PAPER No. 7: Criminalistics & Forensic Physics MODULE No. 31: Black & White & Colour Film Processing and Printing. and Printing

Transcription

1 SUBJECT Paper No. and Title Module No. and Title Module Tag FSC_P7_M31

2 TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. Formation of Black and White Films 4. Black and White structure and forms 5. Film Identification 6. Steps in Black and White film processing 7. Basic Requirements for Black/White film processing 8. Constitute of Black/White film developer 9. Stop Bath after film processing 10. Colour Film 11. Summary

3 1. Learning Outcomes After studying this module, you shall be able to know How Black & White and Colour Film is processed Learn about Black and White film formats Constitute of Black and White film Developer 2. Introduction Light striking on film causes a reaction in light-sensitive layers. The extent of the reaction is dependent upon the amount of light hitting the film. Too much light will over expose the film which will make it fully black (in the extreme case). Too little light will under expose the film and leave little or no image. An exact amount of light needs to strike the film to get the best result. The amount of light required is not same for all films and depends on film speed. 3. Formation of Black & White Films Anti-scratch Layer: This layer protects the emulsion from physical damage. Cross-section of polyester base black-and-white photographic film Emulsion: This layer contains the halides which form the photographic images.

4 Adhesive Layer: This layer bonds the anti-halation and anti curl layer to the base. Film Base: It is transparent layer which supports the image. Anti-Curl/ Anti-halation Coating: This layer prevents the film being exposed from the rear and stabilizes the laminate to prevent curling. 4. Black/White Structure & Forms i. A super coat of gelatine, a few micrometres (one micrometre is millimetre) thick, protects the emulsion from scratches and abrasion marks. ii. The emulsion layer (silver halide suspended in gelatine) is usually nine to 12 micrometres (up to 1 /2,000 inch) thick but may sometimes reach 25 micrometres. iii. A substrate or subbing layer promotes adhesion of the emulsion to the film base. iv. The film base is usually cellulose triacetate or a related polymer. The thickness may range from 0.08 to 0.2 millimetres (0.003 to inch). Films for graphic arts and scientific purposes are often coated on a polyethylene or other polyester support of high dimensional stability v. A backing layer on the rear of the film base counteracts curling. Usually it contains a nearly opaque dye to suppress light reflection on the rear support surface. Such reflection (halation) reduces definition by causing halo like effects around very bright image points. Some film bases are tinted grey to absorb light that has passed through the emulsion layer. 5. Film Identification

5 A cartoon of film contains information like the a) Manufacturer b) Type of film c) Film Speed d) No. of Exposures e) Expiry Date f) Processing Instructions g) Storage Instructions etc. Black & White Film Formats 35 mm films 120 films

6 Spectral Sensitivity of Black & White Film Orthochromatic This emulsion is sensitive from violet to green region of the spectrum and hence can be handled in deep orange light which is safe for the film. Panchromatic -This type of film is all colour sensitive i.e. it responds to all the colour of the spectrum even up to ultra violet rays. So, it should be handled in complete darkness. Infra-Red Sensitive This type of film is sensitive from Ultra Violet to Infra Red region of the spectrum. Black and White Film Latitude The latitude of a film is the amount of exposure error the user can make before the picture degrades noticeably. Black and White Print and Colour print film have quite a high latitude ie. at least a full stop either side of the ideal exposure may be allowed. However slide film has latitude of about 1/3 stop either side of ideal.

7 6. Steps in Black & White Film Processing 1. Working solutions of developer, stop bath and fixer to be prepared. 2. Film from the canister to be loaded in the processing spool. 3. Working solutions to be brought to the required temperature (generally 20 o C for B&W). 4. A pre-wash of the exposed film in plain water at the above temperature is good. 5. The developer has to be poured in the processing tank and agitated appropriately to begin development. 6. The developer should be poured out about 10 seconds before the appropriate development is over. 7. Pour in the stop bath. This is pretty quick and should only take about 30 seconds - 1 minute to do its work. Pour out after the required time. 8. Pour in the fixer solution and leave for the recommended time, commonly 2-5 minutes. Pour out after the required time. 9. Wash the film. There are several methods of doing this, but probably the easiest and quickest if you can't wait to see the results is to fill the processing tank up and invert 5 times. Pour out and re-fill. Invert 10 times. Pour out and refill. Invert 20 times. Pour out. 10. Remove film and place a film clip at either end of the film (one to hang up and the other to weigh down). 11. Squeegee the film to remove surplus water. Leave to dry.

8 7. Basic requirements for Black & White Processing Rolls of exposed Black & White film. Daylight type developing tank. Film can opener. Beaker or measuring cups. Thermometer. Scissors. Containers (of mixed chemicals). Negative carrier. 3 Containers for mixing chemicals. 8. Constituents of Black/White film developer 1 Developing Agent The main chemical which develops the film or paper viz. Metol (C7H10NO)2SO4, Phenidone (C9H10N2O) 2 Preservative This chemical saves the developer from being oxidized by atmospheric oxygen. Viz. Sodium Sulphite Na2SO3 3 Accelerator This chemical (alkali) helps quick completion of development. Viz. Sodium Carbonate Na2CO3 4 Restrained This chemical stops or controls the vigorous chemical reaction and thus protects the unexposed silver from being attacked by the developing agent and thus stops the fog formation. Viz. Potassium Bromide - KBr. 5 Solvent All the chemicals described above are of solid form. So for homogeneous mixing a solvent is required which is generally water H2O. Standard formula of B/W film Developer The standard formula of a Metol Hydroquinone is as follows. The chemicals should be added to water as per the order. Metol - 2 gm Sodium Sulphite - 60 gm Hydroquinone - 8 gm Sodium Carbonate - 30 gm Potassium Bromide - 1 gm Water cc

9 9. Stop Bath after Film Processing When the film development time is complete, either throw away the developing fluid or save for further use. Some manufacturers tell us about how many films each quantity of developer can process, the numbers of cycles affect the development time. When the developer has been removed from the tank, the development process is abruptly stopped by use of a stop solution. The stop solution is mildly acidic and neutralizes the effect of the alkaline developer. It should be left in the tank for around a minute and then poured out, back into the storage bottle. Fixing for Non permanent Image Development of films gives us visible images, but this will decay with re-exposure to light. So we must fix the image by dissolving the unexposed silver halide in Hypo by making a fixer solution as follows:- Sodium Thiosulphate gm Sodium Meta bisulphate - 25 gm Water cc The fixing time in a freshly prepared solution is approximately 5 minutes. Once the solution has been stopped, the film needs to be fixed. Fixing prevents the black and white film from being reactive to light and any more development process. It also fixes the film emulsion into transparent material leaving only the black silver oxide. Fixing should be for around 4-5 minutes depending on the strength and age of the fixer solution.

10 Fully developed but unfixed film 10. Colour Film Same Colour film has a similar sandwich construction but is more complex due to its sensitivity to the three primary colours, individually.

11 Instead of one layer of light-sensitive particles, colour film has three light-sensitive layers. Black-and-white film reacts to short wavelengths, mainly light perceived as blue. If we are taking a picture of something blue, red, and orange, the blue part would be too light, and the red and orange part would look very dark. So, as photographers found out about this problem, they added dye sensitizers to the film to correct the color. Film today, is sensitive to all colors in the spectrum. Today s film is usually coated with three emulsions, which are usually cyan, yellow, and magenta. Each one of the emulsions is sensitive to only one of light s colors. With the emulsion layers, there is also a dye layer, which makes a color that is equivalent to what a human eye sees. How negative colour film works

12 How positive colour film works

13 Color Positive Films actually use all the subtractive color method where instead of Red Green and Blue, it uses Yellow, Magenta & Cyan. In the Cyan, Magenta, Yellow system, the top yellow layer is actually recording the Blue information, the middle Magenta layer records the Green information and the bottom Cyan layer is recording the Red information. But most people relate better to RGB color. So I will describe it that way... Colour Negative Film Processing The most common chemical kit out there is the C-41 Kit. It will include four easy-to-mix powdered chemicals and according to the box, should develop rolls of 35mm film.

14 Mixing Colour Chemicals A narrow neck will help in keeping air out and a wide base will not let them tipping over when working Start by placing the chemical bottles into the hot bath. Insert a thermometer into a developing bottle and wait till it reaches 39.5 C. This is actually half a degree hotter but it was found that in pouring the chemicals into the developing tank, the chemicals will get cool enough to place it right on target.

15 Black & White, Colour Printing on Photographic Paper The main major equipments required for black & white printing are the i. Enlarger ii. Photographic Paper iii. Easel iv. Developing Trays v. Safe Light vi. Chemicals vii. Print Drier. The Enlarger is a major piece of equipment for printing by means of a projector. Basically enlarging consists of projecting an illuminated negative through suitable lens on a light sensitive paper. An enlarger is vertical type consisting of a light house, condenser, negative carrier and extension bellow. An even distribution of light is important so that there should not be any hotspot of light. When a negative is projected through an enlarger, the light loss in the corner of the negative is balanced by the light loss of the enlarger to produce an even distribution of light. The enlarging lens is the most important piece of equipment since the clarity of the print depends on the capability of the lens in producing sharp image. A faster lens, a f/2.8 rather than f/4 permits shorter exposure times and provides a brighter image for focussing. Black & White Photographic Paper Paper Types Chlorobromide Paper: Used both for contact printing and enlarging. By suitable lengths of development, tones can be varies from reddish brown to warm black. Brown safe light must be used while handling this paper. Speed of this paper is 50% higher than chloride paper. This type of paper has more exposure latitude than chloride paper. Bromide Paper: It is the most light-sensitive paper. The main emulsion ingredient is silver bromide in gelatine. It must be handled in safe light of orange.

16 All black & white photographic papers are available in different paper base like Glossy, Matt, and Velvet, Crystal and Semi Matt. and different types of base colour like White, Cream and Yellow. Black & White photographic papers are available in two types of thickness namely 120 gm (single weight) and 240 gm (double weight). Paper Grades Ultra Soft Soft For over negatives Special 6 tones For normal negatives Normal 5 tones 8 or 9 tones 7 tones For under negatives Hard 4 tones Ultra Hard 2 tones (black & white) An Easel Black & White Photographic Paper

17 Paper Developer Normal standard paper developer consists of the following ingredients: Metol gms Dilution 1:1 Sodium Sulphite gms Processing Temperature 80 F Hydroquinone gms Processing Time 3 minutes Sodium Carbonate gms Potassium Bromide gms Water cc Soft standard paper developer consists of the following ingredients: Metol gms Dilution 1:1 Sodium Sulphite gms Processing Temperature 80 F Sodium Carbonate gms Processing Time 3 minutes Potassium Bromide gms Water cc Enlargement from high contrast negatives can be developed in double bath system for better result. In double bath system 30% over exposure is required and development should be reduced to 30% (soft and normal developer). First put in soft developer for 1½ min and then in second developer for 1 minute. No intermediate wash is required.

18 Stop bath This is prepared with 2% Acetic Acid in 100cc of water. It neutralizes the alkalinity of developer in the print. Fixer This is prepared by mixing 250 gms of Hypo, 2.5 gms of Sodium meta bisulphite in 1000 cc of water. Fixing time is 15 minutes in fresh solution and 20 minutes in exhausted solution. Developing Processing Trays (left), Tongs (middle) and Timer (right)

19 Colour Printing In colour printing the prints are made by exposing negatives on colour paper maintaining the same steps with the help of a colour enlarger and colour chemicals. The primary difference between color paper developing and B&W paper developing is the chemicals, composition of paper and temperature. Black & white emulsions have a single layer or two almost-identical layers in the case of variable-contrast paper. Development can be done in cold developer (slow rate) as long as the time of development is increased proportionately. The same is not true for color development. Color papers have three layers of emulsions, each layer responding to and developing into a different color. The main rule in color development is constant temperature and consistent times and a completely black darkroom. Color papers respond to all colors of light. Black & white film does too, but not B&W paper. Any light in a colour darkroom will expose the paper, so enlargement is done in complete darkness. Once a trial print is made, assessment fir colour cast has to be done to evaluate the print s color. For example, the cast may be greenish, which can be corrected by increasing the magenta filter of the colour enlarger. Similarly a cyan cast may be corrected by increasing red which is its complementary colour. Illustration: Additive / Subtractive color wheel. Complementary colors (example: magenta and green) are opposite one another.

20 At first it may be difficult to differenciate between colors, especially between red and magenta, between cyan and blue or between yellow and green. Looking at a print (especially white areas of a photograph or a corrected print) through colored viewing filters will help you to see what each color looks like as a cast. ENLARGER COLOR FILTRATION Once we know the color and strength of the cast, it can be adjusted by filtration in the enlarger to correct the imbalance. While it is possible to print color on a black and white enlarger by placing individual CP or CC filters, enlargers with a dichroic head for color printing are more common. Dichroic heads contain three filters: cyan, magenta, and yellow. Each can be adjusted separately. Collectively, the filters are known as the colorpack. The filtration of each color enlarger varies depending upon the type of enlarger, its age and the age of the bulb. If we use a particular enlarger to print consistently, we can determine a good set of values for that particular color pack and a particular brand of colour film when color correcting a new print. CHANGE THE ENLARGER COLOR FILTRATION Once we have analysed the trial print s colour cast (for example, green) adjustment through filtration in the enlarger will neutralize that cast. The final correct print may come after two to three correction processes.

21 REEVALUATING IMAGE DENSITY Increasing filtration blocks light, decreasing filtration allows light to pass more easily to the print. As a result the image density may change. As a general rule, for every 10 points of filtration you add, add one second of time to the total exposure. For every 10 points of filtration you subtract, subtract one second from the exposure. 11. Summary 1. Orthochromatic emulsion is sensitive from violet to green region of the spectrum and hence can be handled in deep orange light which is safe for the film. 2. Black and White Print and Colour print film have quite a high latitude ie. at least a full stop either side of the ideal exposure may be allowed. However slide film has latitude of about 1/3 stop either side of ideal. 3. Fixing prevents the black and white film from being reactive to light and any more development process. 4. The primary difference between color paper developing and B&W paper developing is the chemicals, composition of paper and temperature.