We have seen before that light, hitting the photographic emulsion spread on the film, modifies the structure of the emulsion, producing an image that is not yet visible [called a latent image) which can be made visible by the developing process.
Not all light wavelengths have the same effect of producing a latent image. The first photographic emulsion that was used in cinematography wasn't sensitive to every colour. The only wavelengths that produced a latent image corresponded to the colour blue [or were in the invisible ultra-violet range], so these early simply photographic films were called "normal" or "blue-sensitive". Does that mean that only blue objects were reproduced? No, you should note that blue is present, even if in tiny amounts, in many colours.
White light includes these blue wavelengths and the blue component of white light will produce a latent image. If, however, red or green light is projected onto a blue-sensitive emulsion no latent image is produced. Therefore, any image obtained with these primitive emulsions didn't permit a complete reproduction of the colours and objects. A good example would be a green parrot with blue wings. Filmed on blue-sensitive film, the negative image would indicate black wings and transparent body. On the positive, the wings will be completely white and the body completely black. The parrot has become a magpie!
It is for that reason that the first improvement to emulsions was to impart a sensitivity to green, the so-called orthochromatic (meaning sensitive to all colours!) film. To return to the parrot: now it will have wings and body that are both grey, since the film is sensitive to both green and blue. It will no longer be a magpie. However the red curtain used as a background will turn out black in the positive as these films had no sensitivity to red.
Eventually emulsions were invented that were not only sensitive to green, but to red as well. These are called panchromatic film. With this film, the parrot now appears in front of a grey curtain. Three different tones of grey that reproduce three different colours: blue, green and red.
Therefore, here we have our first distinction among the emulsions, the black and white cinematographic film: we can have blue-sensitive, orthochromatic and panchromatic films. Naturally, nothing is perfect, not even in the cinema. Just as it is difficult to find a colour that is completely pure, i.e. that doesn't contain even the minimum part of another colour, so also the sensitivity of film isn't 100% restricted to only one colour. Then, as you have seen from the drawing of the visible spectrum, the boundary between one colour and another isn't very precise. Therefore, a blue-sensitive film has very little sensitivity to green, and the same for orthochromatic film to red. Then in panchromatic film the sensitivity to the three colours isn't identical. Research aims at obtaining a perfectly panchromatic film, but it still hasn't got there. For this reason every film will have a different sensitivity to the three colours and this sensitivity can be measured and put on a graph. Here below you'll find illustrative graphs of three types of film. These graphs, which refer to the sensitivity to the three colours of the spectrum, are called spectral sensitivity graphs.
Early panchromatic films did not have sensitivities to red, green and blue that were the same as the human eye so that accurate rendition of colour in terms of shades of grey was not accurate until the 1950's.
Different manufacturers considered the ideal or most practical film to be different in almost every case. The sensitivity of every emulsion type can be measured and displayed on a graph known as a spectral sensitivity graph. The sensitivity or response of a film to various wavelengths throughout the visible spectrum is called Chromatic Sensitivity.
The camera negative should have the best colour reproduction possible and therefore should be panchromatic. Naturally, there are existing negative films that are meant for particular uses that require orthochromatic or blue-sensitive films.
A positive is obtained by printing a negative onto another film. A black and white positive is obtained from a black and white negative, in which there aren't any different colours, but only variations of density or blackness, shades of grey. Therefore, it is not necessary that print film for the positives be panchromatic. Most print films are blue sensitive only. This simplifies their handling as they can be exposed to red and green light without producing any effect and so can be used in rooms in which are lit by yellow light [known as safelights]. Throughout the various procedures of printing, whether of prints, duplicate negatives of internegatives, blue sensitive emulsions are all that is necessary provided the original is an image in terms of shades of grey.
Sound negatives, as will be dealt with in a later section, are also images in shades of grey and should be copied with blue sensitive emulsions.
If however, the positive has been coloured (as often happens for films from the silent era) or is in colour, then it is essential to use a panchromatic film stock to make a copy negative or print.
Obviously, white light must never be used in the handling of film prior to development. The safe light for use with blue-sensitive or orthochromatic film is red light. These films have very little sensitivity to red, but, even though a red lamp emits almost only red light, it also emits small amounts of other colours as well. Even when red light is used, it should always be at low power and for a period well within the fogging threshold of the film stock.
Manufacturers are very specific about the safelight that can be used and their recommendation should be adhered to, rigidly. It is always wise to test the illumination equipment regularly as safelight can be physically damaged or their dyes may be destroyed by heat and allow unwanted wavelengths through. A good method is to take a small piece of raw stock from the reel and leave it on the worktable for a few minutes. As long as would normally be necessary for the work you have to do and then a bit longer and then develop it to test it for fogging.
Complete darkness is necessary for the handling of most panchromatic camera negatives, which have a very high general sensitivity, and of colour internegative or intermediate materials.
Cinematographic film, whether it is to be used for filming or in the laboratory, comes in reels of various lengths. Camera film is sold in reels of 30 or of 120 meters. Film to be used in the lab (negatives, duplicates, intermediates and positives) is usually available in reels of 305 and 610 meters, but some print materials are available in rolls of up to 1200m.
Every reel is contained in a light proof black bag, which is hermetically sealed in a metal can. Opening a can of unprocessed film is one "skill" that is rarely mentioned. In the dark or under the recommended safelight, lay the can on a flat surface, remove the tape round the edge, and remove the lid.
Lift out the bag and lay it beside the can. Take the reel out of the black bag. If you hold it in your left hand with a finger through the centre and so that the free end, the beginning of the film, falls towards you, you should be able to tell which side of the film is treated with emulsion so that you can handle the film correctly.
The emulsion is usually easy to recognise: it is the rougher side whereas the base is perfectly smooth. You can see the difference clearly under incident light, or, if you are in the dark of the developing room you can touch the strip to your lower lip: the emulsion will stick to your lip, the base won't. Usually reels of sensitive material are wound with the emulsion towards the inside.
This is true for 16 mm gauge film as well as 35 mm. 16 mm positive film often has perforations on one side only. If the reel that you've taken out of the can is a 16 mm positive, the perforations could be on one side or the other. Manufacturers supply the film wound in two ways: the A wind and the B wind. If, holding the reel of 16 mm film in your left hand and unwinding the film with your right and with the emulsion on the lower side, you see or feel the perforations on the left side, then it is an A wind. If, however, the perforations are on the right side, then it is a B wind. The same principle is used for other types of film that don't have symmetrical perforations on both sides. The choice of winding needed will depend on the printer or the method of work.