characteristics of films - graininess, density, speed, contrast

1. M.KARTHIKEYAN
ASSISTANT PROFESSOR
DEPARTMENT OF MECHANICAL ENGINEERING
AAA COLLEGE OF ENGINEERING & TECHNOLOGY, SIVAKASI
karthikeyan@aaacet.ac.in
ME8097 NON DESTRUCTIVE
TESTING AND EVALUATION

2. UNIT V RADIOGRAPHY (RT)
1. Principle, interaction of X-Ray with matter,
2. imaging, film and film less techniques,
3. types and use of filters and screens,
4. geometric factors, Inverse square, law,
5. characteristics of films - graininess, density, speed, contrast,
6. characteristic curves, Penetrameters,
7. Exposure charts, Radiographic equivalence.
8. Fluoroscopy- Xero-Radiography,
9. 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.

7. 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.

8. 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.

9.  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.

10.  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 𝑑𝑒𝑛
𝑠𝑖𝑡𝑦
 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.

11.  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.