SENSITOMETRY AND CHARACTERISTIC

CURVEPRESENTED BY

SAROSE BHANDARI

The ability or behavior of film emulsion to absorb different wave length differently is known as spectral response

INTRODUCTION

Spectrum is a display of the different colors arranged in order of their wave length.

The range of wave length visible to human eye is VIOLET 400

nm to RED 700 nm.

Below and beyond this wave length limit it is invisible to human eye.

Basic silver bromide is only sensitive for blue, violet and UV range of spectrum. Example: monochromatic film

SPECTRAL SENSITIVITY

Color sensitizer are added in emulsion to control spectral response.

Photographic emulsion can be classified as

1.Monochromatic(sensitive up to blue color) 2.Orthochromatic(sensitive up to green) 3.Panchromatic(sensitive up to red )

contd….

In screen film system of radiography spectrum emitted by screen phosphor should well respond by film emulsion.

The poorly matched film with screen leads to loss of sensitivity and require increased exposure.

A film which has already exposed to x-radiation is 8 times more sensitive to safelight fogging than is unexposed material because exposed film has reached or exceeded threshold.

Sensitivity is assessed by measurement of the densities produced when a film is subjected to equal exposure of different wavelengths of light.

contd…..

The study of the response of an IR to x-ray or scientific study of light sensitive materials.

It measures film response to exposure and processing.

It is essential tool for maintaining adequate quality control.

Equipments 1.sensitometer (penetrometer) 2.densitometer

sensitometry

Sensitometer :- measures film sensitivity to light , shows the range of densities on an image.

Densitometer :- provides measurement of light transmitted through film.

Measures optical density

SENSITOMETRY CONTD….

Step-wedge Used to monitor film/screen combination. No recommended for processor monitoring.

PENETROMETER

PRODUCING STRIP WITH PENETROMETER

XRAY TUBE

Device uses a controlled light source. Produces same amount of light each time it

is triggered. Available in 11 or 21 step versions.

SENSITOMETER

PRODUCING STRIP WITH SENSITOMTER

SENSITOMETRY STRIPS

Uniform light source and an optical sensor.

Calibration control allows for easy.

DENSITOMETER

READING STRIP WITH DENSITOMETER

It is degree of blackening on a film.

Depend on the level of exposure received by film.

High exposure high density is produced.

It can be quantified by 1.transparency 2.opacity

RADIOGRAPHIC DENSITY

It measures the intensity of light transmitted through the film.

Fractional transmission or transmission ratio is the ratio of transmitted light to incident light.

◦ Transmission ratio=It/Io◦ Percentage transmission=100×It/Io

Transparency

A perfectly opaque area of an image has zero transmission ratio and zero percentage transmission.

A perfectly transparent area of an image has transmission ratio 1 and 100% transmission.

Transmission ratio increases with film blackening.

Transparency contd….

Opacity gives us quantity which increases as blackening of film increases.

It is reciprocal of transmission ratio.◦ Opacity=IO/It

A perfectly transparent area has an opacity 1.

A perfect opaque area has infinite opacity.

opacity

The degree of blackening produced being dependent on the level of exposure received, quantified in terms of opacity and expressed in logarithmic value to the base 10 is referred as optical density or photographic density.

OD=log10 opacity

=log10(Io/It ) It reduces the range of opacity to

manageable proportion.

OPTICAL DENSITY

Density increases with exposure.

Density of an image is approximately proportional to the amount of silver present in emulsion.

Logarithms conveniently express large differences in numbers on a small scale.

If the films are superimposed , the resulting density is equal to the sum of densities.

ADVANTAGES OF LOG CONCEPT OF OD

It refers to the physical measure of exposure expressed in coulomb per kg or roentgen.

Exposure is proportional to the

1.x-ray tube current (mA) 2.exposure time(s)

X-RAY EXPOSURE

To obtain absolute measurements with consistency and accuracy of x-ray exposure, it is impossible. However to obtain relative value is quite possible.

The exposure series can be achieved by one of the two methods

1.Time-scale sensitometry 2.Intensity-scale sensitometry

RELATIVE EXPOSURE

The value of relative exposure may vary from unity to several thousands.

This causes problem of scaling if relative exposure is to be plotted on a graph.

Advantage of LRE

1.Allows a wide range of exposure to be expressed in compact graph.

2.Analysis of curve becomes easier.

LOG RELATIVE EXPOSURE

The study has its origins in the work by Ferdinand Hurter and Vero Charles Driffield (circa 1876) with early black-and-white emulsions

This graph illustrates the way in which a film or film screen combination responds to different level of exposure .

The relationship between the exposure of a film received and the density produced by the exposure can be plotted as a curve known as characteristic curve or DlogE curve or H and D curve.

CHARACTERISTIC CURVE

HOW IS A CHARACTERISTIC CURVE PRODUCED?

There are three basic stages involved

1.Exposing and processing film

2.Measuring the densities produced

3.Plotting the graph

The graph possesses two sharply curved sections known as the toe and shoulder.

The curve is divided into 3 regions

1.The region to the left of the toe 2.The region between the toe and shoulder 3.The region to the right of the shoulder

FEATURE OF CHARACTERISTIC CURVE

This is the region of under exposure. The exposure received by the film in this

region is not sufficient to produce any photographic effect.

The density in this region arises from two sources.A. Base densityB. Fog

1.THE REGION TO THE LEFT OF THE TOE

Due to the absorption of light as it is transmitted through the polyester base.

This will be greater if the base is tinted(color) by a dye during manufacture.

Average base density is 0.07.

A.BASE DENSITY

The density produced by the development of AgX grains which have no diagnostic purpose.

Causes of fog are

1.age veil 2.storage fog 3.safe light fog

4.chemical fog

B.FOG

Total density of Base plus fog is often referred to as Basic Fog.

Its value should not be greater than 0.2

FOG CONTD…..

Gross Fog is subtracted from each value of density measured to plot Net Density against Log related exposure.

Net Density=Gross Density-Gross Fog

NET DENSITY

As exposure increases, the film emulsion begin to respond and its density begins to rise above Gross fog.

This part of the curve is called as Threshold.

THRESHOLD

This is the straight line part of the curve . In this region, change in exposure causes

significant change in density. the two measure consequences of the this region are

A. Contrast B. Latitude

THE REGION BETWEEN THE TOE AND SHOULDER

The exposure variations which constitute subject contrast generate differences in the image density and therefore produces contrast in the radiographic image, known as radiographic contrast.

Although it is possible to have density without contrast but not possible to have contrast without density.◦Depend on A. subject contrast B. film contrast

A.CONTRAST

Depends on differential attenuation of x-ray beam as it passes through patient.

Affected by:-

1.thickness of part 2.density 3.atomic difference 4.radiation energy(Kvp) 5.scatter radiation

A.SUBJECT CONTRAST

Examines how the film respond to the difference in exposure produced by subject contrast.

Depends on:- 1.shape of characteristic curve 2.film density 3.screen or direct x-ray exposure 4.film processing

B.FILM CONTRAST

It is the maximum slope of the curve described by the formula

Gamma=(D2-D1)/(logE2-logE1) where: D2 and D1 are the densities on

the steepest part of the curve resulting from log relative exposure E2-E1

GAMMA

To measure the slope of the curve over useful density(0.25-2), we calculate average gradient which is the slope of the straight line joining two points of the specified densities on the characteristic curve.

G=(D2-D1)/(logE2-logE1) where D1=Net density at 1i.e. 0.25 D2=Net density at 2 i.e. 2.0

AVERAGE GRADIENT

>1: exaggerates subject contrasttypical for x-ray film

=1: no change in subject contrast

<1: decreases subject contrast

AVERAGE GRADIENT CONT..

Gain size distribution in film emulsion.

Whether the film is double coated or single coated emulsion.

Film processing condition, particularly during development.

Characteristic of intensifying screen.

FACTORS AFFECTING AVERAGE GRADIENT

It is an expression of the tolerance of the system to extreme condition of exposure.

The film with more latitude makes the exposure less critical.

It is considered in two parts

1.Film latitude 2.Exposure latitude

B.LATITUDE

It is the difference between upper and lower limits of log relative exposure which produces densities within useful range i.e. 0.25-2.0 above gross fog.

Film latitude varies inversely with the film contrast.

Film latitude=(logE2-logE1) G=(D2-D1)/(logE2-logE1) G=useful density/film latitude Film latitude=1.75/G

1.FILM LATITUDE

This represent the tolerance of a film –screen system to errors in the selection of the exposure factors when the exposure was made.

It depend on the film latitude and subject contrast.

By adjustment of exposure factor prior to making an exposure, we can exercise control over both range and median of log exposure value.

2.EXPOSURE LATITUDE

The median of the log exposure value: This represents the value midway between

the max. and min. log exposure values and depends essentially of the total quality and quantity of x-ray beam i.e. kVp and mAs.

The range of the log exposure: This represents the difference between the

max. and min.log exposure values.

Contd…..

Exposure latitude=film latitude-log exposure range

In steep-range radiography, problems of the exposure latitude can be relieved by high kVp technique which reduce subject contrast.

Contd…..

The speed (sensitivity) of a film or film-screen system is an expression of the x-ray exposure require to produce a given image density.

A high–speed system requires less exposure to produce a specific density than a slower system.

The density level at which the relative speed is assessed is known as speed point.

SPEED AND SENSITIVITY

A value of the relative speed of two film systems can easily be obtained from the characteristic curve of the two systems provided they share a common log relative exposure axis.

In terms of exposure factors speed of system A= mAs for system B

speed of system B mAs for system A to give same density at the same

kVp and FFD

HOW IS RELATIVE SPEED ASSESSED FROM C-CURVE

This is the region of the overexposure.

There are two aspects of the film behavior of this region.

1.Maximam density 2.Reversal

THE REGION TO THE RIGHT OF THE SHOULDER

As the film or the film-screen system is subjected to greater and greater exposure, a point is reached when further increase in exposure can’t result any increase in density. Thus, the film achieves a max. level of density known as Dmax.

In this region, subject contrast do not produce image contrast and no subject detail can be visualized.

1.MAXIMUM DENSITY

In this region characteristic curve has zero gradient, indicate zero image contrast.

D max depend on 1.silver coating weight: The greater the amount of AgX per unit

area of film, the higher is D max.eg. Non screen film

2.processing condition: Exhaustion of developing agent can lead to

a failure to achieve D max.

D Max contd…

The phenomenon when subjected to exposures many times greater than that required to achieve D max, the film emulsion respond in the opposite way to normal, producing a reduction in image density called reversal.

The film is said to be solarized and the reversal part of the characteristic curve is referred to as the region of solarization.

2.REVERSAL

1.Comparision of film performance.

2.comparision of film screen system.

3.monitoring of processing.

4.calibration of step wedge.

Uses of Characteristic curve

1.matching the film-spectral response to intensifying screen spectral response.

2.ensure the choice of safe light illumination appropriate to the spectral response of the film.

Application of spectral sensitivity curve

Very wide latitude

Characteristic curve of CR system

Latitude of CR

Latitude of film

Thank you

Thank you