SENSITOMETRY AND CHARACTERISTIC CURVE
PRESENTED BY
SAROSE BHANDARI
INTRODUCTION
The ability or behavior of film emulsion to absorb different wave length differently is known as spectral response
SPECTRAL SENSITIVITY
SENSITOMETRY AND CHARACTERISTIC CURVE.pptx
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
contd….
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…..
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.
sensitometry
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 CONTD….
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
PENETROMETER
Step-wedge
Used to monitor film/screen combination.
No recommended for processor monitoring.
PRODUCING STRIP WITH PENETROMETER
SENSITOMETER
Device uses a controlled light source.
Produces same amount of light each time it is triggered.
Available in 11 or 21 step versions.
PRODUCING STRIP WITH SENSITOMTER
SENSITOMETRY STRIPS
DENSITOMETER
Uniform light source and an optical sensor.
Calibration control allows for easy.
READING STRIP WITH DENSITOMETER
RADIOGRAPHIC DENSITY
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
Transparency
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 contd….
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.
opacity
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.
OPTICAL DENSITY
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.
ADVANTAGES OF LOG CONCEPT OF OD
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.
X-RAY EXPOSURE
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)
RELATIVE 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
LOG 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.
CHARACTERISTIC CURVE
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.
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
FEATURE OF CHARACTERISTIC CURVE
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
1.THE REGION TO THE LEFT OF THE TOE
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 density
B. Fog
A.BASE DENSITY
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.
B.FOG
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
FOG CONTD…..
Total density of Base plus fog is often referred to as Basic Fog.
Its value should not be greater than 0.2
NET DENSITY
Gross Fog is subtracted from each value of density measured to plot Net Density against Log related exposure.
Net Density=Gross Density-Gross Fog
THRESHOLD
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.
THE REGION BETWEEN THE TOE AND SHOULDER
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
A.CONTRAST
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.SUBJECT 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
B.FILM 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
GAMMA
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
AVERAGE GRADIENT
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 CONT..
>1: exaggerates subject contrast
typical for x-ray film
=1: no change in subject contrast
<1: decreases subject contrast
FACTORS AFFECTING AVERAGE GRADIENT
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.
B.LATITUDE
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
1.FILM 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
2.EXPOSURE 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.
Contd…..
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.
SPEED AND SENSITIVITY
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.
HOW IS RELATIVE SPEED ASSESSED FROM C-CURVE
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
THE REGION TO THE RIGHT OF THE SHOULDER
This is the region of the overexposure.
There are two aspects of the film behavior of this region.
1.Maximam density
2.Reversal
1.MAXIMUM DENSITY
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.
D Max contd…
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.
2.REVERSAL
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.
Uses of Characteristic curve
1.Comparision of film performance.
2.comparision of film screen system.
3.monitoring of processing.
4.calibration of step wedge.
Application of spectral sensitivity 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.
Characteristic curve of CR system
Very wide latitude
