characteristics of films - graininess, density, speed, contrast

characteristics of films - graininess, density, speed, contrast

1. M.KARTHIKEYAN ASSISTANT PROFESSOR DEPARTMENT OF MECHANICAL ENGINEERING AAA COLLEGE OF ENGINEERING & TECHNOLOGY, SIVAKASI [email protected] ME8097 NON DESTRUCTIVE TESTING AND EVALUATION

2. UNIT V RADIOGRAPHY (RT) Principle, interaction of X-Ray with matter, imaging, film and film less techniques, types and use of filters and screens, geometric factors, Inverse square, law, characteristics of films - graininess, density, speed, contrast, characteristic curves, Penetrameters , Exposure charts, Radiographic equivalence. Fluoroscopy- Xero -Radiography, Computed Radiography, Computed Tomography

3. FILM CHARACTERISTICS Wide variety of films is available in the market. These films differ not only in size and general type but also in film speed, film contrast, exposure latitude, and spectral sensitivity. Film speed, contrast and latitude are the three most important characteristics of a radiographic film. A plot of the optical density as a function of the logarithm of the exposure is called the characteristic curve, sensitometric curve or an H and D (Hurter and Driffield ) curve (Figure 6). It describes the response of photographic film to light.

4. The precise shape of the curve depends on the quality of the X-ray beam used for exposure, characteristics of the emulsion and the processing conditions. The primary use of the curve is to describe the contrast characteristics of the film throughout a wide exposure range. The curve has four regions namely base plus fog, toe region, straight-line portion and shoulder region. Dmax represents the maximum density. The base plus fog region represents the inherent density or the initial film density before exposure to x-ray photons. The toe is the low exposure region and the shoulder is the high exposure region of the curve.

5. The region in between the toe and shoulder is known as the linear region. The optical density in the toe region ranges from 0.2 to 0.5. In the shoulder region of the curve, most of the silver halide crystals have been exposed and any additional exposure does not produce much additional blackening. The shoulder region represents the area of high exposure levels. The optical density in the shoulder region is > 2.5. Dmax is the maximum density the film is capable of recording. A radiograph with optical densities in the region of the toe or shoulder furnishes an image with interior contrast.

6. FILM CONTRAST Film contrast is the difference in optical density between two areas in the image. The contrast of a particular radiographic film is fixed by the manufacturer. Film contrast is measured on the slope of the straight – line portion of the characteristic curve at the speed point, which is also called the film gamma. Films with exposures between OD 0.5 and 2.5 exhibit contrast in the diagnostic range. Films with optical density in the toe or shoulder portion of the characteristic curve demonstrate a loss of contrast. Films with steeper straight – line portions have higher contrast.

7. LATITUDE The exposure range over which acceptable optical densities are produced is known as the latitude of the film. Films can have wide or narrow latitude. Latitude and contrast are inversely related meaning that as contrast increases latitude will decrease. Wide – latitude films have low contrast; narrow latitude films have higher contrast. Fig . 7 shows the characteristics curves of films with different latitudes. Film A has narrower latitude and a higher contrast than Film B. Films with narrow latitudes produce high contrast images.

8. Film B has wider latitude and produces lower-contrast images. Generally , higher speed films have higher contrast and narrower latitude; slower speed films have lower contrast and wider latitude .

9. Film speed Film speed or film sensitivity describes the ability of an x-ray film to respond to an x-ray exposure. It is a measure of the position of the characteristic curve at a specific value of the exposure axis. 𝐹𝑖𝑙𝑚 𝑠𝑝𝑒𝑒𝑑 = 1 𝐸𝑥𝑝𝑜𝑠𝑢𝑟𝑒 𝑖𝑛 𝑅 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑 𝑓𝑜𝑟 𝑎𝑛 𝑂𝐷 𝑜𝑓 1.0 𝑎𝑏𝑜𝑣𝑒 𝑏𝑎𝑠𝑒 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 The toe of the H&D curve is short for high speed (fast) film and longer for film with lower speed (slow). The sensitivity of the film is determined by the size of the silver halide crystals, the number of sensitivity specks, and the thickness of the emulsion.

10. Larger crystal size with increased numbers of sensitivity specks suspended in a thick emulsion, all increase the speed of the film. The speed point of a film describes the exposure required to produce an optical density of 1.0 above base. A faster film requires less exposure and lower mAs setting to produce the same optical density.