Sensitometry
Describing photographic
performance
Objectives
The student should be able to:
Describe the importance of the study
Define the relevant terms
Describe the methods of producing a
sensitometric strip
Plot the characteristic curve
Describe the Features of the characteristic
curve
Describe the importance & practical
applications
Why is the study important?
To produce images with optimum contrast
that reveal high details of the object
examined.
Optimum contrast?
Low contrast (long scale contrast)
High contrast (Short scale contrast)
Photographic images
Optimum contrast
High contrast
Low contrast
X-ray images
Introduction
Photosensitive materials (x-ray films) are
used to record the invisible x-ray image
It is required to reproduce all the
characteristics of the invisible image in
visible form.
The films ability to do so depends on its
sensitometric properties.
It is advantages to have a sound
knowledge of sensitometric properties of
x-ray films.
X-ray tube
Plot of incident x-ray
beam intensity
Object
Invisible x-ray
image
Plot of transmitted x-ray
beam intensity
Invisible x-ray image
kV mA Sec FFD
E
B
B1
EM
E
B1
Supporting tissue (m)
B2
E
B2
T2
T1
ET1
EM
T3
Air
Invisible
X-ray
image
ET2
ET3
EA
What is Sensitometry?
The scientific study of the response of
photosensitive material to different levels
of exposures.
How is it done?
Producing a sensitometric strip and plotting
a characteristic curve.
What is Response of exposure &
How is it manifested?

The response is the change that takes
place, after exposure to electromagnetic
radiation (light or x-rays), in the
photosensitive emulsion on the film
 The response of the film to exposures is
manifested as a degree of blackening
produced after chemical processing
How is degree of blackening
quantified/measured?


In Radiography the degree of film
blackening is quantitatively indicated by
the term ‘Optical Density’.
The optical density describes how
much a certain area of the film is opaque
to light incident upon it .
Optical Density
The optical density is expressed
quantitatively as,
Optical Density = Log10 Opacity
It is measured by using the ‘Densitometer’
(The densitometer works on the following
principle)
Consider the light transmitted through an
area of a film
Incident light
intensity (Ii )
Transmitted light
intensity (It)
Transmittance (Transmission ratio)= It / Ii
Opacity = 1 / Transmittance = Ii / It
Density = Log10 Opacity = Log10 Ii / It
Range of densities on a film
Ii
100
4
It
Transmit
tance
0.0001
0.001
0.01
0.1
1
0.01
0.1
1.0
10.0
100
3
2
1
Opacity
10000
1000
100
10
1
0
Density
4
3
2
1
0
Sensitometric strip
A film containing number of areas with
different optical densities from white (fully
transparent) to black (fully opaque)
How to produce a sensitometric
strip and the characteristic curve?
1.Expose a film to different amounts of
known exposures starting from a minimum
and increasing at a known rate (ratio /
wedge factor) up to a maximum.
2. Process the film
3. Measure the densities
4. Tabulate the result & Study the response
by plotting a curve (density Vs Log relative
exposure
Methods of exposing
Time scale method
(using x-ray exposure)
Intensity scale method
i. Using x-ray exposure
ii. Using light exposure
Time scale method
The film is exposed to different quantities of
exposures using constant intensity and
variable duration.
(Quantity = Intensity x Time)
Equipment required
X-ray machine
Cassette with film inside
Lead sheets
Processor
Densitometer
Graph paper
Procedure (time scale method)
X-ray tube
Constants
kV, mA, FFD
Variable
Time
Loaded
cassette
Areas to
be
exposed
Loaded cassette
Lead sheets
Exposure selection
Minimum exposure
Low enough not to produce a measurable
density
Maximum exposure
High enough to produce a density around 3.0
Increment
Wedge factor (Ratio between two exposures)
of 2 is adequate. (2½ can be used to get more
levels)
Alternatively the time steps available in the
machine may be used to get more points on
the graph
Intensity scale method 1
Making a single exposure
using
a calibrated step
wedge
X-ray machine
a loaded cassette
(The intensity of x-rays
passing through the
steps are different &
the duration of
exposure is the same)
Intensity scale method 2
The film is exposed to a series of different
intensities of light for the same duration
using an instrument called the
“Sensitometer”.
The wedge factor is usually 2½
The colour of light should match the
spectral sensitivity of the film
Film obtained
using time
scale method
Plotting the curve
Area
A
B
C
D
Exposure (Time)
0.01
0.02
0.04
0.08
Relative exposure
1
2
4
8
Log relative
exposure
Density
0
0.3
0.6
0.9
0.25
0.3
0.4
0.9
Plotting the curve
Area
E
F
G
H
I
Exposure (Time)
0.16
0.32
0.64
1.28 2.56
Relative
exposure
Log relative
exposure
Density
16
32
64
128
256
1.2
1.5
1.8
2.1
2.4
1.45
2.1
2.5
2.7
2.9
Characteristic curve
Density
4
Shoulder
DMax
3
2
GF = Gross Fog
Straight line
portion
1
GF
Toe
1
2
3
4
Log relative exposure
Characteristic curve
3.00
D-Max
Shoulder
2.50
1.50
Straight line portion
1.00
0.50
GF
Toe
Log Relative exposure
3.48
3.35
3.24
3.10
2.88
2.60
2.48
2.30
2.10
1.88
1.70
1.40
1.10
0.88
0.60
0.00
0.00
Density
2.00
(Features) Information obtainable
Gross fog (Basic fog)
Threshold
Contrast
Latitude (film latitude & exposure
latitude)
Speed & Sensitivity
Maximum density
Reversal
Gross fog (Basic fog) & net density
This is the density of the horizontal part of the
curve at the minimum exposure level
Gross fog = Base Density + Fog
Base Density :- Density produced by the base
material
Fog :- Density produced by the development of
silver halide crystals which have not
received an intentional exposure
Net density = Gross density – Gross fog
Net Density
4
Characteristic curve (with net density)
Shoulder
3
2
Net density = gross density – gross fog
1
Toe
0
1
2
3
4
Log relative exposure
Threshold
The region where the film emulsion begins to
respond to the exposure
Contrast
The rate of change of density for a given change of
log relative exposure
Contrast = ΔD / ΔE
It is given by the slope (gradient) of the straight line
portion of the curve. If it is a true straight line then
the contrast is called Gamma.
Since, in practice, the curve is not an exact straight
line, the average gradient is taken as the contrast.
Characteristic curve
3.00
2.50
2.00
Contrast = ΔD/ ΔE
1.50
ΔD
1.00
0.50
ΔE
2.88
2.60
2.48
2.30
2.10
1.88
1.70
1.40
1.10
0.88
0.60
0.00
0.00
Average gradient
Density
4
3
DY
2
1
DX
B
A
Average gradient = BC/AC
DY - DX
= ----------log EY – log EX
How to select points A and B ?
C
Log EX 1 Log EY 2
3
4
Log relative exposure
Point A ? Point B ?
A : DX = 0.25 above Basic fog
B : DY = 2.0 above Basic fog
 The densities from 0.25 to 2.0 is called
the Useful Density range.
 Useful density range is the density range
in which the differences can be identified
by the human eye.
 The densities which represents different
structures on a radiograph should lie
within this range of densities.
Useful density range
Average
Gradient
=
Range of log relative
exposures that produces
the useful range of
densities
 Useful density range is the range of
densities within which the human eye can
recognize the small differences
That is the range of net densities from
0.25 to 2.0
Latitude
Latitude is an expression of the tolerance of a
system to extreme conditions of exposure.
It refers to the ability of a film or film-screen
system to record successfully a wide range of
exposure .
(considered in two parts)
Film latitude & Exposure latitude
Film Latitude
The difference between the upper and
lower limits of log relative exposure which
produce densities within the useful range
Significance of film latitude
The range of x-ray intensities transmitted
through the body part should lie within the
film latitude, if they are to be viewed as
useful densities on the radiograph.
Any x-ray intensity that falls out side the
film latitude will not reveal any information
& a useful piece of information might be
lost
Film latitude & Average gradient
Density
4
3
DY
2
1
DX
B
When Dx = 0.25+BF & Dy = 2.0
+BF
DY - DX
Average gradient = ----------becomes
log EY – log EX
A
C
Log EX 1 Log EY 2
Film latitude
1.75
= -----------------Film latitude
3
4
Log relative exposure
Exposure latitude
This refers to the freedom of the
radiographer to select slightly different
exposures (to make room for errors) for a
particular examination so that the resulted
densities remains within the useful density
range.
Exposure latitude = film latitude – subject
contrast (log relative exposure range
transmitted from a particular body part)
Subject contrast
kV mA Sec FFD
E
BB
E1
Air
E2
E3
E4
E5
E6
E7
Transmitted
intensities
E1
Relative
Intensity
Log relative
intensity
E2
E3
E4
E5
E6
RE1 RE2
RE3
RE4
RE5
RE6 RE7
e1
e3
e4
e5
e6
e2
e7
e1
Subject contrast
E7
e7
Image contrast
e1
D1 D2
e7
D3
D4 D5
D6
D7
Characteristic curve
Exposure latitude = film latitude – Subject contrast
3.00
e1 ≥ X ; e7 ≤ Y
2.50
2.00
1.50
Film latitude
1.00
Subject contrast
Exposure
latitude
0.50
2.88
2.60
2.48
2.30
2.10
1.88
1.70
1.40
Y
1.10
0.60
0.00
0.00
0.88
X
Speed & Sensitivity
 Sensitivity refers to the exposure required
by a film or film-screen system to produce
a net density of 1.
 Sensitivity is expressed in miliroentgens
(mR)
 A high sensitive (have low mR value for
sensitivity) or high speed system requires
less exposure than that of a low sensitive
or low speed system.
Speed
Numerically the Speed
is proportional to the
reciprocal of the
sensitivity (mR) and
is expressed as
128
Speed = -------------Sensitivity (mR)
1200
Sensitivity
(mR)
0.1
800
0.16
400
0.32
200
0.64
100
1.28
50
2.56
25
5.0
12
10.0
Density
4
Comparison of Speeds of two films (filmscreen systems)
A
3
2
B
Speed A > Speed B
1+BF
Speed A α 1/ ESA
Speed B α 1/ ESB
1
Speed A
ESB
------------- =
------
Speed B
BF
1
2
3
Log ESA
Log ESB
ESA
4
Log relative exposure
As
Log (ESB / ESA ) = Log ESB - Log ESA
Taking logarithms on both sides
Log(Speed A / Speed B) = Log ESB - Log ESA
If Log ESA = a , and Log ESB = b
Log(Speed A / Speed B) = b – a
Then
Speed A / Speed B = antilog (b-a)
Maximum Density (DMax)
The density produced when all the silver
bromide crystals in the emulsion is
exposed and developed
Reversal
This is the region where the density
reduces with the increasing exposure
greater than that produce DMax
Density
4
D
Max
& Reversal
DMax
3
Reversal
2
1
BF
1
2
3
4
Log relative exposure
This is the term
used to describe
the response of
the film to the
range of
wavelengths
(spectrum of
colours) of light
Relative response
Spectral Sensitivity
Monochromatic
film
Orthochro
matic film
1
0.5
300
400
500 600 700
Wavelength (nm)
Uses of the characteristic curve
Information
Gross fog (Basic fog)
Threshold
Contrast
Latitude (film latitude &
exposure latitude)
Speed & Sensitivity
Maximum density
Reversal
Uses
Selection of films
Assessing processor
performance (Quality
control)
Selection of exposure
factors
Comparison of filmscreen systems
Duplication of
radiographs
Summary
Definitions of terms
Methods of producing a sensitometric
strip
Plotting the Characteristic curve
Features of the characteristic curve
The practical applications
Thank You
V.G.Wimalasena, Principal, School of Radiography