Sensitometry or characteristic curve

Radiographic photography
Sensitometry
&
Characteristic Curve
Upakar Paudel
UCMSTH

 Sensitometry as a photographic science has been
credited to two Americans, Ferdinand Hurter and
Vero Charles Driffield, who first proposed a form
of evaluation of the performance of photographic
material in about 1876.
 The main interest in radiography is how the film
will react to (apparently) x-radiation, although
usually the real interest is how the film will react
to the light produced by the IS.
2
History
14 December 2018 Sensitometry & Characteristic Curve

• The study of the relationship between intensity
of exposure of the film and the blackness after
processing .
or
is the measurement of the
sensitivity of the film towards
radiation.
Sensitometry

 The ability or behavior of film emulsion to absorb
different wave lengths differently is known as
spectral response.
 Basic silver bromide is only sensitive for blue,
violet and UV range of spectrum but Color
sensitizers are added in film emulsion to control
spectral response.
4
Spectral response

 Photographic emulsion can be classified as:
 Monochromatic(sensitive up to blue color)
 Orthochromatic(sensitive up to green color)
 Panchromatic(sensitive up to red color)
5
Spectral response

1. Photographic density is normally the
blackness of the film.
2. Characteristic curve is the graph which
shows the relationship between the log of
exposure and the density produced in the film.
7
Sensitometry…

 On the developed film, the deposit of Ag may be
said to have a light-stopping effect or a degree of
blackness; this varies with amount of Ag present,
which in turn increases with increase in exposure.
 Obviously greater blackening of the film allows
less light to be transmitted.
8
1. Photographic density

 It has been explained that the following 3
expressions give the numerical value to the
degree of blackness ….
a. Transparency
b. Opacity
c. Optical density
9
1. Photographic density...

a. Transparency:
It can be expressed by the ratio of the
transmitted light(Lt) through the film to the
incident light(Li).
Transmission ratio =Lt/Li
in percentage , 100Lt/Li
1. Photographic density…
14 December 2018 10Sensitometry & Characteristic Curve

 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 one and 100% transmission.
 Transmission ratio decreases with film blackening
or exposure.
Contd…..
14 December 2018 11Sensitometry & Characteristic Curve

b. Opacity:
 It is the ratio of incident light to transmitted
light
or
is the reciprocal of the transmission ratio.
i.e. Opacity= 1/transmission ratio = Li/Lt
 The blackest part of an x-ray has an opacity
approaching 10,000.
12

 A perfectly transparent area has an opacity one.
 A perfect opaque area has infinite opacity.
 It can be appreciated that it always has the value
greater than unity and it increases with the
exposure.
13
Contd….

c. Density: One method by which the range may
be reduced to more manageable portion is to
convert the opacity into its logarithmic value.
Using logarithmic base 1o, an opacity of 10,000
becomes 4, similarly an opacity of 1(perfectly
transparent) becomes 0.
So, It is defined as the common logarithm of the
opacity,
i.e. density= log10 opacity = log10 Li/Lt, also
known as Optical or photographic density.
14

 It 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.
 The human eye seems to respond to different tones
in a way which is approximately logarithmic. A
density of two looks twice as dark as a density of
one.
15
Why is density appreciated?

NOW,
 The easiest way to appreciate the relationship
between exposures and the results of the
exposures is to make a graph.
 It is found that the most useful graph is obtained
by plotting logarithmic values on both axes.
16

 Logarithmic scales enable a much wider range of
values to be appreciated with understanding; also
equal increments have the same ratio.
 The sensitometric graph which results a curve is
called a characteristic curve.
17

 The term ‘X-ray exposure’ is not synonymous with
exposure factors but it refers to a physical measure
of exposure in units such as coulombs per Kg (SI
unit) or Roentgens (traditional unit).
 It is not easy to measure absolute x-ray exposure,
so we produce relative values of it.
18
X-ray exposure

 If a sheet of screen type x-ray film is divided (say)
10 small areas and each area is exposed with same
tube kv and mA but different exposure times, it is
simple matter to relate the x-ray exposure received
by each area.
 The area which has been given the smallest
exposure is used as the baseline and allocated a
relative exposure of unity, and the other areas,
subjected to higher exposure values greater than
one.
19
Relative exposure

20
Relative exposure

The intensity of radiation producing the
blackest parts of image may be thousands of times
greater than that producing lowest densities. Thus,
the values of relative exposure may vary from unity
to several thousands and it is problematic to plot.
21
Log Relative exposure

 Sensitometer (Penetrameter):- measures film
sensitivity to light,
and
shows the range of densities on an image.
22
Equipments used for sensitometry

 Step-wedge cause the differential attenuation
that creates req. range of intensities.
 Used to monitor film/screen combination.
 Not recommended for processor monitoring.
23
Penetrameter

SENSITOMETRY
RADIATION
STEP WEDGE
DENSITY OBTAINED IN THE IMAGE
.
.
.

 It provides measurement of light transmitted
through film
or
measures optical density.
 Uniform light source and an optical sensor.
 Calibration control allows for easy.
25
Densitometer

14 December 2018 Sensitometry & Characteristic Curve 26

 The relationship between log of the exposure of a
film received and the density produced by the
exposure can be plotted as a curve known as
characteristic curve or D logE curve.
 It is found that the most useful graph is obtained
by plotting Density (a logarithmic value) in Y-axis
and the log of exposure in X-axis.
27
2. Characteristic curve

 It is also called H&D curve after Ferdinand Hurter
and Vero Charles Driffield worked on it.
 This graph illustrates the way in which a film or
film screen combination responds to different level
of exposure .
28
2. Characteristic curve..

 This can be made for any photographic material
and is characteristic of the material under certain
conditions of exposure and processing.
 A series of exposures is done which progress in
steps by a factor on the radiographic material and
then the series of densities can then be measured
by densitometer.
 Progression is in such way that each exposure is
related to the adjacent previous one by a constant
known factor, called Wedge constant(sp.2 or 1.414).
29
2. Characteristic curve…

 There are three basic stages involved
i. Exposing and processing film
ii. Measuring the densities produced
iii. Plotting the graph
30
How to make characteristic curve?

 To generate a characteristic curve we need to
irradiate the film or film-screen system with series
of exposures which progress in known steps so
that the relative exposure received by each step
can be recorded.
 Wedge factor should be constant throughout the
exposure.
31

 It is usual to provide 21 exposure steps.
 The smallest exposure must be such that no
measurable effect can be seen on the film.
 The heaviest exposure should be greater than that
sufficient to activate every silver halide grain in the
emulsion, so that the maximum possible density
produced.
32

Two methods:-
a. Using variation along time scale:
tube kv, mA, focal spot setting, FFD used are
const. but time will vary with wedge constant.
b. Using variation along intensity scale:
using step-wedge (flight of metal strips) of a
material such aluminum, plastic (for soft x rays)
and steel (for hard x rays).
33

Characteristic curve
14 December 2018 Sensitometry & Characteristic Curve 34

 The graph possesses three sections known as :
a. The Toe ( under exposed region)
b. The Shoulder (over exposed region)
c. The Straight line (correct exposed region)
35
Features of characteristic curve
Basic fog
Toe
Straight line portion
Shoulder region
Dmax
Region of solarization

36
 It has small density(around 0.5), even at the
starting point although there has been no exp. or
only a very small exp. which is the result of base
density plus fog.
 Base plus Fog is also called Basic fog which is
usually upto 0.2.
 The density produced by the development of AgX
grains which have no diagnostic purpose is the
Fog.
a. The Toe

Factors that increase basic fog are:
• Ageing of the film
• Poor conditions for storage
• Processing temp. too high
• Overactive developer
• Too long developing time
• Greater film speeds
37
a. The Toe...

38
 The point where the curve just begins to turn up
and the line of the graph causes to be parallel to
the horizontal axis called The Threshold.
 It represents the 1st response of the material to
radiation.
Where,
 Net Density = Gross Density – Gross fog
a. The Toe…

 This is the region of the overexposure.
 There are two aspects of the film behavior of this
region.
 Maximum density
 Reversal
39
b. The Shoulder
Shoulder region

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.
40
b. The Shoulder…

• In this region characteristic curve has zero
gradient, indicate zero image contrast.
• D max depends on
 silver coating weight:
The greater the amount of AgX per
unit area of film, the higher is D max.
Eg. non- screen film.
 processing condition:
Exhaustion of developing agent can
lead to a failure to achieve D max.
41
b. The Shoulder…

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.
42
b. The Shoulder…

 The straight line part of the curve is also called the
region of correct exposure.
 In this region, change in exposure causes
significant change in density. The two measure
consequences of the this region are
 Contrast
 Latitude
44
c. The Straight line
Straight line portion

 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
densities. It depends on:
o subject contrast
o film contrast
45
Contrast

o Subject contrast:
 Depends on differential attenuation of x-ray
beam as it passes through patient.
 Affected by:
 - thickness of part
- density
- atomic difference
- radiation energy (Kvp)
- scatter radiation
46
Contrast

o Film Contrast:
 Examines how the film respond to the difference
in exposure produced by subject contrast.
 depends on:-
- shape of characteristic curve
- film density
- screen or direct x-ray exposure
- film processing
47
Contrast

 Film contrast is equal to the slope of the straight-
line portion of the characteristic curve.
 The slope of the straight-line is called the gamma,
equal to V/H
When gamma=1, low contrast film material
>1, high contrast film material, it
means that large OD differences produced with
small range of x-ray exposure.
 Not approate.
48
Contrast on the curve

 The useful method to specify the contrast is
Average Gradient.
 Is the slope of the line joining two points that
cover the useful densities(0.25-2) on the
characteristic curve.
G bar = (D2-D1)/(logE2-logE1)
where D1 = Net density 0.25
D2 = Net density 2.0
S0, D2-D1 = 2.0-0.25 = 1.75
49
Average Gradient

If,
G bar>1 : exaggerates subject contrast
typical for x-ray film
=1 : no change in subject contrast
<1 : decreases subject contrast
50
Average Gradient

Factors affecting Average Gradient are:
 Grain 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.
51
Average Gradient

 It is the range of exposure (kvp, mA)for having
acceptable image.
 The film with more latitude makes the exposure
less critical.
 It is considered in two parts,
 Film latitude
 Exposure latitude
52
Latitude

 It is the difference between upper and lower limits
of log relative exposures which produce 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)
and, G bar = (D2-D1)/(logE2-logE1)
G bar = useful density/film latitude
Therefore, Film latitude = 1.75/G
54
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.
56
Exposure Latitude

 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.
57
Exposure Latitude…

 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.
58
Speed & Characteristic curve

 To compare different types of films
 To compare different types of screens
 Useful tool to set up exposure devices
 To determine avg. gradient & therefore subject
contrast amplification
 To find film and exposure latitude
 To find absolute value of the speed of the films
 To monitor the performance of auto processor
59
Uses of Characteristic curve

 Very wide latitude
60
Latitude of CR
Latitude of film
Characteristic curve of CR system

 Transmission ratio = transmitted light/incident light
 % transmission = 100It/I0
 Opacity = I0/It
 Optical Density =Log 10 Opacity
 Net density = Gross density- Gross fog
 Average gradient = Useful density range/ film latitude
 Film latitude = Useful density range/Average gradient
where useful density range = 2.0-0.25 = 1.75
 Exposure latitude = film latitude – log exposure range
 _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.
61
Key relationships

1. Radiographic Imaging by DN and MO Chesney
2. Radiologic Science for Technologists by Stewart
Carlyle Bushong
3. www.ndt-od.org/education/resources
4. www.answers.com
5. Radiographic Imaging A Practical Approach by
Derrick P. Roberts and Nigel L. Smith
62
References

Sensitometry or characteristic curve

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