TSK 11 Göttingen 2006 Kouankap & Sylvestre A new practical and econom- ical technique for the obten- tion of high quality thin sec- tion photographs with the help of a simple optical mi- croscope Vortrag Nono Gus D. Kouankap1 Ganno Sylvestre1 Microscopic study is an indispensable step in the determination of the pet- rographic characteristics of rocks. In Cameroon just like in most developing countries, simple polarizing microscopes of the 20th Century constitute the only available material for macroscopic stud- ies. A major difficulty encountered in the use of these instruments resides in the realisation of photographs of the dif- ferent parts of the thin section that re- quire illustrations. The technique that has been used upto now has been inef- ficient and unsatisfactory. Faced with this recurrent situation, we thought of setting up a new, semi-modern, practi- cal and very economical technique for photographing the thin section. The setup that has existed till present day consists of an ‘Euromex’ microscope with eyepiece fixation plates (removable eyepiece plate). The principle consists of removing the eyepiece plate and then fixing a second one which is specially conceived, for, it bears a metallic tube on which can easily be connected an ar- gentic or classical camera, that is, using films. Once the camera is connected, the part of the thin sections to be filmed is carried out through a fastidious set up step of the image, for, it is the eye piece of the camera that is used rather than 1 Université de Yaoundé I, Départe- ment des Sciences de la Terre, BP : 812 Yaoundé-Cameroun that of the camera; the zoom of the cam- era is fixed. Once the setup of the image is over, room light has to be eliminated so as to avoid any influence on the qual- ity of the films. Moreover, the source of light of the microscope should neither be too low, nor too sharp, because on this factor strictly depends the quality of the photographs. All these problems suppose that, for the realisation of good quality photographs, it is better to pro- ceed at night while putting off all room lights (electricity). However, after having thoroughly re- spected all the above constraints and obligations related to light, it is very recurrent that after having realised the pictures, one still obtains empty nega- tives during the process of development. This is because the camera was unable to fix a single image. As a result, in or- der to increase the chances of obtaining images, several photographs are usually taken on one single section, while mod- ifying the intensity of the light source after each film. With this technique, we succeeded in obtaining few images whose quality was highly denatured by light. The few images were then later on scanned so as to have a numerical ver- sion. It should however be noted here that due to the low resolution power of the scanner, the exact nature of the original photograph is no reproduced. Finally, mediocre to poor quality pho- tographs are obtained. This method, which is long, fastidious and expensive, is a serious obstacle in the progress of most research works of geologists in third world countries. This is because is not only a time consum- ing technique, but also a non-reassuring one. The final product is only lightly satisfactory. Because of the above con- straints, it was necessary to revolution- 1 Kouankap & Sylvestre TSK 11 Göttingen 2006 ize the method. Faced with the persisting difficulties, the establishment of a new technique of microscopic photography was then en- visaged The setup is based on the po- larizing microscope, a numerical camera and an adapter. • the polarizing microscope can ei- ther monocular or binocular; • the adapter is a hollow cylinder or a plastic cylinder whose diameter is greater than that of the objec- tive of the numerical camera, with its hight varying between 3 and 5 cm. The adapter should also have a diameter which is slightly greater than or equal to that of the eyepiece of the microscope. The adapter plays the role of extension of the eyepiece thereby facilitating move- ments (translational movements) of the zoom (objective) of the camera. • numerical cameras of all sorts can be used so long as their objectives have diameters which are close to that of the eyepiece of the polariz- ing microscopes. The principle consists to: • Place the thin section on the min- eral plate and chose suitable sec- tions to be filmed • fix the adapter on the eyepiece of the polarizing microscope • insert the zoom (objective) of the numerical camera inside the adapter • Progressively move the camera so that the light source from the eyepiece of the microscope should passe inside the objective of the camera (coincide the objective of the camera with the eyepiece of the microscope. • Once the previous step is properly done, the zone of the thin section to be filmed will be seen on the screen of the numerical camera. At this point in time, the zoom can be readjusted for the best quality pho- tograph to be obtained. The advantages are: • the section to be filmed is visible on the screen of the camera; • after filming, the photo is immedi- ately observable and if its quality is not very good, it is possible to cancel the picture and to repeat the process on the same zone of the thin section; • the pictures obtained are of very high quality and directly transfer- able to a computer without fur- ther scanning, as in the previous case, for the various manipulations. Moreover, with the aid of modern software programs, it is possible to improve the quality of those images in the computer. • The method is relatively easier, faster and cheaper. However, the only obstacle or inconve- nience encountered in the new technique resides in the determination of the scale of the photographs. This is because it is difficult to determine the rate of enlarge- ment or reduction of the camera’s objec- tive as well as that of the computer. In order to overcome the obstacle, the fol- lowing steps have been proposed: • On each section to be filmed, choose a given mineral, mark it 2 TSK 11 Göttingen 2006 Kouankap & Sylvestre and measure its dimensions with the help of the graduations on the crossed nicols. The obtained value is termed initial dimension (Di) or real dimension. • After computer processing and printing of the photographs, the marked minerals are then measured again and the obtained value is termed final dimension (Df ). • The scale of the photo is the ratio of the real dimension to the final dimension E = Di/Df The proposed method enables to obtain high quality photographs, but, it how- ever remains rudimentary, for it requires much skills and manipulations. More- over, the polarizing microscope does not offer durable observations; it will be nec- essary to construct a more practical and less expensive microscope on which is incorporated a numerical camera that will meet up with the expectations of all. Thus, microscopic observations will be carried out for longer periods of time (because the light will no longer exhaust the eyes with time) and in groups, thus permitting constructive discussions. 3