DR. U. NIELSEN Forest Management Inst., Canadian Forestry Service Ottnwc~,Crznadn, KIA OH3 Agfacontour Film for Interpretation

The simplicity of this new enhancement technique makes it available to anyone having access to a darkroom.

INTRODUCTION of equal density are known as equidensities. NE OF THE most important criteria in the Equidensities can also be obtained by 0 interpretation of photographic records photographic methods which do not require is the color or tonal differences between the sophisticated instrumentation. Hut, so far, imaged targets. These differences are pri- these methods have been comples and diffi- marily a consequence of the different light cult to control (for examvle the methods reflection characteristics of these targets. Al- based on the exposure effects known as though tones and, to a lesser extent, colors pseudosolizrizt~tion and Sabnttier e.fect). A often overlap, it has been possible to correlate simpler method involves the principle of these photo signatures with the natureand photographic masks. The original trans- condition of specific objects. However, the parency is exposed on different sheets of con- human eye is very limited in its ability to ventional copying film, whereby the esposurc recognize density differences in a film emul- is systematically changed. The lowest es- sion, not only because these differences may posure will yield a record of the lowest density

ABSTRACT: Agfacontour jilm is a photographic copying material which allows the isolation and perrniznent recording of the different rtensitie~present in ordinary .film negatirles or positizles. These separate records of aretss or lines of equc~l densities (equidensifies) may be fl~rtherprocessed by copying each one of them in different colors. .l color composite cun be produced in which slight r1aric~- lions in density in tlze originaC become readily apparent. This photographic enhancement method was erlaluated using a 70-mm aerial photograph obtained at a scale of 1 :160,000. The enhancements achieved demonsftafe the potential of the Agf(~contoz~rtechnique as czn aid in photo interpretation. be too small, but also because the mass of level only. The next exposure will add a information normally contained in a photo- second density level, and so on. By re-copy- graph often masks the features of interest. One ing the combination of one mask with the solution to this problem is manual or auto- positive of the subsequent mask, any desired matic densitometry. Another possibility is density level can be obtained. It is a simple density slicing, which leads to image enhance- but time-consuming process and registration ment. Density slicing is the separation of the of masks and positives is critical. different densities of a film transparency. These disadvantages have been overcotne It can be achieved, for example, electronically with the development of the Agfacontour with an image scanner, a particular kind of film. It is now possible to obtain equiden- image processor and a television monitor. sities in one single and simple process which The scanning camera records the amount of is similar to conventional photographic copy- light transmitted through the transparency. ing. This paper summarizes the character- This value corresponds to the density of the istics of the Agfacontour filrn and the tech- emulsion being scanned. The image processor niques by which photogral)hic enhancements digitizes this information and assigns an ran be pruduced (Kanz and Schneider, 1970).' identifying color to each one of the density Examples of enhar~cedaerial l)liotogral)lls of levels which are then displayed on the televi- forested and agricultural terrain illustrate sion monitor. These density levels or areas the potential of the method. The Agfacontour film is a black-and-white copying film. Its unique characteristic curve has a positive and a negative branch (Figure 1). The slopes are extremely steep: a negative gamma of about 14 for the positive branch and a positive gamma of about 7 for the neg- ative branch. At a given exposure, the film becomes transparent. Underexposed and overexposed areas remain black. If a trans- parency (negative or positive, black-and- white or, with limitations, color) is copied onto Agfacontour film, only a certain density level of this transparency will permit correct exposure, producing an area- (or line-) equidensity. As there is a stright-line rela- tionship between log exposure and density of the original that will print on the Agfa- contour film. the exDosure necessarv to ob- tain the desked deniity level of the'original can easily be predetermined or, in other words, the position of the equidensity is determined by the exposure. For example, if a one-second exposure is FIG. curves of Agfacontour film (modified from information supplied by Agfa- required to print the density 0 to 0.3, two G~~~~~~), seconds will be required for the next densitv step 0.3 to 0.6, fiur seconds for 0.6 to 0.9, etc. This procedure will produce adjoining ing light, the positive branch of the character- equidensities. If this factor of two is reduced, istic curve receives less exposure and shifts to the equidensities will overlap (Plate 1); if it the right (Figure 1). Because the negative is increased, the equidensities will separate. branch is sensitized to green light, its shift In this latter instance there will be gaps and will be minor. The effect is a narrow equiden- certain density levels of the original will be sity. For the first copy (first order equiden- lost. Without filters and with white-light sity) the width can be reduced to about 0.1 exposure (2,800°K), the width of the equi- density units. density is about 0.8 density units. The com- First-order equidensities can be recopied to plete density range of a normal negative can produce second-order equidensities which therefore be covered with 2 or 3 equidensities. appear as lines (contours) circumscribing the As the positive branch of the characteristic first-order equidensity. The density range of curve has its peak sensitivity in the blue this second copy may perhaps be as low as (420 nm) and the negative branch in the 0.03 density units. Further recopying pro- green (540 nm) band of the spectrum, the duces high-order equidensities with an even width of the equidensities can be changed narrower density ranre. with the aid of filters. Equidensities of one original can be inter- Yellow filters subtract blue from the print- preted separately or they can be reproduced

PLATE1 (upper left). Diazo color reproductions of equidensities. Steps 8 and 10 are adjoining equi- densities, Steps 9 and 10 are overlapping equidensities (see text). Note the third color in the regions of overlap. PLATE2 (upper right). Composite of four eq~ridensitiesobtained from the blue separation. PLATE3 (center lefl). Composite of six equidensities obtained from the green separation. PLATE4 (center right). Electronic enhancement obtained from the green separation. PLATE5 (lower left). Composite of nine equidensities obtained from the red separation. PLATE6 (lower right). Electronic enhancement obtained from the red separation.

AGFACONTOUR FILM FOR THE INTERPRETER

The material chosen for the evaluation of the Agfacontour enhancement technique was multiband aerial photography obtained on July 28, 1971 with 70-mm Vinten recon- naissance cameras from an altitude of 40 000 feet (scale of 1 :160,000). The test area was one of rolling topography mostly covered with a complex mixture of deciduous tree species. Four cameras were operated simulta- neously, three of which produced black-and- white records of the green, red and near- infrared bands. The fourth camera con- tained color-infrared film. The original black-and-white negatives, which were of extremely low contrast and resolution, were first copied onto Kodak- Commercial sheet film. Almost the onlv dis- FIG. 2. Agfacontour screen-equidensities of the cernible change in density was cauiedby central portion of the original green separation vignetting. Black-and-white negatives of (see text). better quality were obtained from the positive color-infrared transparency, which was copied onto Plus-X Pan film using Wratten 47B, 58 in different colors and then be combined into and 25A filters. The resulting records cor- a false-color composite. Agfa-Gevaert recom- respond respectively to the original green, mends two different color methods: the red and near-infrared reflectance of the chromogenic development which yields copies scene. Although vignetting was still apparent, of high resolution, and the Transparex contrast was higher and several density levels method which is much faster, but which yields within the vignetting circles were detectable copies of limited resolution (10 lines per (Figures 3 and 4). millimeter). This second generation material was Another possibility of enhancing the den- copied onto Agfacontour film. Equidensities sity differences of a photographic trans- were produced in 0.15-density unit steps over parency using the Afgacontour film is the the whole density range of the black-and- reproduction of screen-equidensities. A con- white transparencies. Filters used to produce tact screen is placed, for example, between these relatively narrow equidensities were two the original and the Agfacontour film. As the Kodak CC 40Y and a Kodak CC 10Y. All dots of this screen have variable density, reproductions were obtained by contact their size on the copy will be proportional to printing using the light source of an enlarger the amount of exposing light, which in turn with a color temperature of 3,100°K. The depends on the densities of the original. equidensities obtained were copied onto diazo Different patterns are formed which can color films for projection transparencies easily be correlated to the original's density (exposure to ultra-violet light, development if a step wedge is copied at the same time in ammonia vapour). (Figure 2). The width of these screen- In order to provide a standard for compari- equidensities will depend on the density son for the results of the Afgacontour tech- range of the screen dots. It is quite possible nique, the same second generation material to separate density levels of less than 0.1 was also enhanced electronically using a single density units ran^).^ This method was not channel, image density evaluation system. pursued further. Its advantage is that it requires one single exposure only. RESULTS Other characteristics of the Agfacontour The various enhancements produced from film are: emulsion speed-between contact the original black-and-white negative pro- and enlarging papers: base-180 microns, duced few changes and the resultsappeared clear polyester (very stable) ; resolving to be of little practical use. The color com- power-up to 40 line pairs per mm; develop- posites produced showed little more than ment-special developer, otherwise similar to concentric circles of different colors which conventional, black-and-white process. were caused by the increased density towards FIG. 3. G~~~~ separation from the original Fig. 4. Red separation from the original 1: 160, 1 :160,000 scale color-infraredpositive, obtained by 000 scale color infrared positive, obtained by ex- exposure through a Wratten 58 filter. posure through a Wratten 25A filter.

the edges of the photograph. The results for the Agfacontour film. Results would have electronic enhancement were similar. been more striking if photographs of better Enhancements produced from separations quality had been used. Some minor difficulties of the color-infrared film however were much with darkroom equipment also suggest the more successful. Many of the features which posdibility of improvement. This refers mainly could not be detected using the equidensities to the use of a good contact printer connected of one separation could be detected on equi- to a voltage regulator, to temperature control densities of another separation. Photo inter- in processing and to the use of either a con- preters have the options of studying repro- tinuous density wedge or a step wedge grad- ductions separately or combined with one uated in 0.1 density units. The results ob- (Plate 1) or more (Plate 2) equidensities of tained are, nevertheless, encouraging. Equi- either the same or different originals, or they densities can be produced in a very simple can study a complete composite (e.~., manner without the need for so~histicated Plates 3 and 5). instrumentation. If care is taken, results are Many important differences in vegetation accurate and can be repeated. Provided im- types could be delineated using this approach. perfections in the original are eliminated As expected, the separation of coniferous and (such as vignetting), this accuracy permits deciduous tree species was best on equi- objective classifications of density differ- densities produced from the red separation ences within the original. This technique which provided a record of near-infrared re- might represent another step towards rela- flectance. In the centre portion of Plate 5, tively simple automation of photo inter- coniferous species appear white, because the pretation. equidensity corresponding to the highest Only first-order equidensities are of interest density of the original was not included in the in conventional photo interpretation of composite. In contrast, deciduous species vegetation because they can easily be cor- appear yellow and red. Towards the edge of related with the original. Higher-order equi- the photograph coniferous species are rep- densities are not only difficult to interpret resented by magenta. The two groups of but also increase the amount of work con- species were difficult to separate on the siderably. They may have special applica- original (Figure 4). The basic similarity and tions such as in photo-map production. For the differences between Agfacontour and example, it is possible to produce records of electronic enhancements are illustrated by lines which may outline agricultural fileds Plates 3 and 4, and 5 and 6. (Ranz and Schneider, 1970), transportation networks, urban developments, etc. Some other advantages of Agfacontour This study was a preliminary evaluation of equidensities are : AGFACONTOUR FILM FOR THE INTERPRETER

The possibility of combining the information it available to anyone with access to a photo- of several images into one single composite. graphic darkroom. This includes multiband as well as multidate photography. Combination of equidensities REFERENCES obtained from multidate photography would 1. Ranz, E. and S. Schneider, 1970. Der Aquidensi- probably aid in change detection. tenfilm als Hilfsmittel bei der Photointerpreta- The high resolution obtained, which makes it tion. Bildmessung und Luftbildwesen, 2. p. 123- possible to apply conventional interpretation 134. criteria and techniques, such as stereoscopic 2. Ranz, E. Agfacontour-a new film for simple observation. reproduction of equidensities. Mimeograph The simplicity of the technique which makes paper.

Annual ASP-ACSM Convention Washington, D.C. March 1 1 -1 6, 1 973

Illustrated above is the certificate that is available to members of the American Society of Photogram- metry. The original, suitable for framing, is 83 by 11 inches with the name of the member attractively hand engrossed. The price is $2. Orders should be sent to ASP, 105 N. Virginia Ave., Falls Church, Va. 22046.