Radiographic recording Film Materials Film groups n n

Radiographic recording Film Materials

Film groups n n X-ray films • Films exposed to X-radiation alone – (Direct exposure film) • Films exposed to x-radiation and light – (Screen type film) (these are usually of double sided emulsion) Films exposed to light only. E. g. fluorographic films, duplicating films, Cine films (these are of single sided emulsion)

Film structure Double sided emulsion Base Single sided emulsion Base Super coat Emulsion Substratum Anti halation/curl

Film base n n Polyester made from polyethlene terepthalate resin is used now It is extremely strong and untearable Demonstrate high dimensional stability Cellulose triacetate was used before

Functions of film base n n To provide a support for the emulsion layer To transmit light so that an image can be viewed

Desirable characteristics of film base 1. 2. Transparent, free from any defect, uniform transmission of light Strong but flexible to 1. Withstand mechanical stress during processing and handling 2. be made thinner to minimize parallax (0. 18 mm for sheet film, 0. 08 for roll film) 3. 4. Uniform in thickness Dimensionally stable during processing

Desirable characteristics of film base 5. Impermeable to water or processing solutions 6. Non- flammable 7. Chemically inactive 8. Uniform in colour, if a colour tone is added during manufacture

The subbing layer / adhesive layer n This is required, 1. To ensure that emulsion layer adheres to the smooth and shiny base during manufacture 2. To prevent any separation of the emulsion layer from the base during processing n A mixture of gelatin solution and solvent of the film base is used

The emulsion layer n n n This is a layer of silver halide suspended in gelatin It is coated on the subbing layer The shape of crystals is important , it affects the sensitivity of the film and the sharpness of the image Kodak is pioneer in producing flat grain emulsion Light absorbing magenta dye is added to the surface of the grains to increase the sensitivity and to absorb the irradiating light within the emulsion so that unsharpness produce by that way is reduced

The supercoat n n This is a thin layer of gelatin which is applied to the emulsion The prime functions are: • To protect the emulsion from the effect of pressure or abrasion. • To provide suitable surface characteristics; sufficiently glossy to reduce accumulation of dust; not too glossy as to prevent the grip by the rollers of processor.

Non-curl backing n n Present only in single emulsion sheet film. Function is to prevent the tendency of the film to curl when the emulsion swells during processing. This is a gelatin layer coated on a subbing layer It swells equally as the emulsion layer and counteract the swelling of emulsion on the other side.

Anti-halation layer n When a colour dye is added to the anti-curl layer to prevent the effect of halation, it is called an antihalation layer

Halation ? n The ghost image produced out side the proper image, by the light transmitted through the base and reflected back from the base-air interface is called halation.

Light from screen emulsion base True image Halation

Prevention of Crossover effect n n Crossover effect is a type of halation occur when using double sided emulsion films with two intensifying screens. I. e. The light from one screen crosses the base and produces a ghost image on the opposite emulsion To prevent this a colour dye (magenta for green sensitive films) is added to the surface of the emulsion. The dye has the effect of reducing the light scattering within the emulsion and, at the same time limiting crossover and thus image unsharpness

Irradiation n This is the sideways scattering of light within the emulsion itself as a result of the light striking the grains of silver halide This light contributes nothing to the image proper but becomes a source of image unsharpness. The use of thin emulsion layers produce less irradiation.

Films for medical imaging n Duplitized emulsion films • Direct exposure films : - Dental x-ray films, Radiation-monitoring films • Screen type films used with two intensifying screens: - General radiography films n Single emulsion films • Screen type film used with single screen • Photofluorography films : - Cine film, 70 mm & 105 mm roll film, 100 mm sheet film • Cathode ray tube (CRT) photography films • Duplication films • Subtraction films • Laser imaging films

End