B. E, ST

J

JOURNAL

OF THE SOCIETY OF

MOTION PICTURE ENGINEERS

Volume XXXVII July, 1941

CONTENTS

Page

Twenty-Fifth Anniversary of the Society of Motion Picture En- gineers 3

Salute to the SMPE WILL H. HAYS 5

Another Milestone. . „ EMERY HUSE 7

Twenty-Five Years of Service F. H. RICHARDSON 9

Recent Advances in the Theory of the Photographic Process . .

C. E. K. MEES 10

Recommended Procedure and Equipment Specifications for Educational 16-Mm Projection — A Report of the Commit- tee on Non-Theatrical Equipment

Part I. General ^Recommendations 22

Part II. The Optical Properties of Commercially Available

Screens for 16-Mm Projection 47

Part III. Performance Specifications for 16-Mm Projection

Equipment for Educational Service 57

Supplement. Resolution Tests on 16-Mm Projection Lenses 70

Report of the Standards Committee 76

Report of the Theater Engineering Committee 78

Television Report, Order, Rules, and Regulations of the Federal Communications Commission 87

Characteristics of Intermittent Carbon Arcs

F. T. BOWDITCH, R. B. DULL, AND H. G. MACPHERSON 98

Development and Current Uses of the Acoustic Envelope

H. BURRIS-MEYER 109

Current Literature 115

1941 Fall Convention at New York, October 20th-23rd 117

JOURNAL

OF THE SOCIETY OF

MOTION PICTURE ENGINEERS

SYLVAN HARRIS, EDITOR

BOARD OF EDITORS

A C. DOWNES, Chairman

I I CRABTUB A. N. GOLDSMITH E. W. KELLOGG

H K A. M. GUNDELFINGER C. R. SAWYER

A. C. HARDY

Sub*rriptkm to non-members, $8.00 per annum ; to members, $5.00 per annum,

! E tfc

thru annual membership dues; single copies, $1.00. A discount oa MibKriptton or single copies of 15 per cent is allowed to accredited agencies. Order from the Society of Motion Picture Engineers, Inc., 20th and Northampton 4Jton. Pm.. or Hotel Pennsylvania, New York, N. Y.

t*ubit*bed monthly at Easton, Pa., by the Society of Motion Picture Engineers.

Publication Office, 20th & Northampton Sts., Easton, Pa.

General and Editorial Office, Hotel Pennsylvania, New York, N. Y.

West Coast Office. Suite 226. Equitable Bldg., Hollywood, Calif.

Entered a* second class matter January 15, 1930, at the Post Office at Easton, Pa . under the Act of March 3, 1879. Copyrighted, 1941, by the Society of Motion Picture Engineers, Inc.

OFFICERS OF THE SOCIETY

EMERY HUSB, 6706 Santa Monica Blvd., Hollywood, Calif. fut: E. ALLAN WILLIFORD, 30 East 42nd St., New York, N. Y. Yut-Presidcnt: HERBERT GRIFFIN, 90 Gold St., New York, N. Y. •/•.*Ci««rs»c Vic*-President: D. E. HYNDMAN, 350 Madison Ave., New York Vic*-Prtsidenl: ARTHUR C. DOWNES, Box 6087, Cleveland, Ohio.

esident: A. S. DICKINSON, 28 W. 44th St., New York, N. Y. resident: W. C. KUNZMANN, Box 6087, Cleveland, Ohio r: PAUL J. LARSBN. 44 Beverly Rd., Summit, N. J. Trmimrtr: GBORGB FRIEDL, JR.. 90 Gold St., New York, N. Y.

GOVERNORS

BATSBL. 501 N. LaSalle St.. Indianapolis, Ind. DUWUY. 1801 Larchmont Ave., Chicago, 111. •Jon* C. FRAYNB. 6601 Romaine St., Hollywood, Calif.

GOLDSMITH. 680 Fifth Ave., New York, N. Y.

••L HARDY. Massachusetts Institute of Technology, Cambridge. Mass.

RTMR. 5451 Marathon St., Hollywood. Calif

.

195 Broadway. New York. N Y Snoot. 35-11 35th St., Astoria, L. L, N. Y. Term expire* December 31. 1941. •*Ttm eipire* December 31. 1942.

SOCIETY OF MOTION PICTURE ENGINEERS

Incorporated at Washington, D. C. July 24. 1916

Incorporators

C. FRANCIS JENKINS DONALD J. BELL PAUL H. CROMELIN C. A. WILLATT FRANCIS B. CANNOCK W. BURTON WESTCOTT PAUL BROCKETT E. KENDALL GILLETT HERBERT MILES J. P. LYONS

Washington, D. C. Chicago, 111. New York, N. Y. Boston, Mass. New York, N. Y. Boston, Mass Washington, D. C. New York, N. Y. New York, N. Y. Cleveland, Ohio

The object of the Society shall be ... Advancement in the theory and practice of motion picture engineering and the allied arts and sciences, the standardization of the mechanisms and practices employed therein, and the maintenance of a high professional standing among its members.

Membership of the Society

The membership of the Society at the first meeting, held at the Hotel Astor, New York, N. Y., October 2-3, 1916, numbered twenty- six persons, as follows :

C. FRANCIS JENKINS CARL E. AKELEY M. D. COPPLE

DONALD J. BELL H. T. WILKINS F. H. RICHARDSON

PAUL H. CROMELIN R. G. HASTINGS H. T. EDWARDS

C. A. WILLAT H. B. COLES MAX MAYER

FRANCIS B. CANNOCK HARVEY M. WIBLE WM. C. KUNZMANN

W. BURTON WESCOTT H. A. CAMPE A. S. VICTOR

PAUL BROCKETT R. E. VOM SAAL E. M. PORTER

E. KENDALL GILLETT BARTON A. PROCTOR N. I. BROWN

HERBERT MILES HERMANN KELLNER

From this modest beginning the Society has grown to nearly 1300 members distributed all over the world, a tribute to its success in fulfilling the object stated on the previous page. Every branch of the motion picture industry, including both the artistic and the scientific aspects of photography, processing, distribution and pro- jection is well represented in the Society, not only by those directly engaged in these professions but by chemists, engineers, and research workers interested in perfecting the apparatus and materials involved.

Presidents of the Society

C. FRANCIS JENKINS 1916-1918

H. A. CAMPE 1919-1921

LAWRENCE C. PORTER 1922-1923

LOYD A. JONES 1924-1925

WILLARD B. COOK 1926-1928

LAWRENCE C. PORTER 1929-1930

JOHN I. CRABTREE 1930-1931

ALFRED N. GOLDSMITH 1932-1934

HOMER G. TASKER 1935-1936

S. K. WOLF 1937-1938

E. ALLAN WILLIFORD 1939-1940

EMERY HUSE 1941-

SALUTE TO THE SMPE

WILL H. HAYS

When your Society was founded twenty-five years ago, the mo- tion picture, with slow and faltering steps, was just beginning to grope its way into the hearts and affections of the public. The pioneers of that seemingly far away period had enthusiastic confi- dence in this youngster among the arts, but the world at large too often

looked down its nose at the "movies." The child grew and devel- oped, soon was taking prodigious strides, until today the motion pic- ture is the most democratic of the arts of our century, and the uni- versal entertainment of all the people everywhere.

Of the past, with its heartaches and its exhilarations, with its de- feats and its unparalleled triumphs, we can be justly proud, but it is the present and the future that now concern us most.

5

6 SALUTE TO THE SMPE

If we are going to develop this art-industry to its fullest poten- tialities, as I know we are, then the work in no small measure will have to be done by the technicians and engineers of your group. In a basic sense, the motion picture is a mechanical art, the product of technical wizardry. During my long years of association with the industry, I have never ceased to have a feeling of awe on learning of each fabulous scientific advance that has come from the laboratories and workshops, from the minds of men seeking constantly to improve the art. How many times have the unknowing said, "Well, this is it .... nothing more is possible!" And then some Aladdin among you has rubbed a magic lamp and brought forth a new wonder to dazzle and stun the imagina- tion.

You have given the screen voice, color, undreamed-of realism. Knowing what you have done, what you are capable of, I won't even hazard a guess what the motion pictures will be twenty-five years from now when the Society of Motion Picture Engineers cele- brates its Golden Jubilee. But I do know this: whatever the future holds you will contribute greatly to its course.

The motion picture is a collaborative art, requiring many minds and many hands. Some 275 arts and crafts and professions partici- pate in the making of a single film in our studios. It is this coopera- tion of talents, harmoniously integrated, which has made the screen one of the greatest constructive forces of modern times. Our industry must always combine depth of human interest and human under- standing with foresight, tenacity, sound judgment, and unswerving devotion to the public welfare.

The entire industry rejoices to extend greetings and best wishes on this occasion of your Society's 25th Anniversary.

WILL H. HAYS,

President, Motion Picture Producers and Distributors of America,

ANOTHER MILESTONE

EMERY HUSE President

July 24, 1941, marks the Twenty- Fifth Anniversary of the Society of Motion Picture Engineers. In 1916 when a group of twenty-six technical men, headed by Mr. C. Francis Jenkins of Washington, D. C., met with the idea of formulating a motion picture engineering society, little did they realize what might come of their idea. The Society as a mere infant passed through the First World War with only a few scars. As the years passed the Society grew in member- ship and in strength until it eventually became a nationwide organi- zation. Some years after its inception it began to reach out into the world for membershfp and as a result of its far-reaching activities it has become without question the outstanding motion picture engi- neering society in the world today.

Some idea of the growth of the Society, particularly during the past ten or twelve years, can be had if one knows that in 1928, at the time the Pacific Coast Section of the Society was organized, the total mem- bership of the national organization was less than the current mem- bership of the Pacific Coast Section alone. The Society is now made up of two Sections in addition to that on the Pacific Coast — the Mid- West Section, located in Chicago, and the East Coast Section, with headquarters in New York. The East Coast Section is fundamen- tally the parent body of the Society. From the standpoint of mem- bership from all over the world, the Society now boasts of approxi- mately thirteen hundred motion picture engineers.

It is most unfortunate for the affairs of men that the world today is in such a state of turmoil, but it is the purpose of the Society of Mo- tion Picture Engineers during these trying times to maintain its nor- mal activities as far as it is possible to do so. It is firmly believed that in times of war, peacetime activities and efforts must go on and the Society must remain a worthy outlet for the accumulated knowl- edge in the minds of men doing peacetime work, or even war work, provided the latter is connected with motion picture engineering. If

7

8 ANOTHER MILESTONE

we are able to live up to these worthy desires it seems certain that when this period of emergency is over the importance and prestige of the Society will be maintained, and we believe we will have laid a firm foundation upon which a better peacetime program in the field of motion picture engineering may be built.

This issue of the JOURNAL of the Society of Motion Picture Engi- neers, which is dedicated to the Twenty-Fifth Anniversary of the Society, marks a definite milestone in the accomplishments of the Society. It must be proved that these accomplishments have not been made in vain, and it is up to each and every member of the Society to dedicate himself to the perpetuation of the ideals of this Society. This can be done best by looking ahead to the Fiftieth Semi- Annual Convention of the Society which will take place in New York City, October 20 to 23, 1941. We must all put our shoulders to the wheel and see to it that this Convention is the most outstanding ever held by our Society.

President

TWENTY-FIVE YEARS OF SERVICE

F. H. RICHARDSON

Historically speaking, twenty-five years is an infinitesimal portion of time, but in relation to the motion picture industry, twenty-five years covers almost the entire period of birth, growth, and adolescence of the industry. Twenty-five years ago I sat at a meeting with twenty-five other men who had somehow chosen "moving pictures" as their interest and livelihood, and we put this Society to work for us.

We had no idea the Society was going to last for twenty-five years or that it would grow to the technical importance it now holds for the entire industry. We had a job to do, and we set about to do it, and the formation of the Society was one means of helping us to do it.

The Society grew slowly at first, because the industry was flounder- ing about, trying to find itself ; but soon it grew more rapidly as the movies began to expand into the enormous industry we have today; and when sound came into the picture .... It is needless to go into details. Today the Society's influence encompasses the entire world; it has members in all important countries of the world; and consti- tutes the most important source of information on the up-to-date prog- ress and technical developments of motion picture engineering.

I am proud to have been one of the founders of the Society, and all through the years I have tried to contribute whatever I could to the betterment of the industry and of the Society. Projection has been my principal interest, because I started as a projectionist — or "opera- tor," in those days — and I am indeed happy in the fact that, with the aid of a few others, I have been helpful in arousing the interest of the Society in the humble art of "operating moving picture machines."

I trust and feel confident that the Society will continue successfully this work begun so many years ago, and I can but repeat that I am proud to have had a part in all this work. May the Society prosper and find success in all its endeavors.

J

RECENT ADVANCES IN THE THEORY OF THE PHOTOGRAPHIC PROCESS*

C. E. KENNETH MEES**

Summary. — A photographic film consists of a layer of gelatin coated on cellulose base in which are dispersed a great number of very small siher bromide crystals. When exposed to light, electrons are liberated in the crystals and these collect at certain Points, where they are neutralized by silver ions which deposit atoms of metallic siher. This metallic silver deposited in definite specks forms what is known as the latent image, which makes possible the development of the crystal. The surface of each silver bromide crystal in the gelatin layer of an emulsion immersed in the developer is protected by charged layers of bromide and potassium ions. The development of the grain is initiated by the break in this charged layer caused by the presence of the silver latent image. When the developer acts on the silver bromide crystal, metallic silver is produced in a ribbon-like form, a tangled mass of which forms the developed silver grain.

Behind all our technology there lies the basic theory of the photo- graphic process — the chemistry and physics of the formation and structure of the photographic material, its reaction to light, its be- havior in the developer when the image is produced, and the prop- erties of that image.

The science of photography is founded on the two great sister sciences, chemistry and physics, and it was only as our knowledge of these grew that progress could be made on the problems of photo- graphic science. Until recently, photographic science tended to con- sist of a chaos of observations, some of them of real value and others of very doubtful value, with little in the way of theories to connect them properly. It is only within the last few years that fact after fact has been falling into place in an ordered network. At the present time we can say that while much remains to be done, we have a very clear and definite science of photography — something which can be written out and generalized upon and to which the missing parts can be added as more work is done.

* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 6, 1941.

** Kodak Research Laboratories, Rochester, N. Y.

10

<> The Society is not responsible for statements by authors <>

ADVANCES IN PHOTOGRAPHIC PROCESSES 1 1

Strictly speaking, many light-sensitive substances could be used for making photographic images, and the science of photography should be co-extensive with photochemistry itself; that is, with the chemistry and physics of light-sensitive substances. But in practice, this is not the case, and the art of photography is almost entirely con- fined to the use of silver salts, so that the science of photography is necessarily preoccupied with the very complex system of silver halide crystals dispersed in gelatin. Information as to the reactions which go on in the simpler systems used in photochemical investigations throws little light on the photographic process.

If we enlarge a photographic film under a microscope to about the limit of resolution of the microscope ; that is, to some 2500 diameters, we shall find that it consists of a very complex system. On the base, which is cellulose nitrate or acetate, there is coated a layer of gelatin containing silver halide crystals. These silver halide crystals are composed of silver bromide containing a small amount of silver io- dide, and they may be dyed to sensitize them to the longer wave- lengths of light. The crystals vary considerably in size but are of the same general shape. They are triangles and hexagons, which are the natural forms of silver bromide, and they are held in photographic gelatin (Fig. 1). Analysis would show that the film also contains a number of materials — glycerine, hardeners, and other things adapted to control its properties. When this film is exposed to light, the silver bromide crystals are affected in some way by an extraordi- narily small amount of light, and they suffer some change. That change must take place in two steps and not quite instantaneously, although it occurs in a very short fraction of a second. The reason for this conclusion is that the amount of change produced depends somewhat upon the rate at which the light is supplied. This is what is known as the ''reciprocity effect." If the light is supplied rapidly, somewhat more effect is produced than if the light is ap- plied very slowly — as if, for instance, a faucet were running into a bucket and the bucket had a small hole in it. But the analogy is not good because when the exposure is over, the change that has occurred is permanent; the image will keep for long periods. When Andre's photographs were found at the Pole thirty years after his balloon fell on the ice and were developed, they were quite satisfactory, the latent image having been preserved by the cold in spite of immersion in sea water.

The silver bromide crystals in the emulsion depend for their sensi-

12 C. E. K. MEES [J. S. M. P. E

tivity upon the gelatin in which they are suspended. Emulsion makers have known for many years that some gelatins were active and would give sensitive emulsions and that others were inactive. In an arduous research, this was traced by Sheppard to the presence in the gelatin of traces of free sulfur compounds, which are presum- ably derived from the plants which the calves and their mothers ate. When gelatin is made from little animals, like rabbits, which avoid the hot-tasting plants, such as mustard, which contain sulfur, the gelatin does not contain these sulfur compounds, so that it was not

» .v^» P-

*.$> 1

FIG. 1. Silver bromide grains in a photographic emulsion.

improper to state that "if cows didn't like mustard we wouldn't have any movies!" The sulfur compounds in the gelatin react with the silver bromide and produce specks of silver sulfide. These specks of silver sulfide in some way increase the sensitivity of the silver bromide crystal to light.

Recently, a thoroughly consistent theory of the effect of light upon the silver bromide grains has come out of the work of our laboratories and from Professors Gurney and Mott of Bristol, England. In the first place, if we consider the energy diagram (Fig. 2) of a silver bro- mide crystal, we shall find that we have two energy levels, the 5 and

July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES 13

P levels, in which the electrons may be situated. The S band is normally empty and is referred to as a "conduction band." The P band is normally completely filled with the electrons. Upon ex- posure of a silver bromide crystal to light which is absorbed in the long-wavelength end of the characteristic absorption band, the elec- trons are transferred from the lower P band to the 5 band, and the crystal becomes conducting. This property is well known in other materials, as well as in silver bromide, as "photo-conductance," and the silver bromide crystal exposed to light may be imagined to be filled with a sort of gas of conducting electrons. Also, when light is absorbed by the silver bromide, electrons are released. This is the primary photographic process — the thing that happens instantly when light falls on the crystal. The electrons move about with great speed inside the crystal and will very frequently reach the

ZERO POTENTIAL ENERGV

) FILLED

MOMOUCnON

' LEVELS

BC AqS, AqBP ^ ^ ^ ^ ^ Aq SPECK

FIG. 2. Energy diagram of the silver bromide crystal.

boundaries of the crystal, but when they reach a sensitivity speck, they will be trapped by it and the sensitivity speck will become nega- tively charged by the electrons that it has absorbed. Naturally, the sensitivity specks will themselves be giving out electrons slowly if they are at normal temperatures, just as does any other solid body. During an ordinary exposure, the amount of electrons given out by the sensitivity specks will be very small; while those which will be absorbed from the electrons freed by light will be very great. After the formation of the free electrons by light, therefore, the sensitivity specks acquire a negative charge by the absorption of these free elec- trons.

In a crystal, there is always available, of course, a certain amount of silver ions which are formed inside the lattice. As soon as the sensitivity specks acquire negative charges, these silver ions are at- tracted to the specks, each negative charge neutralizes one silver ion and produces a deposit of a silver atom at the sensitivity speck, so

14 C. E. K. MEES [J. S. M. P. E.

that every electron freed by the original light exposure is eventually transformed into a silver atom deposited on a sensitivity speck.

This theory of the effect of exposure was suggested by Sheppard and Trivelli of our laboratory over ten years ago under the title of "the concentration speck theory," but they were unable to give a satisfactory mechanism for the formation of the concentration speck although they saw that in some way the effect of light must be to produce a silver deposit at the sensitivity specks. The new theory of Webb and Gurney and Mott shows that the effect occurs in two stages: first, the release of free electrons, which occurs instantane- ously ; and then the transformation of the free electrons by neutraliza- tion through the silver ions into silver atoms at the sensitivity specks. This accounts for the reciprocity effect. When the light acts, free electrons are formed and go to the sensitivity specks, but the sensi- tivity specks are continually losing electrons and, consequently, if the light is weak, there will not be as many silver atoms deposited at the sensitivity specks as there should be. A certain minimum con- centration of electrons must be built up in the crystal before the elec- trons begin to be trapped by the sensitivity specks. This explana- tion is shown to fit the facts because, when the loss of electrons from the sensitivity specks is reduced by greatly lowering the temperature, the rate at which the light is supplied no longer affects the resulting image.

The action of light, then, on the silver halide crystals is, first, to produce instantaneously a charge of free electrons. Then these elec- trons are attracted to the sensitivity specks, and their charge is neutralized by silver ions, with the result that metallic silver is de- posited around each sensitivity speck and forms the permanent nucleus which we call the "latent image."

The great efficiency of the photographic process is due to the very small amount of work which is done by light in forming an image and the very large amount of work which is done by the chemical de- veloper.

A photographic developer is a reducing agent; that is, it is a sub- stance which is itself oxidized by silver bromide and, in being oxi- dized, reduces the silver bromide to metallic silver. The matter is, however, very complicated, and we are only beginning to understand the behavior of photographic development. In the first place, not all reducing agents, by any means, are photographic developers. If the reducing agents are too strong, they reduce the unexposed

July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES 1.-,

silver bromide and the whole of the film turns black, no image being formed. If the reducing agents are too weak, they will not reduce the silver bromide after exposure. In order that the substance may be a developer, it must have a certain power of reduction or, as we should say in electrochemical terms, a certain "reduction potential." But there are substances which fall in the correct range of reduction potentials, so far as we can measure it, which still are not photo- graphic developers. There are others which are photographic de-

FIG. 3. Filamentary structure of a silver grain (X40,000).

velopers in the sense that they show an image on an exposed film but are not useful photographic developers because they do not develop satisfactory images in a reasonable time.

Our knowledge of the mechanism of development has been greatly assisted by the information as to the structure of the developed silver obtained by the use of the electron microscope. The grains of de- veloped silver show little structure under the highest magnification of the ordinary microscope. It was obvious that they could not be compact masses of silver since their volume is much too great for their mass if the structure was compact, and it was generally thought that the grains had a spongy structure, somewhat similar to that of

16

C. E. K. MEES

[J. S. M. P. E.

I §-

I

—

July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES

17

«RO POTENTIAL

coke. The electron microscope enables photographs to be taken with equally good definition at magnifications about twenty times higher than those which are possible with the ordinary microscope, and when this instrument was applied to the photomicrography of developed silver, it was found that the silver had a most unexpected ribbon-like structure, so that the grains appear like masses of sea- weed (Fig. 3). This filamentary structure of developed silver is very surprising, and the fact that it is so unusual makes possible some de- ductions as to the formation of silver. It might be thought that the ribbons were produced by the formation of silver in interstices in the silver halide grains, but this is seen to be impossible when we examine the development of extremely small silver bromide grains, such as those which are used in emulsions of the Lipp- mann type. Each single crystal turns into a filament of silver, which is much longer than the diameter of the crystal, so that it is evident that filamentary silver must be ejected from the crystal when development occurs. A series of pictures-showing the stages of development of grains are very instructive (Fig. 4). The grains were deliberately selected to be very small and the photographs show clearly the ejection of the ribbons of silver and their growth from the grains until the whole grain has been converted into a spongy mass of silver filaments. It seems to be clear, therefore, that the old idea that the grains dissolved in the developer and then silver was precipitated and coagulated around the exposed crystalline grains is quite incorrect. Instead, we have to imagine that the developer reacts with the exposed silver bromide grain and from it forces filaments of silver arising pre- sumably from the silver silver-bromide interface. As more silver is produced, new spots in the grain become the sources of development until the whole grain is converted into silver.

FIG. 5.

Diagram of grain with pro- tective double layer.

18 C. E. K. MEES [J. S. M. p. E.

In a study of the initiation of development, it must be remem- bered that the problem is not why an exposed grain develops, so much as why an unexposed grain does not develop. If silver bromide is precipitated in the presence of an excess of silver nitrate, it is spon- taneously developable without exposure to light. Moreover, silver bromide even in the absence of free silver and without exposure to light is readily reduced in a developing solution if it is precipitated in the absence of gelatin, and there is no doubt that the adsorption of gelatin to the silver bromide protects it from the action of the de- veloper. This protection may be considered to be due to a nega- tively charged electric layer which surrounds the silver bromide grain formed with an excess of bromide, the function of the gelatin being to protect the charged layer. Dr. J. H. Webb depicts the exposed silver halide grain as a plate, as shown in Fig. 5 in which the charged condition around the grain is represented schematically. The sur- face of the silver bromide grain itself has an excess of bromide ions which give rise to a negatively charged surface. However, just out- side this negative charge, a positive layer of potassium ions must be present to neutralize the negative charge. Without such a neutraliz- ing layer of positive ions, it would be impossible for the surface of the silver bromide grain to be covered with negative bromine ions, since the amount of such a charge in so small a region would give rise to ex- plosive forces. A double charge layer, consisting of negative bro- mine ions on the grain and positive potassium ions in the gelatin just outside, may be considered to exist around the surface of each silver bromide grain. Grains with such a double layer (in solution) would move under an electric field as negatively charged bodies, since the negatively charged grain would be forced in one direction by the field, and the surrounding movable positive ion layer in the opposite direction, but as at any point in the liquid there would be positive ions to form the surrounding positive shell, the double charge layer would be maintained. That the surface charge on the particles and surrounding charge layers do neutralize each other in the manner outlined is proved by the fact that the colloidal suspen- sion does not possess a net charge of either sign, but is neutral as a whole.

It may be assumed that a grain, with its double charge layer, be- haves toward outside charges and also toward charges located inside the grain as a neutral body. An electron placed inside such a double charge layer would experience no force nor, in the same way, would

July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES

19

an electron placed outside such a double layer. However, there is a marked difference in potential between the inside and outside of the grain, and the total jump in this potential occurs in the region between the two charge layers. The potential gradient between these charge layers accordingly gives rise to a strong electrical force between the layers, and an electron placed between them would experience a force toward the outside. It is considered that the double charge layer

UOUBUE CHARGE-' LAVE.R

FIG. 6. Diagram of grain with latent image.

acts in this way as an effective potential barrier to the entrance of an electron into the silver bromide grain of the emulsion and prevents the charged ions of the developer from attacking the grain.

The condition existing in the exposed grain containing a latent image silver speck may be seen in Fig. 6. This shows a greatly en- larged scale model of a charged grain surface with a clump of silver atoms on the surface, which is supposed to represent the latent image produced by exposure to light. The clump shown includes 220 atoms, with approximately the correct spacing. This size was

20 C. E. K. MEES [J. S. M. P. E.

chosen as representing a fair mean of the values given by various workers.

It is assumed that development of a grain is initiated by the break in the double charge layer caused by the silver speck, permitting the negative developer ions to reach this silver speck. The latent image speck is viewed as an electrode penetrating into the grain. The tendency on the part of the developer ions to release electrons to the silver causes electrons to pass to the electrode and charge it nega- tively. This occurs if the electrons of the developer ions are situated in levels above the highest occupied energy levels of the silver metal. The penetration of this negative electrode into the silver bromide grain upsets the neutral electrical condition previously existing in the grain, and there arises an attractive force for the positively charged silver ions in the neighborhood of the latent image speck. Some loose positive silver ions always exist in the crystal lattice owing to temperature motion, and these diffuse to the speck under the attrac- tion of the negative charge there and enlarge the silver speck. Thus, it is supposed that the original silver speck of the latent image com- mences to grow by this mechanism. As this proceeds, the protective double layer is more and more ruptured, and a rapidly increasing area of the silver halide grain is exposed to the attack of the developer. The reduction of the grain therefore proceeds at an ever-increasing rate, and the grain is very soon reduced throughout to metallic silver. In the initial stages of development only, is a silver bromide grain protected from a developer; after the barrier is once penetrated, it rapidly approaches the status of an unprotected grain, which, as pointed out, is developable very rapidly.

This is only a very preliminary sketch of the action of develop- ment. Undoubtedly, the adsorption of the developer to the de- veloping grain plays some part in the reaction. It concentrates the developer ions at the point where they are required and undoubtedly also the actual reaction of the developer with the grain, and its be- havior as a reducing substance is catalyzed by the silver of the latent image.

A great change has taken place in the technic of the motion pic- ture studio in the last twelve years as a result of the application of panchromatic films. Negative films in motion picture work are now invariably panchromatic, and their greatly improved quality compared with the earlier materials is due to the advances that have been made in the preparation of sensitizing dyes. These sensitizing

July, 1941] ADVANCES IN PHOTOGRAPHIC PROCESSES 21

dyes are what are known as "polymethine" dyes, most of them being the class of dyes which are known as "cyanines." There are basic dyes in which the two nuclei are linked by a chain of CH groups. Since many different nuclei can be used, the chain can be of different lengths and various substituents can be inserted in a molecule, so that very many dyes are available, and since they all have properties peculiar to their structure, a wide range of sensitizing can be obtained. The cyanine dyes show very beautiful crystals. They have bright colors, and many of them are pleochroic, so that they show iridescent effects.

In addition to the advances in practical photography which have followed the use of the sensitizing dyes we have achieved a consider- able knowledge of the way in which they work. It seems clear that the optimum concentration for the sensitizing of a silver halide grain is a single layer of dye molecules attached to the whole surface of the grain, as if the flat plate of silver halide were covered with a little velvet pile of dye molecules, all of them firmly attached to the silver halide lattice, but free to resonate to the light which they absorb. The dye molecules appear to be arranged edge-on, so that for the best sensitizing the surface is covered with leaf-like molecules piled edge- wise in as close packing as is compatible with their own structure and the structure of the crystal, forming a parallel pile or edge-on layer one molecule thick. The new surface of the crystal with the dye on it has no affinity for water, but there is an attraction between the molecules oriented in this way, so that colliding particles may tend to aggregate and precipitate. Dyes which otherwise might be sensitizers may fail to sensitize because their molecules are so shaped that they can not form this flat leaf -like structure, and the best sen- sitizers are presumably those which form the structure easily. When the light is absorbed by the dye molecule, it must liberate an electron, but it is not yet possible to decide whether this electron itself acts to form the latent image in the silver bromide or whether the energy is transferred to the silver bromide which liberates the electron into the conduction band.

There are many obscure points which still require elucidation in the theory of the photographic process, but very rapid progress has been made recently and we are beginning to understand the funda- mentals of the process by which pictures are made.

RECOMMENDED PROCEDURE AND EQUIPMENT

SPECIFICATIONS FOR EDUCATIONAL

16-MM PROJECTION*

A REPORT OF THE COMMITTEE ON NON-THEATRICAL EQUIPMENT

MAY, 1941

This report has been prepared in response to a request from the Committee on Scientific Aids to Learning, of the National Research Council.

The report is in three parts. Part I is a general discussion of the problems that enter into the selection and use of 16-mm motion picture equipment for educational institutions. It includes recom- mendations for such comparative tests of equipment as can properly be made without testing laboratory facilities.

Part II is a report on the optical characteristics of the screens available at the present time for non-theatrical projection.

Part III consists of a set of detailed technical specifications de- fining acceptable performance of 16-mm projection equipment for educational uses. The character of these specifications is neces- sarily such that they can be interpreted and applied only by a fully equipped testing laboratory.

Committee on Non-Theatrical Equipment

J. A. MAURER, Chairman

J. G. BLACK R. C. HOLSLAG W. H. OFFENHAUSER

F. E. CARLSON R. KINGSLAKE L. T. SACHTLEBEN

F. M. HALL D. F. LYMAN A. SHAPIRO

J. A. HAMMOND L. R. MARTIN M. G. TOWNSLEY

M. HOB ART R. F. MITCHELL A. G. ZIMMERMAN

PART I

GENERAL RECOMMENDATIONS

Objectives. — In the selection of motion picture equipment for classroom use, the object should be to provide a picture that can be viewed to good advantage by everyone in the classroom. Likewise,

* Presented at the 1941 Spring Meeting at Rochester, N. Y.; received May 27, 1941. 22

NON-THEATRICAL EQUIPMENT REPORT 23

equipment should be selected to provide good reproduced sound in every part of the classroom.

Standards of quality in educational projection ought, if anything, to be higher than those in the theatrical motion picture field. The pupil does not come to the classroom to be entertained, but to learn. In order to learn from the screen, he must watch it diligently, even though he may happen to be seated in a position that affords him only an oblique and distorted view of the picture. In order to learn from the sound, he must be able to understand reproduced speech without effort, and he must be able to obtain a true impression of the character of natural sounds and of the tone qualities of musical instruments when these are used in the films.

In a motion picture theater, if one has to sit in an unfavorable location, as a rule he is subjected to this annoyance for only a single performance. In the schoolroom, however, he may be required to keep the same seat day after day. If this seat does not give him a good view of the picture and a good opportunity to hear the sound, he is under a permanent handicap.

It is because of these considerations that in several instances this report recommends narrower limits than are commonly accepted in theatrical projection practice. The Committee believes that in the educational field there should be no compromise with respect to the conditions that are necessary to secure substantially equal perform- ance for all persons in the classroom.

Basic Steps in Equipment Selection. — Intelligent selection of equip- ment means a great deal more than the mere selection of high-quality equipment. It means the coordinated selection of equipment items in relation to the conditions under which they are to be used. The projector, the unit on which most attention is generally focused, should, as a rule, be the last to be selected.

The first consideration, and one of the most important, is the size of the screen to be provided. This is determined primarily by the maximum distance from which the picture will be viewed by the students.

After the picture size has been determined, the right type of screen surface must be selected, as determined by the shape of the room, or, rather, by the seating arrangement of the spectators in the room.

The picture size and the type of screen surface, together with the degree to which light can be excluded from the room, determine the light output required from the projector. The selection of the pro-

24 NON-THEATRICAL EQUIPMENT REPORT [J. S. M. P. E.

jector should be made from those types which provide as nearly as possible the correct light output.

A similar requirement exists with respect to the power-handling capacity of the sound-reproducing system, in relation to the acoustic properties of the classroom. Fortunately, the power output of the sound-reproducing system can be controlled more conveniently than the light-projecting system of the projector; therefore it is sufficient to ascertain by practical test that the maximum sound power output of the projector selected is sufficient for the room in which it is to be used.

Complete equipment for the projection of films in a classroom in- cludes a suitable stand for supporting the projector firmly in the proper location. Facilities for darkening the room during projection are also needed.

Recommendations with respect to each of these problems will be given, hi the general order in which they have been mentioned.

Picture