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Film Contrast CharacteristicsPerry Sprawls, Ph.D.

Online

Textb

ook

Table of

Contents

CHAPTER CONTENTS

INTRODUCTION AND OVERVIEW

CONTRAST TRANSFER

The Characteristic Curve

Contrast Curve

Gamma

Average Gradient

FILM LATITUDE

Exposure Error

Subject Contrast Range

FILM TYPES

EFFECTS OF PROCESSING

Over processing

Under processing

FILM FOG

Inherent

Chemical

Heat and Age

Radiation Exposure

INTRODUCTION AND OVERVIEWCONTEN

TS

Contrast is perhaps the most significant characteristic of an image recorded on film.

Contrast is the variation in film density (shades of gray) that actually forms the image.

Without contrast there is no image. The amount of contrast in an image depends on a

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number of factors, including the ability of the particular film to record contrast.

Film can be considered as a contrast converter. One of its functions is to convert

differences in exposure (subject contrast) into film contrast (differences in density), as

shown below. The amount of film contrast resulting from a specific exposure

difference can vary considerably.

The General Relationship between Exposure Contrast and Film Contrast

The exposure contrast between two areas can be expressed as a ratio or percentage

difference, as illustrated above. The film contrast between two areas is expressed as

the difference between the density values. The ability of the film to convert exposurecontrast into film contrast can be expressed in terms of the contrast factor. The value

of the contrast factor is the amount of film contrast resulting from an exposurecontrast of 50%. The amount of contrast produced by medical imaging films depends

on four basic factors:

(1) type of emulsion,

(2) amount of exposure,

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(3) processing,

(4) fog.

In this section we consider the basic contrast characteristics of film, how these

characteristics are affected by the factors listed above, and how contrast

characteristics relate to clinical applications.

CONTRAST TRANSFERCONTENT

S

The ability of a film to produce contrast can be determined by observing the

difference in density between two areas receiving a specified difference in exposure,

as shown in the figure above. However, since the amount of contrast is affected by the

level of exposure, a range of exposure values must be delivered to a film to

demonstrate fully its contrast characteristics.

One method of doing this is illustrated in the following figure; this type of exposure

pattern is usually produced by a device known as a sensitometer. In this method, a

strip of film is divided into a number of individual areas, and each area is exposed to adifferent level of radiation. In this particular illustration, the exposure is changed by a

factor of 2 (50% contrast) between adjacent areas. When considering contrast

characteristics, we are usually not interested in the actual exposure to a film but ratherthe relative exposure among different areas of film. In the figure below the exposures

to the different areas are given relative to the center area, which has been assigned a

relative exposure value of 1. We will use this relative exposure scale throughout our

discussion of film contrast characteristics. Note that each interval on the scalerepresents a 2:1 ratio. This is a characteristic of a logarithmic scale. When the film is

processed, each area will have density values, as shown directly below the area. Theamount of contrast between any two adjacent areas is the difference in density, as

shown. In this illustration we can observe one of the very important characteristics of

film contrast. Notice how the contrast is not the same between each pair of adjacentareas throughout the exposure range: there is no contrast between the first two areas,

but the contrast gradually increases with exposure, reaches a maximum, and then

decreases for the higher exposure levels. In other words, a specific type of film does

not produce the same amount of contrast at all levels of exposure. This important

characteristic must be considered when using film to record medical images.

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The Variation in Contrast with Exposure

All films have a limited exposure range in which they can produce contrast: if areasof a film receive exposures either below or above the useful exposure range, contrast

will be diminished, or perhaps absent. Image contrast is reduced when a film is either

underexposed or overexposed.

The Characteristic CurveCONTENT

S

The relationship between film density and exposure is often presented in the form of

a graph, as shown below. This graph shows the relationship between the density andrelative exposure for the values shown above. This type of graph is known as either a

film characteristic curve or an H and D (Hurter and Driffield) curve. The precise

shape of the curve depends on the characteristics of the emulsion and the processingconditions. The primary use of a characteristic curve is to describe the contrast

characteristics of the film throughout a wide exposure range. At any exposure value,

the contrast characteristic of the film is represented by the slope of the curve. At anyparticular point, the slope represents the density difference (contrast) produced by a

specific exposure difference. The same interval anywhere on the relative exposure

scale represents the same exposure ratio and amount of contrast delivered to the filmduring the exposure process. An interval along the density scale represents the

amount of contrast that actually appears in the film. The slope of the characteristic

curve at any point can be expressed in terms of the contrast factor because the

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contrast factor is the density difference (contrast) produced by a 2:1 exposure ratio

(50% exposure contrast).

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A Film Characteristic Curve Showing the Relationship between Density and

Relative Exposure

A film characteristic curve has three distinct regions with different contrast transfer

characteristics. The part of the curve associated with relatively low exposures is

designated the toe, and also corresponds to the light or low-density portions of animage. When an image is exposed so that areas fall within the toe region, little or no

contrast is transferred to the image. In the film shown in the figure in the previous

paragraph, the areas on the left correspond to the toe of the characteristic curve.

A film also has a reduced ability to transfer contrast in areas that receive relatively

high exposures. This condition corresponds to the upper portion of the characteristic

curve in which the slope decreases with increasing exposure. This portion of the curve

is traditionally referred to as the shoulder. In the figure in the previous paragraph the

dark areas on the right correspond to the shoulder of the characteristic curve. The two

significant characteristics of image areas receiving exposure within this range are thatthe film is quite dark (dense) and contrast is reduced. In many instances, image

contrast is present that cannot be observed on the conventional viewbox because of

the high film density. This contrast can be made visible by viewing the film with a

bright "hotlight."

The highest level of contrast is produced within a range of exposures falling

between the toe and the shoulder. This portion of the curve is characterized by a

relatively straight and very steep slope in comparison to the toe and shoulder regions.In most imaging applications, it is desirable to expose the film within this range so as

to obtain maximum contrast.

The minimum density, in the toe, is the residual density, which is observed after

processing unexposed film, and is typically in the range of 0.1 to 0.2 density units.

This density is produced by the inherent density of the film base material and the low-level fog in the film emulsion; it is therefore commonly referred to as the base plus

fog density. The maximum density, in the shoulder, is determined by the design of the

film emulsion and the processing conditions and is typically referred to as the Dmax.

Contrast CurveCONTENT

S

It is easier to see the relationship between film contrast and exposure by using a

contrast curve, as shown below. The contrast curve corresponds to the slope of the

characteristic curve. It clearly shows that the ability of a film to transfer exposure

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contrast into film contrast changes with exposure level, and that maximum contrast is

produced only within a limited exposure range.

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The Relationship of Film Contrast (Solid Line) to Relative Exposure and the

Characteristic Curve (Dotted Line)

The exposure range over which a film produces useful contrast is designated the

latitude. An underexposed film area contains little or no image contrast. Exposure

values above the latitude range also produce areas with very little contrast and havethe added disadvantage of being very dark or dense.

Since the contrast transfer characte