molten meta l and oxides whose t e m p e r a t u r e is main- tained during i ts e jec t ion by the oxygen jet . Although the l a s e r rad ia t ion r a i s e s the su r f ace me ta l to a t e m p e r a t u r e much higher than that r e q u i r e d for burn- ing by exo thermic reac t ion with oxygen, the f i lm showed that the r eac t ion was sus ta ined only for a very shor t t ime a f te r the ces sa t ion of the l ight energy pulse. Cutting r a t e s depend on l a s e r power and thick- ness Table 2. A continuous l a s e r has been developed at SERL for The Welding Ins t i tu te for fu r the r s tudies of meta l cutting. Power output is over 400W from 7m-long tube.

WELDING

The ro l e of the carbon dioxide l a s e r in me ta l welding was d i s c u s s e d by B. F. Scott (Bi rmingham Univers i ty) . When l a s e r rad ia t ion is incident upon a sur face , the the rmal p r o c e s s e s induce va r i a t ions in the phys ica l p r o p e r t i e s of the m a t e r i a l which depend on the power intensi ty and i ts d i s t r ibu t ion within the focused spot. A c r i t i c a l l y defined threshold intensi ty exis t s at which sur face effects cause the apparen t absorp t ion to be high, even in the highly re f lec t ing me ta l s . Below threshold , absorp t ion is low and diffusion los ses f rom the heated region a re s ignif icant . Welds formed in this conduc t ion- l imi ted mode have low depth-width rat io and poor penet ra t ion; the welding speed is low.

Superheating occurs above threshold , allowing vapour nucleat ion within the body of the l iquid phase. A mechan i sm enhancing energy t r a n s f e r is induced and

the p r o c e s s is no longer conduction l imi ted . There is an analogy, a lbe i t incomplete , with the hole t r ans la t ing mechanism observed in e lec t ron beam welding. As t e m p e r a t u r e is i nc rea sed , the re f l ec t iv i ty of most me ta l s for 10.6pm rad ia t ion d e c r e a s e s . With higher absorpt ion , reduced diffusion loss and rap id energy t r ans f e r , the depth- to-wid th ra t io and pene t ra t ion i nc rea se subs tan t ia l ly and t r a v e r s i n g speeds a r e higher.

The superhea t ing threshold has been reached with s o l i d - s t a t e sy s t ems . The achievement of good pene- t ra t ion and weld speed with the carbon dioxide l a s e r depends on the development of devices with higher power in tens i t ies and it could be that the s ingle mode l a se r , when developed, will play an impor tan t ro le in deep welding p r o c e s s e s .

In addit ion to the carbon dioxide l a s e r s a l r eady desc r ibed , a s ing l e -mode l a s e r des igned for p ropa- gation exper iments was on view at the l a bo r a t o r i e s . This fu l ly -eng ineered sys tem was a r r a n g e d for cutting opera t ions of cloth, paper and p las t i c . Radiat ion at 10pro is a lso potent ia l ly useful for d i s tance and veloci ty m e a s u r e m e n t s in engineer ing s t r u c t u r e s and equipment. I n t e r f e r o m e t r y at this wavelength gives m e a s u r e m e n t a c c u r a c i e s of a f rac t ion of a thousandth of an inch with s imple f r inge counting equipment and a sma l l (15cm long) l a s e r giving one watt output is being developed in conjunction with the National Engineer ing Labora to ry and the National Phys i ca l Labora to ry for this purpose .

M. Hi l l i e r

HIGH- SPEED PHOTOGRAPHY CONGRESS

Stockholm, 23-29 June 1968

SINCE 1952 the re have been r egu la r in te rna t iona l conferences on h igh-speed photography. These have been held at approx ima te ly two-yea r ly in t e rva l s and have provided a forum for the exi3hange of informat ion about l a t e s t developments and appl ica t ions . The mos t recen t conference , the eighth, was held at St. E r ik s Massan, Stockholm, and o rgan i sed by the Royal Swedish Academy of Engineer ing Sciences with the R e s e a r c h Inst i tute of Nat ional Swedish Defence. The re were a lmos t 500 p a r t i c i p a n t s f r o m 22 coun t r i e s and over 120 p a p e r s were de l ive red . . These included invi ted l ec tu re s on pho to -e l ec t ron i c image devices , ho log ra - phy and f i b r e opt ics . Twenty four o rgaa i s a t i ons f rom eight coun t r i e s t o o k p a r t in the c o m m e r c i a l exhibit ion.

Though the pape r s p r e sen t ed were ga thered under ten s e p a r a t e headings mos t p r o g r e s s had been made in the f ield of pho to -e l ec t ron i c image dev ices and holo- graphy.

IMAGE CONVERTERS AND ELECTRONIC SHUTTERS

One of the mos t i n t e r e s t i ng of the 17 pape r s was given by A. V. K r a u s e and W. P. Raffan 1 who desc r ibed

a new image tube known as a ' shu t t e r p l ana r image conve r t e r ' . This had been devised to take advantage of the efficient total absorp t ion t y p e of photocathode. Because it r e q u i r e s a sol id s u b s t r a t e the photocathode is made in the form of a mesh through which the pho to-e lec t rons can t r ave l to the sc reen . Shuttering is accompl i shed by an e l ec t rode roughly analogous to the gr id of a t r ade valve. In this case however the ' g r id ' is s i tuated on the o p p o s i t e s ide of the cathode and cons is t s of a t r a n s p a r e n t conducting l ayer .

G. Esha rd and R. P o l a e r t 13 d e s c r i b e d equipment in which p rox imi ty diodes with $20 and S1 photocathodes could be manufactured using the photocathode t r a n s - fer method. In another paper T. H. Bulpit t 6 de sc r ibed a convenient por tab le c a m e r a sys t em which r eaches far towards the goal of r eco rd ing on f i lm each photo- e lec t ron leaving the photocathode. The sys t em con- s i s t s of a two- s t age image conve r t e r tube containing an image conve r t e r and a p rox imi ty diode in one envelope with in teg ra l f ibre optic r ead-ou t .

Other pape r s in this sec t ion were devoted to the deve- lopment of s y s t e m s desc r ibed at e a r l i e r c o n g r e s s e s . F o r example R. W. Smith 2 de sc r ibed the l imi ta t ions of an image tube which had been fea tured in the p r e -

Optics Technology November 1968 4.7

vious cong re s s . This tube cons i s t s of a dynamic e l ec t ron image s t o r e working in conjunc t ion with a cascade image i n t ens i f i e r . In effect a h igh - speed event is s t o r ed as a s t r e a m of e l e c t rons which is then sampled p iece by piece. Smith p r e s e n t e d f r a m i n g s equences taken at 5 x l0 s f r a m e s per second with this tube.

ROTATING MIRROR CAMERAS

A paper by R. E. Rowlands and o the r s 72 d e s c r i b e d the deve lopment of an u l t r a - h i g h - s p e e d mul t ip le f r a m i n g sy s t em. Th i s used a s e q u e n t i a l l y - m o d u l a t e d ruby l a s e r in con junc t ion with a s t r e a k c a m e r a . Mul t ip le l ight pu l ses f rom the l a s e r p rov ide the shut - t e r i n g ac t ion and exposure t i m e s a r e shor t enough for no image mot ion compensa t i on to be r e q u i r e d . A big advantage is that the exposure t ime is not affected by the f r a m i n g ra te .

P. B. N. Nu t t a l l -Smi th 34 d e s c r i b e d a ro t a t ing m i r r o r f r a m i n g c a m e r a with cont inuous acces s at an a p e r - t u r e of about f /18 being des igned and cons t ruc t ed at the Royal A i r c r a f t E s t a b l i s h m e n t . The image t r a n s - fer s y s t e m uses an a r r a y of fixed p lane m i r r o r s and a s ing le t r a n s f e r l ens . T h i s l ens ro t a t e s with the ro t a t ing m i r r o r . The des ign p rov ides 240 f r a m e s and f r a m i n g r a t e s f rom 2 500 to 250 000 f r a m e s pe r second, that is to say, the speed r ange i m m e d i a t e l y above that of the ro t a t ing p r i s m type of c a m e r a . The m a j o r advantage Of this des ign is i ts cont inuous acces s - - i t is not n e c e s s a r y to wait for a p a r t i c u l a r phase of the ro ta t ion of the m i r r o r as i t is in mos t o ther ro ta t ing m i r r o r s y s t e m s .

TIME RESOLVED SPECTROSCOPY

W. A. Wa l l e r 44 d e s c r i b e d the use of a s t o r ed charge image r e a d e r with a t i m e r e s o l v i n g g r az ing s p e c t r o - graph. The s p e c t r a l image is s to red on the photo- cathode su r f ace in the fo rm of an a r r a y of pos i t ive cha rges . These cha rges can be s canned and readout n o n - d e s t r u c t i v e l y .

A. Bardocz 4s d e s c r i b e d a nea t m e c h a n i c a l shu t t e r device which can be p laced be tween the l ight s o u r c e and the s l i t of an a s t i g m a t i c s p e c t r o g r a p h to p rov ide t ime r e so lu t ion . The shu t t e r takes the fo rm of a ro t a t ing d i sc upon which an A r c h i m o d e s s p i r a l s lo t has been machined . The s t e e p e r the s p i r a l the g r e a t e r is the t ime r e s o l u t i o n of the s y s t e m .

FLASH LIGHT SOURCES

T h e r e were many pape r s on f lash l ight s o u r c e s inc lud ing v i s ib l e , i n f r a - r e a d , u.v. and x - r a y s o u r c e s as well as l a s e r s . F o r example F. D. H a r r i n g t o n d e s c r i b e d a t ime r e s o l v e d spec t ro scop i c i nves t iga t ion of a quar tz l i n e a r f lash tube and the re were m a n y o ther pape r s devoted to the spec t ro scop i c s tudy of va r i ous types of f lash tube.

A tube cons i s t i ng of compact m e a n d e r - s h a p e d cap- i l l a r i e s with a f lash du ra t ion of app rox ima te ly a m i l l e s e c o n d and a r ad ia t ion beam of about f ive d e g r e e s was d e s c r i b e d by F. F r u n g e l and H. G. Pa tzke 57. It was c l a imed capable of e m m i t t i n g th rough an RG780 f i l t e r suf f ic ien t ene rgy to adequate ly expose c o m m e r -

c ia l ly ava i l ab le b lack and white i n f r a - r e d f i lms to p e r m i t i - r f l a sh photography at 420ft at f /5 . Appl i - cat ion to c r i m e de tec t ion was men t ioned .

F i e l d E m i s s i o n C o r p o r a t i o n exhibi ted the i r 600kV, 3ns x - r a y f lash uni t which could p rov ide s i m u l t a n e o u s x - r a y and e l ec t ron beam r ad iog raphs . Objec ts made to f l u o r e s c e unde r e l ec t ron i r r a d i a t i o n could a lso be s i m u l t a n e o u s l y photographed by the v i s i b l e l ight thus emi t ted . A pape r on this sub jec t was g iven by J . L. B r e w s t e r and o thers . 61.

S. K. Handel and B S tenerhag 65 sa id that by exploding a thin tungs t en wi re e l e c t r i c a l l y in vaccuo, u l t r a - f a s t and r e p r o d u c i b l e x - r a y pu l ses of 20 x 10 -9 sec half- width a re emit ted . They d e s c r i b e d an inves t iga t ion of this phenomenon and the r e s u l t s ob ta ined ind ica tes that the x - r a y pu l se can p e n e t r a t e about 10mm of a l u m i n i u m at 25 kV in i t i a l capac i to r vol tage.

HOLOGRAPHY

In holography l ight waves l eav ing an ob jec t a r e r e - corded in both ampl i tude and phase . Th i s is done by i l l u m i n a t i n g the ob jec t cohe ren t ly and hea t ing the s c a t t e r e d l ight waves with a cohe ren t r e f e r e n c e wave to fo rm an i n t e r f e r e n c e p a t t e r n on a photographic plate. When the p la te is developed and i l l u m i n a t e d by a r e p l i c a of the r e f e r e n c e wave the objec t waves a re r e c o n s t r u c t e d . The se t r ave l out f rom the p la te as if they had n e v e r b e e n i n t e r r u p t e d and can be made to do p r a c t i c a l l y eve ry th ing the o r i g i n a l waves could have done. In p a r t i c u l a r when the waves fal l on the eye its lens focuses them and one s ee s the o r i g i n a l object . In p r i n c i p l e t he r e f o r e ho lography al lows a rap id ly changing event to be ' f r o z e n ' and then r e c o n - s t r u c t e d as a s t a t i c t h r e e - d i m e n s i o n a l s cene . Mea- s u r e m e n t s can then be made on the r e c o n s t r u c t i o n at l e i s u r e .

Ten pape r s were devoted to h i g h - s p e e d holography mos t u s ing the o r ig ina l Gabor ' i n - l i n e ' s y s t e m with pu lsed ruby or ne odymi um l a s e r s as l ight s o u r c e s and exposure t i me s in the 10 to 30ns r ange . Some working m u l t i f r a m e s y s t e m s were p r e s e n t e d . F o r example th ree f r a m e s 2 .5 × 107 f r a m e s pe r second u s i n g opt ica l de lays 7o and ten f r a m e s at 104 f r a m e s pe r second us ing a ro t a t i ng p r i s m 76.

Severa l p a p e r s pointed out the advan tages of mul t ip le exposure holography. A paper f r o m AWRE 79 d e s - c r i bed ho l og r a ms made of explos ive phenomena in which the pu l sed l a s e r was s y n c h r o n i s e d to the f i r ing of an explos ive charge . R .O. B u z z a r d s5 d e s c r i b e d the advantages of r e c o r d i n g wavef ron t s ho lograph ica l ly p r i o r to the use of a sha rp focus ing Sch l i e ren sys t em developed by Kant rowi tz and T r i m p i . This de tec t s only the dens i ty g r ad i en t in a given p lane but is in- h e r e n t l y m o r e di f f icul t to ad jus t than a conven t iona l Sch l i e ren s y s t e m and in any case is l i m i t e d to exam- ining only one p lane dur ing a given tes t run . Both of these d i sadvan tages can be o v e r c o m e us i ng holography The r e c o n s t r u c t e d waves a r e used for ad jus t ing the Sch l ie ron knife edges and for e xa mi n i ng the va r i ous p lanes of i n t e r e s t . Because i ts effect can be mon- i to red d i r ec t l y knife edge a d j u s t m e n t is g r ea t ly s im- pl i f ied.

An app l ica t ion of ho lographic t echn iques in opt ica l i n f o r ma t i on p r o c e s s i n g to p rov ide b e t t e r d i sp l ays of m u l t i - i m a g e p i c t o r i a l i n f o r m a t i o n was demons t r a t ed

48 Optics Technology November 1968

at the A l d e r m a s t o n s tand in the exhibi t ion . A l a rge n u m b e r of o r d i n a r y photographs (for example taken of an objec t f r o m d i f fe ren t angles) we re in a second step combined ho lograph ica l ly onto a s ing le p la te which when viewed u n d e r p r o p e r condi t ions p r e s e n t s a t h r e e - d i m e n s i o n a l image of the object . The p r i n - ciple advantage is that say photographic technique can be used for the f i r s t s tep, whe rea s the ho lographic p r o c e s s in the second s tep can a lways be c a r r i e d out under c o n t r o l l e d l a b o r a t o r y condi t ions . Ho log rams of l a rge s c e n e s as wel l as x - r a y h o l o g r a m s produced in this way were d i sp layed . A l a rge n u m b e r of p i c t u r e s taken of an ob jec t at d i f fe ren t t imes could in the s a m e m a n n e r be combined on to a s ing le h o l o g r a m which when v iewed f r o m d i f fe ren t angles would r e c o n s t r u c t images of the ob jec t as r e c o r d e d at d i f f e ren t t imes . This migh t f ac i l i t a t e the v iewing and eva lua t ion of high speed as well as t ime lapse photographic r e - cord ings .

Towards the end of the C o n g r e s s L. J. P o l d e r v a a r t 10o gave a v e r y well p r e s e n t e d con t r i bu t i on on the u se of synchro s t r o b e t e l e v i s i o n in the i nves t i ga t i on of pe r iod ic p r o c e s s e s up to 1MHz. The l ight s o u r c e (a Nonol i te with an exposu re of 20 nanoseconds ) , c a m e r a and m o n i t o r a r e s y n c h r o n i s e d by a s igna l f rom the object . The s y n c h r o n i s i n g f r equency is ad- jus ted to the work ing f r equency of two m o n i t o r s y s t e m by a f r e q u e n c y d iv ide r so as to p rov ide a s t i l l p i c tu r e . A de lay l i n e a r l y v a r y i n g with t ime r e v e a l s the ob jec t in slow mot ion .

A. De Velpi 132 sa id that r e c o r d i n g d ig i ta l data with the aid of h i g h - s p e e d c a m e r a s had r e a c h e d a new and s ign i f i can t phase . He m a i n t a i n e d that in c e r t a i n app l ica t ions photographic r e c o r d i n g is now s u p e r i o r to m a g n e t i c r e c o r d i n g and d e s c r i b e d e x p e r i m e n t a l work which suppor ted this view.

When bi t r a t e s of a few hundred thousand pe r second a re exceeded the l i m i t i n g c h a r a c t e r i s t i c s of magne t i c med ia b e c o m e data s a t u r a t i o n s lowing, bit dropout , high cos t of r e a d - i n un i t and i ncompa t ib i l i t y with c o m p u t e r i n - p u t fo rma t . De Volpi r e p o r t e d on a ' d ig i t a l pho tographic data r e c o r d i n g s t a t i on ' deve loped at the A r g o n n e Nat iona l L a b o r a t o r y which c i r c u m - vents these p r o b l e m s and can a c c u m m u l a t e data at 3 × 108 bi ts p e r second--above the top l i m i t on c o m - m e r c i a l m a g n e t i c equipment . The s t a t ion f e a t u r e s a h i g h - s p e e d f r a m i n g c a m e r a s y n c h r o n i s e d with a pane l of 720 neon l amps . To ta l s t o r age capac i ty of the HYCAM f r a m i n g c a m e r a was sa id to be 30 b i l l ion bi ts for a 20 - second e x p e r i m e n t at 5 000 f r a m e s pe r second.

Though the C o n g r e s s as a whole can c e r t a i n l y be counted a s u c c e s s the way in which p a p e r s were de l i ve r ed lef t some th ing to be de s i r ed . Most au thors were a l lowed only ten m i n u t e s p r e s e n t a t i o n t ime and to a s s i s t the i n t e r p r e t e r s the s p e a k e r s we re e n c o u r - aged to r ead d i r e c t l y f rom the i r w r i t t e n pape r s . This r e s u l t e d in a f la t p r e s e n t a t i o n and ( p a r t i c u l a r l y in the ca se of p a p e r s in G e r m a n ) to a c o n s i d e r a b l e loss of i n t e l l i g ib i l i t y as the i n t e r p r e t e r s r epea t ed ly fa i led to keep up with the s p e a k e r s . Another c r i t i c i s m is that al though au tho r s f rom the Soviet Union p rov ided about a q u a r t e r of the p a p e r s , R u s s i a n was not an official c o n f e r e n c e language.

The next C o n g r e s s wil l be held in Denver , Colorado in 1970.

J . D. R e d m a n

REFERENCES

I. Electron Tubes for Fast Shuttering, A. V. Krause and W. P. Raffan (Twentieth Century Electronics Ltd)

2. 'Recent Developments of the High-Speed Image Intensifier Framing Camera', R. W. Smith (Imperial College, London)

6. 'Image Converter Camera System for Single Photoelectron Recording' T. H. Bulpitt, (TRW Inc .)

13. 'Tube obturateur electrofiique pour des temps de pose dune nano seconde', G. Escha'rd et Remy Polaert (Laboratoire d'Electronique et de Physique appliques, France)

34. 'Rotating mirror framing cameras with con- tinuous access, ' P. B. N. Nuttall-Smith (RAE)

44. 'A Time Resolving Grazing Incidence Spectro- graph', W. A. Waller, AWRE, Aldermaston)

48. 'Mechanical Device for High-Speed Spectro- graphy', A. Bardocz, (Institut fur Plasmaphysik, Garching bei Munchen, Germany,.

50. 'Time-Resolved Spectroscopic Investigation of a Flash Lamp, F. D. Harrington, (U.S. Naval Research Lab. Washington)

57. 'Long Distance Infrared Flash Photography', F. Frungel & H. G. Patzke, (Impulsphysik GmbH, Hamburg)

]1. '3 Nanosecond Exposures with X-Ray, Electrons and Light, 'J. L. Brewster, J. P. Barbour, F. J. Grundhauser, W. P. Dyke, (Field Emission Corporation, USA)

65. 'A new Method for Production of Ultrafast Reprgducible X-Ray Pulses', S. K. Handel and B. Stenerhag, (University of Uppsala, Sweden)

72. 'Ultrahigh-speed Framing Photography Employ- ing a Multiply-Pulsed Ruby Laser and a 'Smear Type' Camera: Application to Dynamic Photo- elasticity', R. E. Rowlands, C. E. Taylor* and I. M. Daniel L (*I.IT Research Institute, Chicago, Illinois ~ University of Illinois, Urbana, Illinois)

76. 'Holographic Recording of Rapid Transient Events and the Problems of Evaluation of the Reconstructions' J.W.C. Gates, R. G. N. Hall, and I. N. Ross, (National Physical Laboratory)

79. 'The Use of Holography to Measure the Position and Velocity of Rapidly Moving Objects.' J. D. Redman and N. A. Lowe, (AWRE, Alder- maston)

85. 'Three-Dimensional Schlieren System', R. O. Buzzard, (KMS Industries Inc., Michigan)

100. 'The Use of TV in High-Speed Cinematography Synchro-Strobe TV', L. J. Poldervaart, Th. J. Verhaegh, A. P. J. Wijnands, (Technological University, Eindhoven)

132. 'Digital Data Recording Using High-Speed Framing Cameras', A. De Volpi, (Argonne Lab. Argonne, Illinois)

Optics Technology N o v e m b e r 1968 49