Ninth European conference on optical communication

through the basic principles of photoelasticity using transparent models in which, by viewing through polarizers, fringes can be seen related to the stress patterns in the subject under stress. They use two materials, one is Araldite and the other a soft plastic. Their respective stress velocities are 2000 and 200 m s-L Thus the slower material permits lower camera speeds. They also have used cine-camera and strobes at 8000 pps and showed good pictures in the slow material of models of various forms and stress. To reach higher camera rates they are making a Cranz-Schardin system with nine channels using a Pulse-Photonics spark array. Each spark will be of 0.5/,ts duration and 2.5 J power: they intend to use a megahertz rate. The final trio of papers were all in the same "road" as they were all about forms of transport and the necessary tests to try to make them as safe as is mechanically possible. It was not for the faint hearted as the things they do to vehicles are too close to the things vehicles can do to us. First was Mr G. Tidbury for himself and his colleague Mr R. Fry on "High speed photography in the development of bus superstructure strength regulations" at CIT Cranfield. Principally he showed a film of the study of materials and structure shapes with records of deliberate crashes of buses of various periods and designs. The camera speed was normally 500 pps and the buses were often slid down ramps with obstructions introduced to cause them to roll over. They included examples in which they failed to reach the intended result showing that sometimes it is not so easy to overcome the inherent stability of some vehicles. They gave particular care to the effect of striking curbs and motorway barriers, rolling over and the effects on the passenger space, and the behaviour of springs and other components. Some of the tests were at the Motor Industry Research Establishment from where came our next paper. This was by Mr E. Bone who told us of their facilities. They do not only concern themselves with road vehicles but have studied helicopters and his film began with a rare event, the deliberate dropping of a helicopter. He outlined the amount of instrumentation involved in a single vehicle test. Obviously each vehicle is not of much value afterwards and, as prototype models cost into five figures, it is necessary to record as many facets and acquire as much data as possible. This means a lot of instrumentation an~d good control. To study a car they use 160 kW of lighting, some stationary and some attached to the test vehicle. They use Photosonics and Stalex ruggedized cameras, especially for those on the test car. Cine speed is

normally 1000 pps. The still photographers have become especially skilled at taking shots at the crucial moment in a collision test - the moment about 20 ms after impact Excellent film and still examples were shown. The final speaker was Mr S. Petty of the Transport and Research Labs on "The use of film analysis in car impact testing'. TRL has just celebrated its 50th anniversary but is at present contracting down to a staff of 700 or less. They consider all vehicles down to cycles but he selected one topic to illustrate analysis: the 40% frontal impact between private cars, the most dangerous accident for drivers and passengers. First they determined typical results using two cars and then sought an obstruction so that only one car needed to be destroyed in each test. Their normal instrumentation includes video, chronographs, accelerometers and cine cameras,-with a preference for the Millikan with claw for its ruggedness and frame stability for good analysis. 400 pps is the standard camera rate onto Videochrome positive colour film at 160 ASA. For quality pictures they use 70 m m film at 50 pps and action stills, where experienced operators are essential, and even on some occasions in danged For instant study they use the Polaroid cine system which is instant enough. Correlation of car and d u m m y details are essential and much use of code marking is made, so that accurate velocities and directions are calculable. For example they have shown that in an accident, car seat belts stretch up to 150 ram. Try your seat belt that much looser, and see where you may be when it matters. After lunch at St Peters, with the manufacturers exhibition, the members had the choice of going home, visiting the Engineering Science Labs or, as I did, visiting the Oxford Scientific Films Laboratory. Having often admired their magnificent nature films on television a visit to their base was a real pleasure as well as an education, particularly with their "ground level" insect eye-view system where a matchbox is a sky-scraper and small animals become prehistoric monsters. They are now also applying their skills in advertising films and cartoon-like productions involving optical table top cine mixing of subjects of disparate sizes into realistic scenes at apparently matching sizes. Their films are amazing especially when the background skills and tricks are also seen, and even more so when one has the background knowledge to appreciate the small margin between success and failure. G.H. Lunn

Ninth European conference on optical communication Geneva, Switzerland, 23-26 October, 1983 Approximately 1000 delegates attended the ninth ECOC in Geneva. The conference was spread over three days and included papers on fibre technology, fibre splicing and coupling, long and short wavelength diode lasers, integrated optics and fibre systems. The first session on long wavelength diode lasers was

OPTICS AND LASER TECHNOLOGY. FEBRUARY 1984

opened with an invited paper by Ikegami and Motosugi of N I T on single longitudinal mode lasers. Particular attention was given to distributed feedback (DFB), distributed Bragg reflector (DBR) and multi cavity lasers, The authors concluded that the DFB was simplest in structure and operation and was a likely

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