In processing by reversal, the aim is to produce a positive image directly. There are various ways of doing this, but with amateur movie films it is usual to employ a special type of emulsion, and to treat this with suitable chemical solutions to bring about the desired result. Briefly, the steps involved are as follows.
After exposure in the camera, the film is first developed to a negative in the normal way. It is then washed, but instead of being fixed, it is treated with a form of bleaching solution which dissolves the silver grains making up the grey negative image formed by the developer. It is important to note that the bleaching solution does no more than this, and leaves untouched the undeveloped - and still light-sensitive - silver salts in the emulsion which, in the normal way, would be removed by treatment in a hypo fixing bath.
Where once the negative image existed in the emulsion, the action of the bleaching solution leaves only gaps - more or less transparent according to the actual density of the negative image from point to point. The rest of the sensitive emulsion is still unaltered.

The next step is to re-expose the film so that the whole of the remaining sensitive emulsion is affected. This is done, not in a camera as in the first exposure, but in the open, to a naked lamp or white light for a sufficiently long period. The film is then put through the developer once again and a second silver image produced. A final treatment with hypo is sometimes given, but is scarcely necessary since no undeveloped silver salts remain to be dissolved away from the film.

Reference to Fig. 1 will help to explain how this process results in a direct positive image. Let us imagine that we are to photograph in our cine camera an object containing the three tones as shown at stage A. We will assume that conditions of exposure and development of the film are such as to give a negative image on the film as shown at stage B. For the sake of simplicity, this negative image shows complete blackening of the emulsion in the upper portions which received the greatest exposure, an intermediate degree of blackening in the lower portion where the exposure was less, and no blackening at all in the central region where little or no light was reflected from the original. Since we are not going to fix this negative in hypo, unexposed sensitive silver salts will be left in the film, in amounts reciprocally proportional to the extent of the exposure received by the film. Thus, the amount of this unexposed sensitive silver salt will be greatest in the central region of the image, somewhat less so in the lower portion and completely lacking in the upper portion.

With the film still handled in the safe lighting of the darkroom, it is now subjected to the action of the silver bleach. Since this removes only the developed silver image, we are left, as at stage C of Fig. l, with a completely clear area in the upper portion of the image, a partially clear area in the lower portion and a relatively opaque central region. The opacity in the central and lower portions of the image, is due, of course, to the unaffected silver salts present in the emulsion and is normally removed by the action of the fixing bath.
The film may now be handled in the open light, and the next stage in the process consists of completely fogging the remaining silver salts in the film by exposure to a suitable source of light. All that remains to be done now is to develop up these fogged silver salts to produce the effect shown at stage D; the greatest silver density is obtained in the central region with relatively less in the lower portion and none at all in the upper portion of the image. In other words, we have produced a direct positive of the original object.
The above description is admittedly sketchy and in practice there are a number of auxiliary points to be watched. Their actual nature depends on the particular bath compositions, but in any case, they do not affect the principle of the process outlined.

Fig. 1 : The Mechanism of Reversal Processing
Stage A : Original subject, composed of three tones. As seen by camera.   
Stage B : After development ; silver image shown by hatched lines ; unexposed silver by dots.   
Stage C : After bleaching ; silver image removed, leaving only unaffected silver salts in amounts depending on negative image density.  
Stage D :  After exposure of unaffected silver salts and further treatment in developer, to produce a positive directly.

Types of Solutions Used

Although any standard negative developer can be used for the first development of the exposed reversal film, it is found that best results are given by one containing silver halide solvent. It is usual, therefore, to add a salt such as sodium thiocyanate. The same solution can be used for the second development of the film, but as the silver halide solvent serves no useful purpose here, any standard metol-hydroquinone developer will do instead.

For the silver bleach, either acid bichromate or acid permanganate will serve, and treatment in these is usually followed by a short wash and subsequent immersion in a weak solution of sodium sulphite or bisulphite to remove the yellow colour imparted by the bleach. A further short wash follows before the second exposure is given.

Approximate time in minute s

1  First development (with silver halide solvent)  5-10  
2  Harden (formalin)  3  
3  Rinse in running water  2  
4  Bleach out silver negative image  7-10  
5  Clear away yellow stain (sulphite)  3  
6  Rinse in running water  2  
7  Second exposure - to white light
8  Second development  3-10  
9  Rinse in running water .  2
10  Fix (acid hypo  5  
11  Wash in running water  20  
12  Dry

Particulars of typical formulae used in the above operations are given on the Processing Formulae page.
After the second development is complete, comes a water rinse followed by a few minutes' treatment in an acid fix, and the usual thorough wash. Drying must be carried out in a dust-free atmosphere, as any particles that cling to the surface of the wet film will remain to show as unsightly blemishes in the much magnified image on the screen.

The chemical action of the acid bleach has a distinctly weakening action on the gelatine of the film emulsion. It is, therefore, a usual precaution to harden the gelatine before projecting it to the bleach, and for this purpose a solution of formalin is satisfactory.

The various operations involved in processing a reversal film are tabulated on the previous page. Approximate times for each stage are shown, but they depend ultimately on the actual bath composition in use, as well as on the type of film to be processed.
Errors in Exposure
Since the extent of the second exposure in reversal processing depends very largely on the exposure received by the film in the camera, it is useful to examine here the results obtained by under- and over exposure in the camera. How far these variations in exposure can be tolerated is dealt with below in 'Compensated Second Exposure.' Fig. 2 shows in diagrammatic form the general results of under- and over exposure of a reversal film. By working through each stage of the process as in the explanations of Fig. 1, it will be seen that the following results are obtained.
Under exposure
Because an undesirably thin negative image is formed in the first place, insufficient silver is dissolved away from the film during the bleaching process. The excessive amount of silver salts left to be developed after the second exposure, tends to give veiled highlights and clogged shadows. The whole picture looks flat, lifeless and too dark.

Over exposure

The negative image formed is undesirably heavy and too much silver is dissolved away from the film by the bleach. The unaffected silver salts left are insufficient to give a full-bodied positive, and the final picture has burnt-out highlights and poor detail in the shadows.

How far it is possible to correct for these exposure errors once the film has been processed it is doubtful to say. Generally speaking, however, little or nothing can be done with the overexposed film. It is possible to improve it by intensification, but usually only at the expense of increased grain or colour change. Reduction of the under exposed reversal film is rather more likely to be successful, and a short treatment in dilute ferricyanide solution, followed by a short fix in plain hypo solution, is generally sufficient to bring about the desired result.       

Fig. 2 :  The effects of Under- and Over exposure on Reversal Processing.   Under exposure :  results in lost highlights and clogged shadows.  Over exposure :  results in burnt-out highlights and lost shadow detail
Compensated Second Exposure

A more elegant way of correcting the errors of under- and over exposure is that known as `compensation of second exposure. This is the technique used by most commercial film processing laboratories, and by its means quite considerable variations in exposure can be successfully handled.
How it works is shown in principle in Fig. 3. As in Fig. 2, the effects of under- and over exposure should be traced through until stage C is reached. Now, we have already seen that with under exposed film the image at stage C will be carrying rather more than its desirable share of sensitive silver salts; on development these give too dark a picture. What is needed is some way to expose just enough of these silver salts to give a picture of the correct density; any silver salts remaining after development can then be removed in the usual way by fixation in hypo. Conversely, with over exposed film, we need to be able to give an exposure rather greater than average in order to bring up the density of our picture to the desired level.       

Fig. 3 :  Compensation for Exposure Errors by control of Second Exposure in Reversal Processing. Under exposure in the camera is compensated by decreasing the extent of the normal second exposure; over exposure is compensated by increasing the extent of the normal second exposure.

This problem has been solved by the provision of automatic means for quickly assessing the average density or `weight' of each picture at stage C in the processing cycle, and adjusting the intensity of the second exposure lamp appropriately. Obviously, for this procedure to be as effective with overexposed as for under exposed films, the average, non-compensated exposure must be so regulated that it always leaves part of the silver salts, present at stage C, unexposed. This is the general rule and it explains why in practice final fixation of the film is required.

The application of the compensated second exposure technique to the continuous processing of reversal films is a straightforward procedure, and commercial systems are described in the patent literature. By its use, the effective exposure latitude of the reversal film is considerably extended, and few amateur movie makers realise how much they owe to compensated second exposure devices in the avoidance of wasted shots in their films.

Where for special purposes, such as making fades, etc., the introduction of this compensated second exposure would prove objectionable, a note to this effect should accompany the film when it is sent for processing. In such a case, a non-compensated second exposure is given instead.
Compensated second exposure is not given to reversal colour films (e.g. Kodachrome) because of the technical difficulties involved in processing these films and this means, of course, that the correct exposure estimation of colour film is all the more important for consistently good results to be obtained.
Grain in Reversal Films
One result of reversal processing is a relatively finely grained image in the positive. Here is why. The fast emulsions used on camera reversal films contain a high proportion of the large extra-sensitive silver halide grains or salts. When developed to a negative, these tend to produce a coarse-grained structure in the image. In the reversal procedure, it is precisely these large grains which are removed, as silver, by the bleach, leaving the relatively slow, smaller grains of silver salts from which to make the final positive image, so that in general the overall grain structure of the reversal positive is pleasingly fine.