Recent developments

Recent developments

Recent developments Laser photo-coagulator hospital at UK l-------- Ultra-v13let spectrograph Field emlssicn A C.W.argon ion laser photo-coagul...

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Recent

developments

Laser photo-coagulator hospital

at UK

l--------

Ultra-v13let spectrograph

Field emlssicn

A C.W.argon ion laser photo-coagulator developed by Coherent Radiation, has been supplied to the Moorfield eye hospital for the treatment of neovascularization. It is the first of its kind in the UK. Neovascularization is a poliferation of blood vessels on the nervehead and/or vitreous humour. In a recent instance where the treatment was successfully used at the Palo Alto Medical Clinic, a patient with neovascularization due to diabetic retinopathy underwent two treatments with the 125 mW argon ion laser. Two months after the first of these treatments the neovascularization was observed to have disappeared completely. Meyer-Schwickerath’s pioneering work in the late 1940’s and early 1950’s led to the first effective use of light in the treatment of certain eye diseases. By 196 1, laser developments made it possible to consider the use of intense laser light sources instead of the xenonarc source that was then being used for photo-coagulation. The first laser photocoagulator, which used a pulsed ruby laser, was successful in the treatment of a number of ocular diseases. Because of its red colour however, the ruby laser was ineffective in the treatment of most opthalmic vascular diseases. The argon laser has high spatial coherent, good wavelength selectivity, and high continuous power. Thus the argon laser can be applied to any peripheral retinal problem in which the xenon-arc is conventionally used, and can produce similar results with lower power introduced into the eye. Since its output can be focused to very small spot sizes (smaller than any other laser or the xenon-arc), the

Electra-Optics

I

Schematic

diagram

beam pum-

argon laser can also be applied to the treatment of macular diseases. In addition the argon laser can be used to obliterate undesired vessels, in the plane of the retina, on the nervehead and in the vitreous humour. Coherent Radiation L td, PO Box 12, Royston, Herts, UK Ultra-violet

laser

A laser which emits at 0.1161 pm, claimed to be the shortest wavelength yet achieved in a laser, has been developed by scientists at International Business Machines Corporation. The new laser, reported by Dr. Rodney T. Hodgson and Dr. Russell W. Dreyfus in the February 28 issue of Physical Review Letters is an advance over the previous record task about 0.16 pm achieved by Hodgson in 1970. Both lasers use hydrogen gas as the active medium, but the new laser is pumped by a high-power electron beam rather than by a gas discharge. The electron beam delivers 4 GW in a 3 ns pulse of 400 kV electrons. The beam current of about 10 kA m-* is generated by a field-emission electron source, and is contained along the axis of a tube with 1 cm internal diameter by an axial magnetic field.

Conference and Exhibition

The 1st European Electra-Optics Markets and Technology Conference and Exhibition will take place between 12th and 15th September 1972 at the Palais des Expositions, Geneva, Switzerland.

of electron

in Geneva

the electro-optics field. Subject coverage ranges from advances in holography down to simple video recorders for home use. Couriers for the event are Swissair.

The event has been arranged to appeal Mack-Brooks Exhibitions L td, both to technical and marketing people in 7, London Rd, St Albans, Herts, UK

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Optics and Laser Technology

June

1972

molecular

hydrogen

laser

The tube contains hydrogen gas at a pressure of 20 to 100 torr Collisions between electrons and hydrogen molecules in the ground electronic state raise the molecules to high-lying electronic states. Stimulated emission occurs from these states to upper vibrational levels of the ground electronic state without the need for mirrors. In conventional molecular spectroscopy, the present 0.1161 - 0.1240 pm lines represent wavelengths appearing in the Werner bands. Early experiments with the laser indicate the output power at 0.1161 pm is at least 500 W cmM2. Since the electrons in the beam lose only a small fraction of their energy to the hydrogen gas in the laser, it should be possible to increase the power output by merely increasing the length of the laser. IBM Corporation, Research Division, BOX218, Yorktown Heights, N.Y. 10598 USA Optical information

store

Dr Irwin Schneider, a scientist at the US Naval Research Laboratories, has patented a method of storing information using optical materials. In the new method, anisotropic colour centres in crystals such as the Na centre in optical materials like sodium doped potassium chloride are optically biased. The colour centres are first oriented in a common lattice direction with polarized light, and information is then stored by reorienting the colour centres with radiation with the same wavelength but different polarity. Information can then be read with polarized light at a longer wavelength without destroying the information. Dr. Schneider said optical materials have a greater infor-

Introduction to the theory of Atomic Spectra I. I. Sobelman The physical principles and theory of atomic spectroscopy are systematically covered through a presentation based on the modern system of the theory of angular momentum. Partial Contents: Part 1: The Hydrogen Spectrum; Systematics of the Spectra of Multi-Electron Atoms; Spectra of Multi-Electron Atoms; Part 2: Angular Momenta; Systematics of the Levels of Multi-Electron Atoms; Hyperfine Structure of Spectral Lines; Relativistic Corrections; Part 3: The Atom in an External Field; Interaction of Atoms with an Electromagnetic Field; Broadening of Spectral Lines; Fundamentals of Excitation of Atoms; Subject Index. f 78.00 hard cover 632 pages

Mechanics and Electrodynamib Volume I of A Shorter Course of Theoretical Physics L. D. Landau (deceased) and E. M:Lifsh\tz The study of the analytic properties of the scattering amplitude has been a useful tool in understanding elementary particle interactions. The authors review the progress that has been made in this study through the majorization technique of Feynman diagrams. The development is careful and rigorous with proofs of the fundamental theorems, and the results are applied to physical processes. Comparisons are made between the majorization method and the alternative field theory and Landau curve theory methods. The new approach of this book is the use of the majorization technique to study analyticity and a second new feature is the study of Cutkosky rules and their application to the box diagram. f3.00 hard cover 280 pages

Polarized Light and Optical Measurement

OPTED

Holography System

STRESS ANALYSIS NON DESTRUCTIVE TESTING VIBRATION ANALYSIS SURFACE DYNAMICS CONTOURING THERMAL DEFORMATION

The OPTEL holography system has been designed to meet all the above requirements. It is a flexible system engineered to provide simplicity of set up providing maximum resolution. Available with the system is a purpose built holography table as well as a wide range of HeNe Lasers up to 15 mW. Our technologist will be pleased to call and give you a practical demonstration of the system as applies to your requirements.

D. Clarke and J. F. Grainger Contents: Preface; The Description of Polarized Light; Some Interactions of Light and Matter; Optical Elements Used in Polarization Studies; Measurement of the State of Polarization; The Role of Polarization in Optical Instrumentation. Appendix 1: Historical Notes on Polarization; 2: Matrix Multiplication; References; Index. 196 pages f3.50 hard cover

Sole distributors:

Survey and General Instrument Co. Ltd Optical and Mechanical Division Fircroft Telephone: Cables:

Way,

Edenbridge,

0732-71

Optic Sales,

2434

Kent (3 lines)

Edenbridge,

Kent

0136

Optics and Laser Technology

June

1972

709

Photocathode

Screer

-----_

---_ ---

The John Hadland Ima-Con Camera

mation storage capacity than magnetic core memories. Naval Research Laboratory, Code 2650, Washington DC 20390, USA

Design award for John Hadland John Hadland’s Ima Con HE 700 camera which can take a photographic exposure in 0.00000005 s was among the products which have won 1972 Council of Industrial Design awards for capital goods. The ultra high speed camera is used for research into transient events such as laser-generated plasmas and sparks. The Imacon system turns light into a beam of electrons which can then be controlled by an electric field (see illustration). Light from the object to be photographed is focused through a conventional lens system onto a photo-cathode, which converts the light beam into an electron beam. The electron beam is in turn focused onto a phosphor screen which converts the electron beam back into light. Blocking the path between the photo-cathode and the phosphor screen is the aperture plate which has a narrow slit for the electron beam to pass through. In front of this aperture plate are a pair of shutter plates which, when a cycling electrical current is applied to them, make the electron beam move up and down across the aperture plate. Thus repetitive shuttering is achieved as the electron beam can only get through the aperture plate when it is passing across the slit. But as the electron beam is moving up or down when it passes through the slit it produces a blurred

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Optics

and Laser Technology

picture on the phosphor screen. This is resolved by a pair of compensating plates which are applied with a current of exactly the same frequency and amplitude as that applied to the shutter plates but of opposite phase, thus arresting the up or down movement. A third pair of deflector plates - called the shift plates - are fed with a voltage which changes between each pulse of electrons and thus separates each discrete picture by moving it to a different part of the screen. John Hadland (P.I.) L td, Bovingdon, Herts, UK

Absorption band clue to fibre losses Researchers at Corning Glass Works and Bell Telephone Labs have identified three spectral regions where optical attenuation in glass fibre wave guides is 20 dB/km or less. But if low attenuation in commercially important regions of the spectrum is to be attained, then the OH ion concentration of the glass used must somehow be reduced. The three attenuation minima are at 633 pm, 1060 pm and around 800 pm. High absorption lines are found at 725 pm, 875 pm and 950 pm. Unfortunately, these lines all lie near the wavelengths used by semiconductor emitters of commercial importance. A spokesman for Corning said that as the absorption lines are almost certainly due to OH ions in the glass, some means of reducing OH concentration needs be found. The detailed origin of extraneous scattering is not yet known. Corning Glass Works, Corning, New York 14830,

June

1972

USA

High gain image intensifier Physics Exhibition

at

A TV image intensifier with a maximum gain of 100,000 and so compact that two could be incorporated in a pair of goggles was one of the main exhibits from Mullard at the recent Physics Exhibition (Alexandra Palace, London March 13th - 16th). The XX1 241 can be used in automatic gain control systems, and as each channel electron multiplier element has its own saturation level bright areas are not enlarged on the screen, thus eliminating dazzle. Also on show on the Mullard stand were, triglycine sulphate infrared detectors. One of the more unusual exhibits was featured on the Agricultural Research Council stand. The Meat Research Council are using a laser diffraction method to determine the toughness of meat. A low energy laser beam is aimed at the meat fibres, which act as a diffraction grating, and the resulting diffraction patterns can be used to measure mean fibre length.

Laser lighthouse Helium neon lasers are being used as the source of illumination in a new unmanned miniature lighthouse designed by Decca Survey. The lighthouse is 6ft 10 in high and contains two 50 mW helium neon lasers. One is used continuously and the other is in standby, tube No 2 being activated automatically on the failure of tube No. I. Each tube possesses two heaters. Again, the standby heater comes into action automatically when the

regular heater fails. The laser beam is diffused and will neither damage the retina nor impair sailors’ night vision. 2 mW gas laser According to a spokesman for Decca Survey, the lighthouse has already undergone a trial period of 1 year with the Australian Government ~who will give a verdict on the lighthouse shortly. In the meantime the UK Trinity House organization which is responsible for commissioning lighthouses and navigational buoys, is conducting its own evaluation programme on the lighthouse. Decca claim the laser lighthouse costs only a tenth of the cost of an equivalent conventional lighthouse. Advantages claimed are good range and exceptionally good penetration of fog. Also available from Decca Survey is a laser marine channel light which is expected to find applications in harbour approach navigation systems. Decca Survey, Survey House, Kingston Road, Leatherhead, Surrey, UK Laser in geodetic survey Distances are being measured by a laser geodimeter as part of a 10 county federal geodetic survey at present underway in southern Texas, USA. The idea of using a laser in conjunction with a standard Model 4D geodimeter (which times a light pulse to reach a distant retrodirective reflector and return) was first investigated in 1965. Experiments using a 50 mW laser as the light source showed that the light loss in a Kerr cell can be as much as 90% and the response of the IP21 photo-multiplier at low light levels at 0.6328 pm was very low. Therefore, the transmitter and receiver had to be redesigned. The transmitter of the geodimeter was removed to provide space for the laser, KDP cell, and collimator. These were remounted to permit easy access to components. The receiver changes included addition of a 56TVP photo-multiplier and coated receiver optics. The laser used in the present survey is a 2 mW He-Ne gas laser, used with an external dc. power supply. It can be used to transmit either with confocal or hemispherical mirrors at the ends of the light path. Experiments have shown that the systems are equally accurate but the hemispherical mirrors give lower noise level and greater return

KDP cell

Collimator

To distant reflector -

Laser geodimeter

light signal. The laser geodimeter developed at the US Department Commerce.

was of

US Department of Commerce, National Ocean Survey, Rockville, Md 20852 USA Good prospects for optical industry In a recently published analysis of the US commercial optics industry, Creative Strategies Inc, have concluded that only two companies derive more than $100 million in annual revenue from the sale of commercial optics products. These are American Optical, a division of Warner-Lambert, and Bausch & Lomb Overall growth is estimated at 7% per year but certain subgroups within the industry, the report claims, are finding specific markets with much greater growth potential. Three major groups within the commercial optics industry were defined in the study. These are the ophthalmic group, including prescription eyeglass lenses, frames and contact lenses; the non-prescription sunglasses industry; and the optical instrument and lens industry. According to the report, new markets are appearing in the area of optical instruments and lenses due to developments such as electro-optics, lasers, optics, optical coatings and films. The study infers that success in the optical components industry lies in identification and solution of specific problems rather than to the production of exotic instruments. User applications for optical instruments and lenses include medical, pollution measurement, and information storage and retrieval systems. Revenues in the sub-group are estimated at $488 million in 197 1,

and growth at 9% is predicted for the 1972-1976 period. The report costs $360. Creative Strategies Incorporated, The Executive Building, 1032 Ehvell Suite 100, Palo Alto, California 94303, USA Laser television uses 0.3 mm radiation A television display of radiation from an HCN laser is reported in the 27 January 1972 issue of Electronics Letters. It is the first time that a moving-picture display has been obtained of radiation at this wavelength (0.337 mm), which cannot be detected by conventional image-forming devices. The display provides a 25 frames s-l picture on a television monitor of radiation from 0.3 mW HCN laser, using a special vidicon tube. The tube has a target of triglycine sulphate, the rear face of which is scanned by an electron beam. Various lenses of polyethylene and TPX were used to produce an image of the radiation from a 2 m HCN laser on the 16 mm diameter vidicon target. The display system could assist the development of laser systems operating in this wavelength region which could have practical applications in future, for example, seeing in fog. The Camera system and the vidicon tube were developed by the English Electric Valve Company, under a UK Ministry of Defence contract, and the experiment using the HCN laser was carried out at the UK National Physical Laboratory. IEE Publishing Department, PO Box 8, Southgate House, Stevenage, Herts, UK

Optics and Laser Technology

June

1972

111

Ultra-violet

telescope in orbit

A satellite carrying a joint UK/Belgian ultra-violet telescope was launched into earth orbit for the European Space Research Organization (ESRO) by NASA on March 11 th. The aims behind the 220 Million project are to investigate the ultra-violet emissions of young stars (i.e. extremely hot stars in the region of 10 years old) and to look for hitherto unnoticed ultra-violet emitters in space. Another field of study will be the unexplained clouds of interstellar dust (megallanic clouds) which are scattered through the galaxy. Previous observers have noted that this dust exhibits strong absorption edges in the ultra-violet, and by studying the fine structure of the absorption spectrum, scientists hope to gain a good idea of the clouds’ composition. The satellite’s orbit will precess during the experiment. This will enable the satellite’s equipment to observe the whole sky during the six months of its life. In the ecliptic polar regions where the precessing orbit recrosses its previous paths, the telescope will be keeping track of any temporal variations in the ultra-violet emissions of individual ultra-violet emitters. The satellite will be fitted with high resolution sun-sensors and horizon scanners to keep it in the correct position and altitude to enable acurate measurements to be made during flight. This means the craft will only be operational during the six months when the earth is not blocking the

sun’s rays. When reemerges from it is theoretically system could be

the satellite the earth’s shadow possible that the restarted again.

The ultra-violet telescope, which is designated S2/68, is coupled to a photometer spectra-photometer combination and mounted without independent pointing capability within the orbiting satellite. It will be used to make photometric measurements of any stellar radiation of appropriate wavelength and intensity which appears within its field of view. The telescope, an off-axis reflecting instrument, is arranged to bring incident stellar radiation to a focus in the plane of a set of entrance slots of the spectrophotometer combination. The instrument’s field of view is defined by the dimensions of these entrance slots. One narrow slot feeds, via a Fabry mirror, the single channel photometer whose pass-band is defined by the combination of a photo-multiplier cathode and filter. Two adjacent shorter slots are positioned such that certain attitude information may also be obtained from the photometer channel. A larger slot is used to feed the three channel spectra-photometer. In this part of the instrument Light passing through the entrance slot is allowed to fall on an ellipsoidal secondary mirror designed to image the primary mirror on a plane-reflecting diffraction grating positioned to direct dispersed light at three appropriately positioned exit slots. Light passing through each exit slot is redirected by a Fabry mirror onto the window of a photo-multiplier. The telescope field of view is made 17 arc-minutes wide so that each object is seen at least three times thus providing against complete loss of observational data even in the event of telemetry problems in two consecutive orbits.

The TDIA

satellite

during solar panel

deployment tests at European Space Research and Technology Centre at Nordwijk, Netherlands

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Optics and Laser Technology

With orbital movement of the telescope, primary stellar images move across the wide entrance slot causing the dispersed spectrum to move across the exit slot array, thus providing spectrum scanning without the need for any moving parts. The spectra-photometer fully covers the wavelength range from 0.13 pm to 0.26 pm, each of the three detector channels yielding approximately 20 data points for each scan of the primary star image. This data is in the form of photon counts over an inte-

June

1972

gration interval of 148 ms, each count being the number of photons in a passband of approximately 35 A half-width. The derived signals are processed and fed to the spacecraft for en-coding, storage and telemetry. By closing a shutter in front of the instrument’s entrance slots, by telecommand, it is possible to determine dark count of the detectors in orbit. Since the experiment is viewing from a continuously sunlit spacecraft, in a direction normal to the sun-line, there is a requirement to attenuate scattered sunlight. For this purpose a system of baffles is incorporated in the telescope whilst the spacecraft is relied upon to provide a system of primary baffling.

European Space Research Organisation, 114, Avenue Charles de Gaulle, 92 Neuilly-sur-Seine, France

High power chemical laser A high power chemical C.W. laser generating 1.8 kW with a lasing region rrsasuring 7 in X 3 in X 0.5 in developed by scientists at the Aerospace Corporation, Los Angeles. The laser which mixes hydrogen and flourine, incorporates a narrow tube which flares out into a flattened mixing region at one end (7 in long X 0.5 in high). The chemically inert gas, helium, is pumped into the end of the narrow neck at a pressure of l-2 atmospheres, where it passes through an intense electric arc which heats the helium atoms to nearly 3000°C. The hot gas then enters a second chamber where SF, proceeds into the flared mixing region and expands out of an array of small nozzles into a lasing chamber. Here a screen of nozzles sprays the mixture with hydrogen gas. At a distance of about 3 in from the nozzle openings, hydrogen and fluorine from the SF6 combine to produce infra-red radiation. Chemical conversion efficiency is said to be about 10 to 20%. The resulting radiation is extracted as a laser beam from the lasing chamber with two 4 in square mirrors made of a berylliumcopper alloy coated with gold. Nitrogen was also tested as the diluting medium in the Aerospace laser, but helium gave a more intense beam; replacement of hydrogen with the isotope gives deuterium a longer wavelength. Full details appear in the March issue of Journal of Applied Physics. American Institute of Physics, 335 East 45 St, New York 10017,

USA

US Army study stressed materials by hologram The US Army has ordered a pulsed ruby laser holography system from Apollo lasers. It is to be used to study materials subject to high rates of stress within a range of environments which the US Army describes as typical of those experienced by Army field equipment. Apollo Lasers, Inc., 6365 Arizona Circle, Los Angeles, California 90045, USA Lens centring gauge An electro-optic lens centring gauge which does not require a skilled operator has been developed at Sira Institute.

Sira’s lens centring gauge uses the exchange spindle methods favoured by most lens manufacturers and can be adapted for use with most edging machines. Designed round the Loh spindle, it can be supplied to suit Bothner and other kinds of bearing. It measures the centring errors and displays them on a cathode ray tube. The lens is fastened with pitch or wax to a bellchuck carried on a spindle. This unit is then put in the centring gauge and the lens is moved about on a bellchuck until centred. The operation is carried out by tilting the lens about the centre of curvature of its lower surface. Centring accuracy is expressed in terms of tilt of the upper face and the instrument has a constant sensi-

Lens centring

gauge

tivity independent of the lens radius of curvature. The lower surface of the lens can be checked after the upper surface has been centred. When the spindle carrying the bellchuck and lens is clamped into the instrument, it rotates. The detecting unit is then lowered until the focus meter reading is a maximum, and as the spindle rotates the centring error shows up as a circular trace on the cathode ray tube. The spindle is temporarily stopped with one hand and the lens is moved with the other SO that the spot moves towards the centre of the circle, When the spindle is released the error trace on the tube will be reduced in diameter. The process is repeated until the centring error is reduced to an acceptable level. The instrument can handle lenses up to 1 Sin thick, 4.75in diameter and 0.2in minimum radius of curvature. Sensitivity is about 10 seconds of arc, and maximum initial error is 10 degrees. Sira Institute, South Hill, Chislehurst, Kent, UK Rotating prism could replace solid state Q-switches

The Ferranti laser scanning analyser, first developed to inspect food wrappings for contamination, has been found to be applicable to a wide variety of industries. Here scanning analyser Model 7 1A is seen checking paper for flaws at the mill of a Swedish papermaker. On the left is the control console/data processing unit; mounted on the machinery is the optical head containing the 2 mW He-Ne laser.

Despite the current preoccupation with the reduction or even elimination of moving parts wherever possible, a group of researchers at the National Physical Laboratory has recently developed a mechanical Q-switch which they think is competitive with electrooptic devices such as Kerr and Pockels cells. Advantages claimed are low cost, high optical transmission and good homo-

Optics and Laser Technology

June

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773

geneity. In addition the optics are claimed to be highly resistant to laser damage and temperature distortion. The laser beam is passed through a pentagonal silica prism which is mounted inside a rotating air turbine. The axis of rotation goes through the centre of the pentagonal end faces of the prism, and the light enters and leaves by these end faces at an angle, reflecting internally from the rectangular faces as the prism rotates. There are three versions of the device. The smallest is only 60 mm square by 30 mm high and can be run at speeds of up to 250,000 r.p.m. The others are somewhat larger and run at 120,000 and 90,000 r.p.m. respectively.

detection and assessment of ocean fish. An additional 40 mm image intensifier section coupled to the camera tube provides sensitivity to low-light levels. Besides detecting fish from the plankton glow, the camera can record the outline of the school. By analysing the size and shape of the outline, scientists claim to be able to determine the species of the school. RCA, 30 Rockefeller Plaza, New York, N. Y. 10020 USA

Rotation of the pentagonal prism gives pulses at rates up to 20 kHz or more. Maximum beam diameter passed by the prism of the 250,000 r.p.m. unit is 4mm but this is adequate for uniphase operation of ruby lasers. The other units have a 10 mm optical aperture. The unit has been used in high speed holographic recording. National Research Development Corporation, PO Box 236, Kingsgate House, 66-74 Victoria St London SWI, UK Infra-red eye for hire An airborne thermographic service that gives early warning of dangers such as imminent power cable failure, gas leaks, crop disease or river pollution has been formed by Rank Precision Industries (Solus Schall Inspection) and Management Aviation Ltd. The equipment which is mounted on board a helicopter (Fig 1) consists of a thermographic camera and a TV monitor. Fig 2 shows a picture of a power cable with overheating shown up as a white spot on the television monitor. Rank Precision Industries, Phoenix Works, Great West Rd, Brentford, Middlesex, UK Tracking fish by TV A new RCA low light level airborne TV camera that can detect the dim glow of sea plankton being distrubed by fish is being used for night-time ocean surveys. The camera, which employs a 40 mm silicon intensifier target camera tube, is being flown at altitudes up to 6000 ft by the US National Marine Fisheries Service in a new approach to the

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Optics and

Laser Technology

Fig. 2

Fault in overhead

transmission

line shows up on the TV picture

Errata Readers may have been confused by errors which occured in Mr. D. Denby’s paper, ‘A holographic interferometer comparable with an in-line reference field laser speckle interferometer’. (November issue, p 220) The penultimate sentence in Mr Denby’s third paragraph should read as follows: ‘In the case where the imaging axis is normal to the hologram plane, and the reference beam is inclined at an angle 01to this axis, the amount of lateral blur, Ax, of image points at a distance 2 from the hologram plane due to a wavelength spread Ah is given to a good approximation by: etc.

Ax = Zsincusah h ’

Another error occurred in R. L. Van Renesse’s paper ‘Photopolymers in holography’ in the February issue. The correct symbol for diffraction efficiency is Q, and not { (p 26, paragraph 2), and the unit on the vertical axis of Fig. 2 should have been dq and not ~1/2

June 1972