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C o u ~ m i s s i o n 1 : P h o t o g r a p h y a n d i t s t e c h n i c s .

Some Bemarks on Pholography in Aerial Surveyiny (40. i. f,:. ~.)

By A. oou Octeucrauts, Stockholm.

l 'hotogrammetry as a science may be analysed from various points of view. It may be seen as the result of several more abstract types of knowledge, aiming at the determinat ion of sides and dimensions from photographically obtained central perspectives, lint it can also be analysed with regard to the different stages of the work.

In this respect we might distinguish between obtaining, reading, and measuring o1' the photographs, i. e. first the photographing, then the qualitative, and, lastly, the quant i ta t ive use of the results. These different types of work need a differel~t pre l iminary study.

Now most of those w-ho work in and on photogrammetry have b e e n - - a n d a r e - . mathematicians and geodesists. Therefore the photographic pa r t of the work has mostly been left to photographers and assistants. This should not, however, continue; the very best results can only be obtained if every part of the work is performed in the very best way.

The editor of this review has asked me to write down some views on this question. l ie told me that he considered other parts of the work to have reached a certain degree or perfection, especially the mathematical questions; now it was necessary to analyse the photographic work and to find the best way to carry out the various processes.

Of course, this is not the place for a treatise on photography - - and yet one cannot ignore any part of it, if one wants to tu rn out a photogram giving the truest and easiest reading and the most correct measuring. Nevertheless, some principles may be explained.

Further , it is necessary to analyse the photographic work, in order to estimate the errors introduced thereby. Genera l ly the photogram is considered as a correct reprensentat ion of a perspective (certain wide angles only excepted). For su& work, where the photogram is used :for the reconstruction of the original bundle of rays, this assumption may be permitted, but it is not possible to do so in cases where the photo- gram itself is used as a result, like in rectifying. Every error then appears in ttie results and has to be taken into account. If the best results are desired the camera and its performance cannot Be neglected.

In photography there are two parts, the optical and the photochemical; of these I will concentrate on the latter. Some paris of the optical system are, however, of direct importance in practice, especially the focussing.

In most cases photogrammefrieal work :is executed while focussing on infinity. This may be all r ight if the photographs are taken vert ical ly from great heights, but it is not all right for obliques.

When analysing this :fact, one has also to consider the Scheimpflug theorem: it applies here as welt as in the case of rectifying. First the centre point should be focussed on the axis; then the angle of the negative should be adjusted so as to satisfy the Scheinqfflug conditions.

One cannot, however, use different focussing for every exposure, bat has to be satisfied with one cal ibrat ion only. Further , in order to avoid being dependent on the way of hohling the camera, it will be necessary to adjust the plate perpendicular to the optical axis.

For this adjus tment the quali ty of the objective should now be considered; if the focal plane is curved, a mean focal distance should be chosen at which the best' definition is obtained. Then the variat ion of distances to the objects has also to be considered,

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because the render ing may become indistinct for nearhy objects in obliques or if c.g. the vertical photographs show mountainous regions.

This would not be an evil in itself if the points to be measured could still be distinguished, but it should be considered that often the image of such points may at the same time be diffused and displaced. Fur thermore the density centre of the diffusion disc may or may not correspond to the centre of displacement. These questions were investigated by me in a paper, published in i926, p. 1, in the Proceedings of the Swedish Aeademy of Military Seienee. The calibration should therefore first of all aim at the best possible inf ini ty focussing, the adjus tment of which with respect to the field curvature of the objective is obtained by means of a collimator; then a displacement correction should be introduced depending on the purpose for which the camera is i n t ended . In order to secure the correct calibration the following should be taken into account: the enlargement to be used, the height from which the photographs are taken (determining the required definition) the speed of the airplane (determining other sources of inevitable diffuseness).

In the photochemical work, the light-sensitive material used is o f great ilnportance. When we compare the present possibilities afforded by objectives and film sensitivit.v with the conditions existing when I started working in photography, the possibilities with regard to short exposures have been increased almost a thousandfold.

We have also to consider ordinary sensitivity, gradation and contrast, colour sensitivity, halation and graininess, as well as detail rendering.

The sensitivity has been increased year by year - - part ly in general, part ly for eolour only. Usually such improvements were accompanied by an increase in graininess which was a decided disadvantage. Every detail has to be built up by a certain number of grains or clumps in eonsequenee of which the pereeptible linear details grow with the graininess so that the degree of enlargement is more limited.

However, means have been found to keep the graininess down without loss of sensitivity. To this bas been added a great increase i n eolour sensitivity, lJp to a few years ago the orthochromatic emulsions were t reated with the same eohmr sensitizers as fifty years ago; in the last few years new and better sensitizers have been developed. I consider, however, these emulsions as things of. the past; if we wish to get the same colour-rendering with panchromatic emulsions as with orthoehromafie material, a greenish or green filter should be used instead of a pure yellow. We have further to differentiate between a rendering that approaches the sensitivity of the eye as closely as possible (orthopanehromaey) and another type which aims at obtaining a clear image even if the air: is not clear (telepanchromacy) so that we have to aeeept a less correct eolour rendering. An extreme type of rendering is found in the infra-red sensitivity whid,_ renders many greens almost white, whilst all blue is cut off, e. g. in a wood most needles are rendered dark, whilst most leaves become white. This may cause a eertain difficulty when interpreting, but seareely when measuring. These infra-red emulsions are better suited for stereoplotting than for rectifying. Their relatively l o w sensitijcity is a grcal difficulty, but when pl~otographs are taken in good light ~ i.e. sunlight without elouds - these emulsions might and are used even with advantage, for distant photography by obliques. Greater difficulties arise when we use these emulsions in a hazy atmosphere. At present the exposure time is about ~/2s--V~0 see. at 1:4,5 but with further improvements in the sensitivity the exposure and diaphragm may be considerably reduced.

One thing, however, is certain - - we ha,/c gained very much by the increased red- sensitivity of the panchromatic emulsions, as it has enabled a paler :filter to be used, giving better eorrections whilst the exposure time is only a little increased.

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Sensitivity, however, is not everything; the image has to be built up with a cerlain relation between light intensity and density in the fihn, whirl1 is called gradation, or me~e simply contrast. We have hard (conirasty), normal, and soft emulsions.

Often aerial photography deals with very monotonous objects; there are many details, but they are not very distinct. When photographing the photo-technicians are t rying to make the photographs of such details as clear and eontrasty as possible. On the other hand, when applying this procedure very much detail in other parts of the image, par t icular ly in the shadows, is lost. Shadows e. g. from a steep mountain or a cloud whidl cause ugly spots in the rectified map usually do not mar the plotting. Often, however i~ hard emulsions, they leave empty spots without any deiail, if one does not overexpose and apply an adjusting developer, but overexposure is not always possible.

For g.reater enlargement exact details in shadows are a necessity; at present everT aerial survey method strives to cover in each photograph.as great an area as possible, i1~ order to keep down the unproduct ive time of orientation. This, too, would require a moderate contrast in emulsions for aerial surveying work.

On the other hand there is not much lati tude needed, because there is seldom a chance of overexposing, especially if appropriate :filters are used. In obliques, however, haze :is often the cause of different exposures at different distances.

We have already spoken about eolour sensitivity, but there is also the question of real eolour rendering. Former colour plates and films were at the same time slow and gram~ : now a eolour fihn of the new type is speedy, more free from grain than almost any other material, and more or less automatieal ly processed. Colour photographs could eertainly be better in terpreted and read than black and white photographs. I remember a small 6 X 6 c m camera, constructed by Hugershoff, for whidl I suppose it would not be h~l possible to obtain eolour fihns from Agfa or Kodak.

I have repeatedly pointed out that everybody wants to work with enhtrgement. For su& purposes it is most important to have a material quite free from halation. The old saying that films are eo ipso free from it does not hold good. There exists a l inearly small but nevertheless intense halation which is not so much noticeable in contact printing, L,'dt whidl can cause much trouble with increasing enlargements.

Iil this ease also graininess increases in importance. A good deal of research work has been earr.ied out with regard to the grains or clumps of grains in emulsions, but the f a d of real importance, whether we study them in the negative or in the enlargemel~ts, is the space between the clumps where rays of light can penetrate. Here we ought t~ have good details, but if our i l luminat ion is not good, diffraction and the like can spoi! our work. Our material should be fine-grained, although that in itself is not enough. We must develop the emulsions so ~hat we do not spoil the work by causing the clumps to grow together and such types of lamps etc. should be used, which produce as litt!c diffraction as possible. To unders tand this one only needs compare a negative held up against a strong light with a negative held against light reflected from a white paper.

In the processing of photogrammetrie photographs I should like to stress especially one .point in part icular: with an ord inary alkal ine developer perhaps only two-ihird:.~ of the latent image is effeeted. One can e.g. expose a film, cut it in two, develop one half in the alkaline developer, the other half in a concentrated developer with very little alkali: time of development: 3--30 minutes; then the difference ~-ill show clearly.

About M of the light enter ing the emulsion passes through and is not absorbed. The remainder is distr ibuted in the depths of the layer; it should, however, specially be pointed out that any developer which is more alkaline, especially if applied after soaking the film in water or during the desensitizing' process, has only a superficial effect and does not penetrate into these deep layers. One can certainly reduce the exposure time and yet

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get ~, better result, especially ill graininess if a deep-working deveh)per is used. OJ~e can either use any ord inary developer and improve it by taking away all the alkali and half the water or apply a special developer; it is, however, of the utmost importance to get one that keeps well.

I have found the following type and used it for a long time; it keeps in an open ~cssel a couple of months and is concentrated enough to w o r k well for that time:

Metol Adurol / Chlorquinol Sodium sulphite i. t ryst . Bromide of potassimn Pinakrypto lgreen I : 1000 Water up tc

15 gins, 4,0 gins,

350 gms, 15 cm a (sul. 1 : 10),

150 em a, 3000 cm ~.

There are new developers with regenera t ing solutions used for D P plants whidl are vtten very useful - - e. g. the new Kodak D 20 and D 20 R.

For these types of developers the developing time is 15--30 minutes. If a speedy rcsutt is wanted, it is bet ter first to soak 2--3 minutes in the solution and then activate the process during 2 - - ' lminu tes in a 5% solution of crystal l ised soda. No rinsing between, only after the soda bath and Before the acetic acid stop bath (preferably also before the acid f ixing bath).

Developing is done

in a dish:

on the band (Correx):

o n / ' e e l s :

in processing madlines

analogous to cinefilm machines :

only for a small number of plates or films;

w h i d / c a n be satisfactory, but which must be done very care- fully. The celluloid band is easily damaged and not cheap;

perhaps the best method, especial ly if a motor is used and the frequently used first-water bath whidl gives a super- ficial development is avoided. As a first bath a non-alkaline solution ought to be used.

certainly a very good way, but not cheap.

When fixing, it is impera t ive not to exceed the time necessary, because the finer the grain the greater the danger to lose the fine detail required. There is, however , a special danger if the f ixing bath is not fresh and strong; an old bath fixes slowly, cuts away details. and leaves residues which are very difficult to wash out.

Washing should be done thoroughly. Its effectiveness not only depends on tile quant i ty of water used, but even more on its beil~g regular ly changed. Once in a while a plate or a film should be stained with red ink and the time should be ched¢ed needed for washing out the stain.

in summer there is a certain danger of the processing solution and the washing water having ve ry different temperatures , e.g. 20--22 ° and 8 ° C, as may often happen. Then one has to arrange for a slower change from the processing solution to the water - - or else the gelatine layer, though not floating off - - may yet be loosened, so that local dis- placements may occur. This may also happen when hardening or a ret iculat ion ,s.i!I appear whi& destroys details.

When drying care should be taken to get a uniform drying speed, otherwise differences in density will a p p e a r which may cause faults. Special care seems to be necessary with ex t reme wide-angle-photographs where the way through the emulsion is increased by more than half and where not 0nly a diffusion, but also a displacemen¢ may take place.

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When washing and drying it may be useful to remember that washing is done 1)y first letting the fixing bath drip off, then washing it off from the surface and lastly washing the remaining fixing bath out of the emulsion. Similar ly the drying is effectcd by first letting" the water drip off, [hen mopping the surface and lastly by evaporating tile water out of the emulsion, paper fibres etc. If eadl p'receeding stage is done with care, the last and final procedure is more easily carried out.

For positive work plates, films or paper can be used. Plates are safest, because there the base :is at once dlemically indifferent and medmnical ly unshr inkable . Films have a continuous but mostly regular shrinking; paper behavers much more irregularly in this respect. If an original length is given as 100 ~*hen w.et, paper expands to 101 and shrinks when dry to about 99,5. It igh glossy paper may expand when dry to any length up to 103, the subsequent shr inking is slow and uncertain.

The shrinkage or expansion of paper is always different along and across the fibre~*; not so nmdl though with papers (negative- or docnment-) without a bary ta base, There are papers on the market with thin metal laid in; these are ahnost unshrinkable.

Contact copying from films or plates should be done in paral lel light; for enlarge- ments ordinary projection apparatus should be used and of course equipped with identical lenses also corrected for short distances. Parallel light is necessary in the first case, because all but the smallest plates of ordinary qual i ty show a marked concavity which might deform the image.

It is also very often necessary to correct the concentrat ion of light in the ceatre of the wide-angle negatives. In contact copying this may be done by using only one small lamp (opal) placed at a distance from the negative about equal to the focal distance in question and no diffusing glass between them. In project ion one can use a piece or b la& paper with a round hole which is moved up and down the i l luminat ing beam and thus giving increased exposure in the centre.

It should always be remembered thai a half-tone negative is best made by over- exposure and by a shortened development in the right type of developer, whilst the time of exposure for a bromide or chloride paper copy should be kept low and its development be sufficient and thorough; blad< and white negatives are treated like a paper copy.

The. photographic taboral~ory of a photogrammetrieal establishment should preferably consist of several smaller rooms, not of one or two big ones. If possible one ought to have separate rooms for:

loading and unloading work; negative development; rectifying; processing of bromide copies; contact copying and processing (dtlorobromide and dfloride work); daylight work.

The walls are best painted in yellow, e.g. chromium-yellow. Lighting: a direct or indirect ceiling light, and a special light where necessary.

The dark rooms should also be well heated and ventilated. Gas is always dangermls :for the materials and should be avoided if possible. An easy and yet lightproof passage from the daylight rooms to the darkrooms is essential.