molten metal and oxides whose temperature is main- tained during its ejection by the oxygen jet. Although the laser radiat ion ra i ses the surface metal to a temperature much higher than that required for burn- ing by exothermic react ion with oxygen, the fi lm showed that the react ion was sustained only for a very short t ime after the cessat ion of the light energy pulse. Cutting rates depend on laser power and thick- ness Table 2. A continuous laser has been developed at SERL for The Welding Institute for further studies of metal cutting. Power output is over 400W from 7m-long tube.
The role of the carbon dioxide laser in metal welding was d iscussed by B. F. Scott (Birmingham University). When laser radiat ion is incident upon a surface, the thermal p rocesses induce var iat ions in the physical propert ies of the mater ia l which depend on the power intensity and its distr ibut ion within the focused spot. A cr i t ica l ly defined threshold intensity exists at which surface effects cause the apparent absorption to be high, even in the highly ref lect ing metals. Below threshold, absorption is low and diffusion losses from the heated region are signif icant. Welds formed in this conduction- l imited mode have low depth-width ratio and poor penetration; the welding speed is low.
Superheating occurs above threshold, allowing vapour nucleation within the body of the liquid phase. A mechanism enhancing energy t ransfer is induced and
the process is no longer conduction l imited. There is an analogy, albeit incomplete, with the hole translat ing mechanism observed in electron beam welding. As temperature is increased, the ref lect iv i ty of most metals for 10.6pm radiation decreases . With higher absorption, reduced diffusion loss and rapid energy transfer , the depth-to-width ratio and penetration increase substantial ly and t ravers ing speeds are higher.
The superheating threshold has been reached with so l id -state systems. The achievement of good pene- tration and weld speed with the carbon dioxide laser depends on the development of devices with higher power intensit ies and it could be that the single mode laser , when developed, will play an important role in deep welding processes .
In addition to the carbon dioxide lasers already descr ibed, a s ingle-mode laser designed for propa- gation experiments was on view at the laborator ies . This ful ly-engineered system was arranged for cutting operations of cloth, paper and plast ic. Radiation at 10pro is also potential ly useful for distance and velocity measurements in engineering st ructures and equipment. Inter ferometry at this wavelength gives measurement accurac ies of a fraction of a thousandth of an inch with s imple fringe counting equipment and a smal l (15cm long) laser giving one watt output is being developed in conjunction with the National Engineering Laboratory and the National Physical Laboratory for this purpose.
M. Hi l l ier
HIGH- SPEED PHOTOGRAPHY CONGRESS
Stockholm, 23-29 June 1968
SINCE 1952 there have been regular international conferences on high-speed photography. These have been held at approximately two-year ly intervals and have provided a forum for the exi3hange of information about latest developments and applications. The most recent conference, the eighth, was held at St. Er iks Massan, Stockholm, and organised by the Royal Swedish Academy of Engineering Sciences with the Research Institute of National Swedish Defence. There were almost 500 part ic ipants f rom 22 countr ies and over 120 papers were del ivered. . These included invited lectures on photo-e lectronic image devices, hologra- phy and f ibre optics. Twenty four orgaaisat ions f rom eight countr ies tookpart in the commerc ia l exhibition.
Though the papers presented were gathered under ten separate headings most progress had been made in the field of photo-e lectronic image devices and holo- graphy.
IMAGE CONVERTERS AND ELECTRONIC SHUTTERS
One of the most interest ing of the 17 papers was given by A. V. Krause and W. P. Raffan 1 who descr ibed
a new image tube known as a 'shutter p lanar image converter ' . This had been devised to take advantage of the efficient total absorption type of photocathode. Because it requires a solid substrate the photocathode is made in the form of a mesh through which the photo-electrons can travel to the screen. Shuttering is accomplished by an electrode roughly analogous to the grid of a t rade valve. In this case however the 'gr id ' is situated on the oppos i te side of the cathode and consists of a t ransparent conducting layer.
G. Eshard and R. Po laert 13 descr ibed equipment in which proximity diodes with $20 and S1 photocathodes could be manufactured using the photocathode t rans- fer method. In another paper T. H. Bulpitt 6 descr ibed a convenient portable camera system which reaches far towards the goal of recording on fi lm each photo- electron leaving the photocathode. The system con- s is ts of a two-stage image converter tube containing an image converter and a proximity diode in one envelope with integral f ibre optic read-out.
Other papers in this section were devoted to the deve- lopment of systems descr ibed at ear l ie r congresses. For example R. W. Smith 2 descr ibed the l imitat ions of an image tube which had been featured in the pre-
Optics Technology November 1968 4.7
vious congress. This tube consists of a dynamic e lectron image store working in conjunction with a cascade image intensi f ier . In effect a h igh-speed event is stored as a s t ream of e lectrons which is then sampled piece by piece. Smith presented f raming sequences taken at 5 x l0 s f rames per second with this tube.
ROTATING MIRROR CAMERAS
A paper by R. E. Rowlands and others 72 descr ibed the development of an u l t ra -h igh-speed mult ip le f raming system. This used a sequent ia l ly -modulated ruby laser in conjunction with a s t reak camera. Mult iple light pulses from the laser provide the shut- ter ing action and exposure t imes are short enough for no image motion compensat ion to be required. A big advantage is that the exposure t ime is not affected by the f raming rate.
P. B. N. Nutta l l -Smith 34 descr ibed a rotat ing mi r ro r f raming camera with continuous access at an aper- ture of about f/18 being designed and constructed at the Royal A i rc ra f t Estab l i shment . The image t rans - fer system uses an ar ray of fixed plane mi r ro rs and a s ingle t rans fer lens. Th is lens rotates with the rotat ing mi r ro r . The design provides 240 f rames and f raming rates from 2 500 to 250 000 f rames per second, that is to say, the speed range immediate ly above that of the rotat ing pr i sm type of camera. The major advantage Of this design is its continuous access- - i t is not necessary to wait for a par t i cu la r phase of the rotat ion of the mi r ro r as it is in most other rotat ing mi r ro r systems.
TIME RESOLVED SPECTROSCOPY
W. A. Wal ler 44 descr ibed the use of a stored charge image reader with a t ime reso lv ing graz ing spectro- graph. The spectra l image is stored on the photo- cathode surface in the form of an ar ray of posit ive charges. These charges can be scanned and readout non-destruct ive ly .
A. Bardocz 4s descr ibed a neat mechanica l shutter device which can be placed between the light source and the s l i t of an ast igmat ic spectrograph to provide t ime resolut ion. The shutter takes the form of a rotat ing disc upon which an Arch imodes sp i ra l slot has been machined. The steeper the sp i ra l the greater is the t ime resolut ion of the system.
FLASH LIGHT SOURCES
There were many papers on f lash light sources including v is ib le, in f ra - read , u.v. and x - ray sources as well as lasers . For example F. D. Harr ington descr ibed a t ime resolved spectroscopic invest igat ion of a quartz l inear f lash tube and there were many other papers devoted to the spectroscopic study of var ious types of f lash tube.
A tube consist ing of compact meander -shaped cap- i l la r ies with a f lash durat ion of approximately a mi l lesecond and a radiat ion beam of about five degrees was descr ibed by F. F runge l and H. G. Patzke 57. It was c la imed capable of emmit t ing through an RG780 f i l ter suff ic ient energy to adequately expose commer-
cial ly avai lable black and white in f ra - red f i lms to permi t i - r f lash photography at 420ft at f/5. Appl i - cation to c r ime detect ion was mentioned.
F ie ld Emiss ion Corporat ion exhibited their 600kV, 3ns x - ray f lash unit which could provide s imultaneous x - ray and e lectron beam radiographs. Objects made to f luoresce under e lectron i r rad iat ion could also be s imul taneous ly photographed by the v is ib le l ight thus emitted. A paper on this subject was given by J. L. Brewster and others. 61.
S. K. Handel and B Stenerhag 65 said that by exploding a thin tungsten wire e lect r ica l ly in vaccuo, u l t ra - fas t and reproducib le x - ray pulses of 20 x 10 -9 sec half- width are emitted. They descr ibed an invest igat ion of this phenomenon and the resu l ts obtained indicates that the x - ray pulse can penetrate about 10mm of a lumin ium at 25 kV init ia l capacitor voltage.
In holography l ight waves leaving an object are re - corded in both ampl i tude and phase. This is done by i l luminat ing the object coherent ly and heating the scattered light waves with a coherent re ference wave to form an in ter fe rence pattern on a photographic plate. When the plate is developed and i l luminated by a rep l ica of the re ference wave the object waves are reconstructed. These travel out f rom the plate as if they had neverbeen interrupted and can be made to do pract ica l ly everything the or ig ina l waves could have done. In par t i cu la r when the waves fall on the eye its lens focuses them and one sees the or ig ina l object. In pr inc ip le therefore holography allows a rapidly changing event to be ' f rozen ' and then recon- st ructed as a stat ic th ree-d imens iona l scene. Mea- surements can then be made on the reconst ruct ion at le i sure .
Ten papers were devoted to h igh-speed holography most us ing the or ig inal Gabor ' in - l ine ' system with pulsed ruby or neodymium lasers as light sources and exposure t imes in the 10 to 30ns range. Some working mul t i f rame systems were presented. For example three f rames 2.5 107 f rames per second us ing optical delays 7o and ten f rames at 104 f rames per second using a rotat ing pr i sm 76.
Several papers pointed out the advantages of mult iple exposure holography. A paper f rom AWRE 79 des - cr ibed holograms made of explosive phenomena in which the pulsed laser was synchron ised to the f ir ing of an explosive charge. R.O. Buzzard s5 descr ibed the advantages of record ing wavefronts holographical ly pr ior to the use of a sharp focusing Schl ieren system developed by Kantrowitz and Tr imp i . This detects only the density gradient in a given plane but is in- herent ly more diff icult to adjust than a convent ional Schl ieren system and in any case is l imi ted to exam- ining only one plane dur ing a given test run. Both of these disadvantages can be overcome us ing holography The reconst ructed waves are used for adjust ing the Schl ieron knife edges and for examining the var ious planes of interest . Because its effect can be mon- i tored d i rect ly knife edge ad justment is great ly s im- plif ied.
An appl icat ion of holographic techniques in optical in format ion process ing to provide better d isplays of mu l t i - image p ictor ia l in format ion was demonstrated
48 Optics Technology November 1968
at the A ldermaston stand in the exhibit ion. A large number of o rd inary photographs (for example taken of an object f rom dif ferent angles) were in a second step combined holographical ly onto a s ingle plate which when viewed under proper condit ions presents a th ree-d imens iona l image of the object. The pr in - ciple advantage is that say photographic technique can be used for the f i rs t step, whereas the holographic process in the second step can always be car r ied out under contro l led laboratory condit ions. Holograms of large scenes as well as x - ray holograms produced in this way were displayed. A large number of p ic tures taken of an object at di f ferent t imes could in the same manner be combined on to a s ingle hologram which when viewed f rom different angles would reconst ruct images of the object as recorded at di f ferent t imes. This might fac i l i tate the viewing and evaluation of high speed as well as t ime lapse photographic re - cordings.
Towards the end of the Congress L. J. Po ldervaar t 10o gave a very well p resented contr ibut ion on the use of synchro s t robe te lev is ion in the invest igat ion of per iodic p rocesses up to 1MHz. The l ight source (a Nonolite with an exposure of 20 nanoseconds) , camera and moni tor are synchron ised by a s ignal from the object. The synchron is ing f requency is ad- justed to the working f requency of two moni tor system by a f requency div ider so as to provide a st i l l p icture. A delay l inear ly vary ing with t ime revea ls the object in slow motion.
A. De Velpi 132 said that record ing digital data with the aid of h igh-speed cameras had reached a new and s igni f icant phase. He mainta ined that in cer ta in appl icat ions photographic record ing is now super io r to magnet ic record ing and descr ibed exper imenta l work which supported this view.
When bit ra tes of a few hundred thousand per second are exceeded the l imi t ing character i s t i cs of magnet ic media become data saturat ion slowing, bit dropout, high cost of read- in unit and incompat ib i l i ty with computer in -put format. De Volpi reported on a 'd igital photographic data record ing stat ion'developed at the Argonne National Laboratory which c i rcum- vents these prob lems and can accummulate data at 3 108 bits per second--above the top l imi t on com- merc ia l magnet ic equipment. The stat ion features a h igh-speed f raming camera synchron ised with a panel of 720 neon lamps. Total storage capacity of the HYCAM f raming camera was said to be 30 bi l l ion bits for a 20-second exper iment at 5 000 f rames per second.
Though the Congress as a whole can certa in ly be counted a success the way in which papers were del ivered left something to be desi red. Most authors were al lowed only ten minutes presentat ion t ime and to ass i s t the in terpreters the speakers were encour- aged to read d i rect ly f rom their wr i t ten papers. This resul te...