and magnification of the image formed by lenses
Calculation of the position of image formed by a lens
Calculation of the magnification of the image formed by a lens
Dioptric power of lenses (vergence)
Physiological optics (optics of the eye)
2 Visual acuity, contrast sensitivity and tests for potential vision
Visual acuity
Measurement of visual acuity
Contrast sensitivity
Tests for potential vision
3 Errors of refraction and binocular optical defects
Emmetropia and ametropia
Hypermetropia and related conditions
Myopia
Astigmatism
Binocular optical defects
4 Asthenopia, digital eye strain, anomalies of accommodation and convergence
Asthenopia and digital eye strain
Accommodation
Convergence
5 Clinical refraction: Determination of the errors of refraction
Introduction
Objective refraction
Subjective refraction
Determination of the muscle balance
Summary of clinical refraction
6 Keratometry, corneal topography and aberrometry
Corneal optics
Optical principles and nomenclature of techniques to study corneal shape and curvatures
Keratometry
Corneal topography
Slit-scanning corneal tomography system
Aberrometry and wavefront technology
7 Spectacles
Introduction
Spectacle frames
Spectacle lenses
Optical centration and decentration
Glazing
Verification of spectacles
Spectacle-related asthenopia
8 Contact lenses
Introduction
Contact lens manufacturing
Tear film and contact lens interactions
Optics of contact lens
Indications and contraindications of contact lens use
Design description and parameters of a contact lens
Rigid contact lenses
Rigid lens problems
Scleral rigid gas-permeable lenses
Soft (hydrogel) contact lenses
Hybrid contact lenses
Extended wear lenses
Rigid versus soft contact lenses
Disposable contact lenses
Special contact lens fitting situations
Therapeutic contact lenses
Cosmetic soft lenses
Complications of contact lens wear
Contact lens solutions and care of contact lenses
9 Intraocular lenses: Optical aspects and power calculation
General considerations
Optical aspects of IOLs
Calculation of IOL power
Optimization of IOL power
Important considerations and final selection of IOL power
10 Low vision management
Low vision
Magnitude of global vision impairment and blindness: Some facts
Low vision aids
Evaluation of the patient with low vision
Prescription of LVAs
11 Refractive surgery
Refractive surgery: An introduction
Patient selection and preoperative evaluation for refractive surgery
Refractive surgery for myopia
Keratorefractive procedures for myopia
I Incisional corneal procedures for myopia
II Earlier lamellar corneal refractive procedures for myopia
III Laser ablation corneal refractive procedures for myopia
IV Newer lamellar corneal refractive procedures
V Corneal refractive therapy (orthokeratology)
VI Intracorneal implants
B Lens-based refractive surgeries for myopia
C Combined lens-based and cornea-based refractive procedures
Summary
Refractive surgery for astigmatism
Refractive surgery for hypermetropia
A Keratorefractive procedures
B Intraocular refractive procedures for hyperopia
Summary
Refractive surgery for presbyopia
Future refractive surgeries
12 Optical instruments and techniques
Introduction
Optical instruments and techniques for anterior segment evaluation
Slit-lamp biomicroscopy
Introduction
Parts of a slit-lamp
Technique of biomicroscopy
Gonioscopy
Principle of gonioscopy
Direct gonioscopy
Indirect gonioscopy
Aims and indications and types of gonioscopy
Optical pachymeter
Specular microscopy
Optics
Types of specular microscopes
Procedure
Methods of analysis
Commercially available specular microscopes
Clinical uses of specular microscopy
Comprehensive anterior segment analyser
Confocal microscopy of cornea
Optical instruments and techniques for posterior segment evaluation
Ophthalmoscopy
Distant direct ophthalmoscopy
Direct ophthalmoscopy
Monocular indirect ophthalmoscopy
Binocular indirect ophthalmoscopy
Biomicroscopic examination of fundus
Fundus camera and related devices
Fundus camera
Wide-field retinal imaging systems
Fundus imaging with a smart phone
Rtx1 adaptive optics retinal camera
Laser scanning imaging techniques
Scanning laser ophthalmoscopy
Confocal scanning laser ophthalmoscopy
Spectralis HRA plus OCT
Retinal thickness analyser
Scanning laser polarimetry
Optical coherence tomography
Basic principle and OCT machine
OCT for posterior segment imaging
Procedure
Normal OCT scan of retina
Colour coding in the OCT scan
Interpretation of retinal scan
The macular scan
OCT scan protocols in macula
Optic disc scan
RNFL assessment with OCT
Clinical applications of posterior OCT scan
I Macular disorders
II OCT in glaucoma
III Enhanced depth OCT: Imaging for choroid
IV OCT with surgical microscope
Swept-source OCT
En face OCT
OCT angiography
Conventional OCT versus OCT–SLO
Limitations of OCT
OCT machine for anterior segment imaging and biometry
Optical devices for eye surgery
Optical instruments for refraction
Smart phones in ophthalmology
Index
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
Y
Z

Citation preview

Modern System of Ophthalmology (MSO) Series

Theory and Practice of

OPTICS and REFRACTION

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Modern System of Ophthalmology (MSO) Series

Theory and Practice of

OPTICS and REFRACTION FIFTH EDITION AK Khurana ms, mams, cto (London) Professor and Head Department of Ophthalmology SGT Medical College, Hospital and esearch nstitte Grgram, Haryana, D ormer Senior Professor and Head egional nstitte of Ophthalmology Postgradate nstitte of Medical Sciences ohta, Haryana, D Assisted by Aruj K Khurana dnb (Sanar ethralaya, Chennai), fico (arayana ethralaya, engalr), fvr ssistant Professor, itreoetinal Serices Department of Ophthalmology SGT Medical College, Hospital and esearch nstitte Grgram, Haryana, D Bhawna Khurana ms, dnb, fico, fnn, frcs ssociate Professor, Sint, Paediatric Ophthalmology, and Ocloplasty Serices Department of Ophthalmology SGT Medical College, Hospital and esearch nstitte Grgram, Haryana, D

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Dedicated to My parents and teachers for their blessings My students for their encouragement My children for their help My grandchildren for their patience And my wife Professor Indu Khurana, for her understanding

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Preface Since the invention of eyeglasses, in Italy in the year 1289, efforts have been made to improve their prole as well as the technique of prescrib ing eedless to say that still the refraction continues to form the bread and butter for ophthalmologists as well as the optometrists So, the fth edition of “heory and ractice of ptics and efraction” has been updated to provide basic principles and the recent ad vances in the eld of optics and modalities of correcting refractive errors In addition, the nowledge of basic principles of light and op tics are essential to learn the art of refraction So, n effort has been made to sillfully inter mingle these two essential ingredients to the advantage of readers, and a detailed coverage has been given to the clinical refraction, ie determination of the errors of refraction ow ever, to eep this boo primarily practically oriented, detailed theory and mathematical foundations and calculations have been pur posely ept aside he optical aspects of mo dalities of correcting the refractive errors, vi spectacles, contact lenses, refractive surgery and intraocular lens implantation have been discussed at length anagement of low vision, which is a challenge, has also been included odern ophthalmic and optometric practice is virtually impossible without the use of sophis ticated optical instruments escription of such advances is beyond the scope of this basic boo on optics and refraction owever, one separate chapter has been devoted to the description of optical instruments and procedures which form

an essential part of the eamination of the eye in modern ophthalmic practice Salient Features of Fifth Edition • Fifth edition continuous to be a part of odern System of phthalmologyS Series • Chapter Layout depicts list of contents and highlights the topics covered in the beginning of each chapter • Text matter is designed to meet the needs of students in ophthalmology and optometry, as well as practicing ophthalmologists and optometrists • Text is arranged in a userfriendly manner with various levels of headings, subheadings, bold face and italics • High quality coloured photographs and line diagrams, provide vivid details and also pro fusely illustrate the tet • Revision of text has been done in each chapter to include the recent advances Sincere efforts have been made to verify the correctness of the tet owever, in spite of best efforts, ventures of this ind are not liely to be free from human errors, some inaccura cies, ambiguities and typographic mistaes sers are, therefore, requested to send their feedbac and suggestions he importance of such views in improving the future editions of the boo cannot be overemphasied eedbacs received will be highly appreciated and duly acnowledged A K Khurana

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Acknowledgements Acknowledgement needs to be made to all those who have been instrumental in making this revised fth edition a reality. Surely, I owe sincere thanks to them all. Those who need special mention rof S Titiyal and rof amarata sharma from r  entre for phthalmic Sciences, AIIS, ew elhi rof S hull, S haar, rof Saniv ittal, AIIS, ishikesh, rof irti Singh and rof Subhash adeya, A, ew elhi rof arsh ahadur and rof enu hasmana from IS, olly rant, ehradun rof anisha upta, SIS, eharadun rof S Singh and rof amaleet Singh from Allahabad rof upali hopra and rof itin atra, , udhiana rof  uliani, , ew elhi rof ursatinder Singh, o, eptt of phthalmology Adesh edical ollege and ospital, Shahbad r S hauhan, Senior rof and ead, I, IS, ohtak rof Anand Aggarwal, ead, eptt of phthalmolgy, ovt edical ollege, atiala rof rempal aur, ead, eptt of phthalmolgy, ovt edical ollege, Amritsar rof aneet aur from S  ewat, r Sumeet handua,  pthalmology, alpana hawla edical ollege, arnal r ahipal S Sachdev, edical irector, entre for Sight, ew elhi and r agan Sahni, ontact ens Specialist, ew elhi. ooperation and help received from faculty and residents, especially rof  hugh, rof eera Sharma, r Supriya, r arvinder, r arman and r Aman, of epartment of phthalmology, ST edical ollege and ospital, urugram is appreciated.

I am thankful to rof rmial hawla, and r hetan hhikara from I, IS, ohtak for updating chapter on ptical Instruments and Techniues and r Ashwani hai, ecturer, ptometry, ST niversity for contributing tet on, “Spectacle-related Asthenopia”. I want to put on record the remarkable hardwork put in by r Aru  hurana and r hawna  hurana in completion of this edition. y task has been made untiring by the love and moral support, in addition to the editorial help, rendered by my daughter r Arushi and Son-in-aw r urukripa, Asst rofessors of edicine, ayo linic ollege of edicine and Science, ochester, innesota, SA and my wife r Indu hurana, rofessor meritus, ST niversity, and ormer rof of hysiology and ean-cum rincipal, orld ollege of edical Sciences and esearch, urawar, haar, and ormer Sr rof and ead, epartment of hysiology, IS, ohtak. I also thank Sh anmohan Singh hawla, anaging Trustee, rs adhupreet hawla, hairperson, Sh am ahadur ai, hancellor, rof Sham al Singla, Advisor, rof  alra, ice-hancellor, rof eena aiyar, ecutive irector, rof en SS ochar, ean S, r  hanna, ead  ept, ST niversity for providing atmosphere conducive to such academic activities. The enthusiastic co-operation received from the team of lsevier India esprecially Shabina asim, ead, perations and ditorial, Anand  ha, ontent roect anager is duly acknowledged.

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Contents Preface

vii

Acknowledgements

ix

Chapter 1

Elementary and Physiological Optics

Chapter 2

Visual Acuity, Contrast Sensitivity and Tests for Potential Vision

4

Chapter 3

Errors of efraction and inocular Optical efects



Chapter 4

Asthenopia, igital Eye Strain, Anomalies of Accommodation and Convergence



Chapter 5

Clinical efraction etermination of the Errors of efraction

1

Chapter 6

eratometry, Corneal Topography and Aerrometry

1

Chapter 7

Spectacles

1

Chapter 8

Contact enses



Chapter 9

ntraocular enses Optical Aspects and Poer Calculation

1

Chapter 10

o Vision anagement

1

Chapter 11

efractive Surgery



Chapter 12

Optical nstruments and Techniues

44

Index

1

555

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1

Elementary and Physiological Optics Chapter Outline

ELEMENTARY OPTICS Light • Nature of light • Properties of light • Visible light and the eye Measurement of Light Physical Optics Phenomena ase on ae Optics • Interference • Diffraction • Polarization Phenomena ase on uantum Optics • Transmission and absorption • Scattering of light • Photoelectric effect • Laser • Fluorescence

ELEMENTARY OPTICS Optics is the branch of physics that studies the behaviour and properties of light including its interaction with matter and the construction of instruments that use or detect it. Elementary or Classical optics deals with the basic fundamentals of optics in the form of two models—physical and geometric optics. Elementary optics is thus concerned with: • Light, its nature and properties,

eometrical Optics • ectilinear propagation of light • eection of light • efraction of light PYSIOLOICAL OPTICS OPTICS O TE EYE • ye as an optical instrument • omponents of the eye’s optical system • Schematic eye • The reduced eye • etinal image size • atoptric images • es and isual angles of the eye • ptical aberrations of the normal eye

• Physical optics, which takes into consideration the basic dual nature of light, and • Geometrical optics, refers to ray optics, which uses the geometry of straight lines. Before describing the geometrical and physical models of optics, it will be worthwhile to know about nature and properties of light.

LIGHT ight is a form of energy whose interaction with retina gives the sensation of sight. hus, light is

2

Theory and Practice of Optics and Refraction

the visible portion of the electromagnetic radia tion spectrum which ranges from short ioniing radiations  3   to the longest radio waves  3  meter ig. .. ight lies between the ultraviolet and infrared portions, from  nm at the violet end of the spectrum to  nm at the red end ig. . and able .. he white light con sists of seven colours denoted by ‘BO’ vi olet, indigo, blue, green, yellow, orange and red.

Table 1.1 Wavelength of different rays S. no.

Types of rays

Wavelength (in nm)

.

osmic rays

 3 

.

Electronic rays

. 3 

.

amma rays

 3  to .

.

rays

. to .

.

ltraviolet rays

. to 

.

n NATRE O LIGHT o understand the nature of light, various theo ries have been put forward from time to time. t present, it is universally accepted that like matter, light also has dual nature i.e. it pos sesses the characters of both the wave and the particle. he two characters of its dual nature are complementary.

Cosmic rays

-rays

X-rays

Ultraviolet rays

 to   to 

b. ndigo

 to 

c. Blue

 to 

d. reen

 to 

e. ellow

 to 

f. Orange

 to 

g. ed

 to 

.

nfrared rays

 to  3 

.

ireless rays ertian rays

 3  to  3 

a. hort

 3  to  3 

a. ong

 3  to  3 

Electromagnetic oscillations

Over  3 

Eectroanetic ae Natre of Liht

Electromagnetic wave nature of the light implies that it is a portion of spectrum of electromag netic radiations – nm. ight behaves as a wave as it passes through air, vacuum or other transparent media, including the transparent ocular tissues. uygens in  proposed that light moves in the form of waves from the lumi nous source. hese waves consist of crests and troughs. t any instance, a trough or a crest is circular in shape. he locus of points in the same phase at a particular time is called a wavefront. he shape of the wavefront depends upon the nature of source. or waves from a point source in air, the wavefronts are spherical. f the source is a long slit, the wavefronts are cylindrical.

isible rays a. iolet

.

t long distances, they appear plane. n im portant characteristic of wave motion is that it transmits energy, not matter. Phenomena explained by wave nature of light. ave nature of the light eplains the following phenomena: • ropagation of light through vacuum, • eection of light,

Light

Infrared rays

Invisile

Violet nm



Radio waves Short Long

Invisile

Indigo 

Micro waves

Blue 

Green 

ellow 

range 

Red 



ig  S  pectrum of electromagnetic radiation. Note the very small portion occupied by visible light.

Eeentary and Physiooica Optics

• efraction of light, • henomenon of interference, • henomenon of diffraction and • henomenon of polariation. Partice Natre of Liht

ight ehibits some characteristics of particles photons when it is being absorbed or when it is being generated in a light source. n order to eplain the photoelectric effect of light, Einstein in  proposed that light of a given freuency n consists of uanta photons with the same energy E 5 hn, where h is the lanck’s constant. hus, quanta or photons can be considered the units in which the energy of electromagnetic radiation is measured. he energy of an individual photon is directly pro portional to the freuency and inversely propor tional to the wavelength. herefore, the energy of a photon at  nm is twice as great as that of a photon at  nm. or eample, red light is innocuous, ultraviolet light produces burns and rays produce severe damage to the tissues. Phenomena explained by particle nature of light. article nature of light eplains the following phenomena successfully: • hotoelectric effect of light, • cattering of light, • Emission of light and • bsorption of light. Note. n fact, the photoelectric effect of light eplained by its particle nature is responsible for seeing things. hen a light photon is ab sorbed in one of the sensitive cells of the retina, the chemical change induces an electrical signal to the brain and one perceives the light. n PROPERTIES O LIGHT ome of the important properties of light are summaried as follows:  ight is propagated as electromagnetic

waves i.e. it does not reuire medium for its propagation. 2 peed of light in free space i.e. vacuum is  3  ms , miless.  t is transverse in nature and so can be polaried.



 t is not deected by electric and magnetic

¤elds.  hen light passes from one medium to the other, velocity and wavelength change, amplitude may decrease or remain constant, but frequency and colour of light do not change i.e. colour of light is determined by its freuency and not by wavelength. or eample, if red light passes from air to water or glass, its velocity and wavelength in water or glass will be different from that in air, but freuency and colour re main the same.  elocity of all wavelengths of light in free space is same and is eual to  3  ms. ow ever, in a medium, the speed of light is different for different wavelengths.  he speed of light in a medium is lesser than in vacuum. hen the same light passes through different media, its speed will vary depending upon the density of medium the denser the medium the lesser will be the speed of a given light.  ight of a single wavelength is called monochromatic light. hite light is heterochromatic.  ight ehibits phenomena like reection, refraction, absorption, diffraction, interfer ence and polariation. hese will be discussed later. n ISILE LIGHT AN THE EYE • The media of the eye are uniformly permeable to the visible rays between  and  nm. • ornea absorbs rays shorter than  nm. herefore, rays between  and  nm only can reach the crystalline lens. • The normal human eye is insensitive to wave lengths between  and  nm ultraviolet rays because these are absorbed by the crys talline lens of the eye. n aphakic eyes, the light rays between  and  nm can also pass on to the retina. herefore, the aphakic eyes are sensitive to those wavelengths which give rise to the sensation of blue or violet co lour. ence, the newly aphakic patients often complain that everything looks bluer than visualied before the operation. • The eye is most sensitive to yellowgreen light, i.e. light of wavelength  nm. he sensitiv ity of the eye decreases on both sides of this

Theory and Practice of Optics and Refraction



Sensitivity of eye

power of light produced by a light source can be measured in the following terms:

nm

400

424 455 492 575 585 647 700

ig  S  ensitivity of eye to visible spectrum of light.

wavelength, so it is minimum for violet and red light ig. .. • Persistence of the eye is . s i.e. if the time in terval between two successive light pulses is lesser than . s, eye cannot distinguish them separately. • ange of sensitivity. he human eye can detect energies of a few photons per second up to bright sunlight, a difference of  in sensitivity. • echner’s law. he relative sensation of an increase in sensitivity is proportional to the log of the change, and so by increasing the intensity of a lamp from  to  footcandles, the same sensation of change as that from  to  footcandles is given. his law applies for four orders of magnitude. • Weber’s law. he change of brightness neces sary to be noticed is proportional to the origi nal brightness i.e. D 5 , where D is the least amount of change of intensity notice able,  is a constant and  is the brightness of the light. herefore, the change necessary be fore a difference is noticed in a bright light source is larger than in a dim one.

MEASREMENT O LIGHT he uantitative measurement of light is carried out in two different ways. n  RAIOMETRY adiometry refers to the measurement of light in terms of its power generatedemittedirradiated by a source of light. ts basic unit is watt. he

• adiant u. t refers to the amount of light emitted from a source and is measured in watts or oules per second • adiant intensity. t refers to the intensity of light emitted from a source and is measured in watts per steradian • rradiance. t refers to the amount of light fall ing on a surface and is measured in watts per square metre • adiance. t refers to the amount of light re ected from a surface and is measured in watts per steradian per square metre n 2 PHOTOMETRY t is the measurement of light in units and is based on the response of the eye. hus, photom etry uses the eye as the comparison detector. hotometric measurement terms are as follows: Luminous ux. t refers to the total ow of light in all directions from a source of light. he unit of measurement of luminous u is called lumen Luminous intensity old name: candlepower. t refers to the light emitted from a source in a given direction. he unit of light intensity is candela. andela is the modern unit of luminous in tensity. t is mere precisely de¤ned replacement for the old unit, the candle which is based on a standard wave candle. he de¤nition of can dela is based on a standard electrical ¤lament lamp.  candela 5  lumen per steradian the stera dian is the measure of cone of light.  point source with output of  candela emits a total of  i.e. ,. lumens. llumination or illuminance on a surface. t re fers to the light arriving at a surface, i.e. the num ber of lumens per suare metre incident on that surface lumenm. he old names for this unit lumenm were the metrecandle and lu. he illumination E of a given surface is as follows: • nversely proportional to the suare of dis tance d of the surface from the light source.

Eeentary and Physiooica Optics

• irectly proportional to the angle of incident light i on the surface, i.e. E 5  cos id, where  is the luminous intensity. Luminance of a surface is the total amount of light reected or emitted by the surface. wo sets of units are in use as measure of luminance: • Lambert One lambert is de¤ned as the lumi nance of a perfect diffuser surface emitting  lumencm • ootlambert is more commonly used unit for luminance. t is de¤ned as the luminance of a surface reecting or emitting  lumenft  footlambert 5  candela ft f a source has a known output in watts, we can determine its output in lumens, provided we know the spectral properties of the lamp, i.e. power at each wavelength. he output at each wavelength is multiplied by the sensitivity of the eye at the wavelength and the results are summed to obtain the total response of the eye to light from that source. or eample, if the source is monochromatic, with a wavelength at the peak of eye’s photiopic sensitivity,  nm, the conversion factor is  lumens per watt. t



other wavelengths, the factor is less, falling to approimately  at  and  nm. rightness is not a precisely de¤ned term, but it refers in general to the sensation produced by a given illuminance on the retina. postilb is de¤ned as the luminance of a perfectly diffusing surface that is emitting or reecting  lumen per suare metre. t is encountered in pe rimetry, where the luminance of the background and the targets is often speci¤ed in apostilbs. n RAIOMETRIC ERSS PHOTOMETRIC TERMS he commonly used radiometric and photo metric terms are summaried in able . n APPLIE ASPECTS he clinician should be familiar with freuently recommended levels of illumination employed by illumination engineers. t should be noted that the recommended footcandles are a mea sure of the luminous power impinging on a surface, not that which is reected into the eye. n ideal   lamp bulb provides about  footcandles of illumination  ft away and  footcandles  ft away.

Table 1.2 Principal types of light measurement and their relationships Description

Radiometric measure

hotometric measure

Type

nits

Type

nits

uantity of light leaving a source or passing through a region of space

adiant u

atts 5 oulessecond

uminous u

umen  candle emits  p  m

ight emitted per unit solid angle

adiant intensity

attssteradian  sr 5  unit of solid angle

uminous intensity candlepower

 candela 5  msr

uantity of light per unit area incident on a surface or at an image

rradiance

attssuare metre

lluminance

 lu 5  lumensuare metre

ight reected or emitted by a surface, per unit area and per unit solid angle

adiance

attssteradian suare metre

uminance

 footcandle 5  lumensuare foot  apostilb 5 p lumen suare metresr  footlambert 5 p lumensuare footsr

etinal illuminance

rolands luminance of  candlesuare metre viewed through  mm pupil

lluminance at the retina, adusted for pupil sie

Theory and Practice of Optics and Refraction



ecommended levels of illuminance illumina tion for some common purposes are as follows: • Of¤ce, kitchen • eading • efracting lane n all chart n roector • Operating table

 footcandles  footcandles – footcandles  footcandles  footcandles

PHYSICAL OPTICS hysical optics takes into consideration the ba sic dual nature of light i.e. the light possesses the characteristics of both the waveform and the particle photon or uantum. herefore, the physical optics can be divided into two parts: • wave optics and • uantum optics. n AE OPTICS ave optics concerns with eplanation of the observed phenomena such as interference, diffraction and polariation. n it, light is treated as a wave with the following characteristics: • Wavelength L of a light wave is de¤ned as the distance between two symmetrical parts of the wave motion ig. .. • mplitude  of a light wave is the maimum displacement of an imaginary particle on the wave from the base line ig. .. • Phase One complete oscillation of light waves is called a cycle ig. .. ny portion of the cycle is called a phase. A A X

Y A

Minima

Base line

Wavelength (l)

B

ig  L  ight as a waveform depicting: A, basic characteristics; , phase differences.

• Phase difference refers to the fraction of a cycle or wavelength by which the two waves of eual wavelength travelling in the same direc tion are out of step with each other ig. .B. • oherent and incoherent light ight waves that are out of phase are called incoherent, while the light composed of waves eactly in phase is termed coherent. henomena based on ave optics • nterference, • iffraction and • olariation. n INTERERENCE hen two or more wave trains of light of the same freuency travelling in almost same direction su perimpose, the resultant intensity in the region of superimposition is in general different from the sum of the intensities due to individual waves. his modi¤cation in the distribution of intensity of light in the region of superimposition is called in terference. epending upon the way the waves superimpose, the interference is of two types:  Constructive interference. hen the waves

superimpose in such a way that their maima and minima correspond with each other full phase difference, the resultant intensity is greater than the sum of the intensities due to separate waves ig. .. his phenomenon is called constructive interference. 2 estructive interference. hen the waves superimpose in such a way that the maima of one corresponds with the minima of other half phase difference, the resultant intensity is lesser than the sum of the separate intensities. his phenomenon is termed as destructive in terference ig. .B. hus, due to the phenomenon of interference, we get intensity maima due to constructive inter ference and intensity minima due to destructive interference, which are called bright and dark fringes, respectively. he array of such fringes is labelled as interference pattern ig. .. t has been observed that to get interference effects, the waves must be coherent. he best condition for interference to occur is when the

Eeentary and Physiooica Optics

A



B

ig    henomenon of interference: A, constructive interference; , destructive interference for eplanation, see tet. Screen (side)

Screen

R

P

Q

Maxima

Minima

ig  nterference pattern consisting of bright due to intensity maima and dar due to intensity minima fringes.

light is monochromatic, i.e. a narrow band of wavelengths. But interference can also be obtained with white light under optimum conditions. Coherence is the measure of the ability of two light beams to produce interference. oherence is of two types:  patial coherence refers to the ability of two

separated portions of the wave  and  in ig. . to produce interference.

2 Temporal coherence is a measure of the ability

of a beam to interfere with another portion of itself  and  in ig. .. emporal coherence is improved by using a ¤lter to select a narrow band of wavelength. Cinica Sinicance of Interference

estructive interference occurs within the stroma of the cornea. he collagen bundles of the stroma are so spaced that any light



Theory and Practice of Optics and Refraction

deviated by them is eliminated by destructive interference.

A S

henomenon of interference has a wide range of applications.  few important ones are as follows: • olography utilies the phenomenon of inter ference to produce threedimensional images. • Laser interferometry is based on interference. ith its use, it is possible to predict the visual potentials in patients with hay media due to cataract. • nti-reection coating on the spectacle glasses utilies the principle of destructive interference. • t is used to determine the refractive inde or thickness of transparent sheets. • Ecitation lter and the barrier lter used in uorescein angiography fundus camera are based on the phenomenon of interference. • he socalled cold mirror has a multilayered coating based on interference ¤lter that is designed to reect the visible cold light and transmit the infrared wavelength. • ptical coherence tomography O is the most recent innovative clinical application of interference. he O scanner is basically a ichelson interferometer see page , ig. .. n IRACTION hen an opaue obstacle or an aperture is placed between a source of light and a screen, in accordance with rectilinear propagation of light, usually a sharp shadow or an illumi nated region is obtained on the screen, as shown in igure . and B. hen the sie of aperture is larger than the wavelength of the light, there occurs rectilinear propagation of light ig. .. owever, if the sie of the obstacle or aperture is comparable with the wavelength of light, the light deviates from rectilinear propagation near the edge of the obstacle or aperture and enters the geometrical shadow ig. .. his aring out or en croachment of light in the shadow one as it passes around the obstacle or through small aperture is called diffraction ig. ..

Light

Appications of Interference

No light

Aperture

No light

B

A

Light

A

Shadow

S

Light

Obstacle

B

B

Light

Light l