SensitometrySensitometry

Describing photographic Describing photographic performanceperformance

ObjectivesObjectives

The student should be able to:The student should be able to:Describe the importance of the studyDescribe the importance of the studyDefine the relevant termsDefine the relevant termsDescribe the methods of producing a Describe the methods of producing a sensitometric stripsensitometric stripPlot the characteristic curvePlot the characteristic curveDescribe the Features of the characteristic Describe the Features of the characteristic curvecurveDescribe the importance & practical Describe the importance & practical applicationsapplications

Why is the study important?Why is the study important?

To produce images with optimum contrast To produce images with optimum contrast that reveal high details of the object that reveal high details of the object examined.examined.

Optimum contrast?Optimum contrast?

Low contrast (long scale contrast)Low contrast (long scale contrast)

High contrast (Short scale contrast)High contrast (Short scale contrast)

Photographic imagesPhotographic imagesOptimum contrastOptimum contrast

High contrastHigh contrast

Low contrastLow contrast

X-ray imagesX-ray images

IntroductionIntroduction

Photosensitive materials (x-ray films) are Photosensitive materials (x-ray films) are used to record the invisible x-ray imageused to record the invisible x-ray imageIt is required to reproduce all the It is required to reproduce all the characteristics of the characteristics of the invisible imageinvisible image in in visible form.visible form.The films ability to do so depends on its The films ability to do so depends on its sensitometric properties.sensitometric properties.It is advantages to have a sound It is advantages to have a sound knowledge of sensitometric properties of knowledge of sensitometric properties of x-ray films.x-ray films.

X-ray tube

Object

Plot of incident x-ray beam intensity

Plot of transmitted x-ray beam intensity

Invisible x-ray image

Format

ion o

f Invi

sible

x-ra

y im

age

Invisible x-ray imageInvisible x-ray image

B2 T1 T2 T3

E

E B1 E B2

ET1EM

ET2

ET3

EA

kV mA Sec FFD

BB1 Air

Supporting tissue (m)

EMInvisible X-ray image

What is Sensitometry?What is Sensitometry?

The scientific study of the The scientific study of the responseresponse of of photosensitive material to different levels photosensitive material to different levels of exposures.of exposures.

How is it done?How is it done?

Producing a Producing a sensitometric stripsensitometric strip and plotting and plotting a characteristic curve.a characteristic curve.

What is Response of exposure & What is Response of exposure & How is it manifested?How is it manifested?

The response is the change that The response is the change that takes place, after exposure to takes place, after exposure to electromagnetic radiation (light or x-rays), electromagnetic radiation (light or x-rays), in the photosensitive emulsion on the filmin the photosensitive emulsion on the film

The response of the film to exposures is The response of the film to exposures is manifested as a manifested as a degree of blackeningdegree of blackening produced after chemical processingproduced after chemical processing

How is degree of blackening How is degree of blackening quantified/measured?quantified/measured?

In Radiography the degree of film In Radiography the degree of film blackening is quantitatively indicated by blackening is quantitatively indicated by the term the term ‘Optical Density’.‘Optical Density’.

The optical density describes how The optical density describes how much a certain area of the film is opaque much a certain area of the film is opaque to light incident upon it .to light incident upon it .

Optical DensityOptical Density

The optical density is expressed The optical density is expressed quantitatively as,quantitatively as,

Optical Density = LogOptical Density = Log1010 Opacity Opacity

It is measured by using the It is measured by using the ‘Densitometer’‘Densitometer’

(The densitometer works on the following (The densitometer works on the following principle)principle)

Consider the light transmitted through an Consider the light transmitted through an area of a filmarea 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 filmRange of densities on a film

IIii IItt TransmitTransmittancetance

OpacityOpacity DensityDensity

100100

0.010.01

0.10.1

1.01.0

10.010.0

100100

0.00010.0001

0.0010.001

0.010.01

0.10.1

11

1000010000

10001000

100100

1010

11

44

33

22

11

00

4 3 2 1 0

Sensitometric stripSensitometric strip

A film containing number of areas with A film containing number of areas with different optical densities from white (fully different optical densities from white (fully transparent) to black (fully opaque)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 exposingMethods of exposing

Time scale methodTime scale method

(using x-ray exposure)(using x-ray exposure)

Intensity scale methodIntensity scale methodi. Using x-ray exposurei. Using x-ray exposure

ii. Using light exposureii. Using light exposure

Time scale methodTime scale methodThe film is exposed to different The film is exposed to different quantitiesquantities of of exposures using constant intensity and exposures using constant intensity and variable duration.variable duration.(Quantity = Intensity x Time)(Quantity = Intensity x Time)

Equipment requiredEquipment requiredX-ray machineX-ray machineCassette with film insideCassette with film insideLead sheetsLead sheetsProcessorProcessorDensitometerDensitometerGraph paperGraph paper

Procedure (time scale method)Procedure (time scale method)Constants

kV, mA, FFD

Variable

Time

Lead sheets

Loaded cassette

X-ray tube

Loaded cassette

Areas to be exposed

Exposure selectionExposure selectionMinimum exposureMinimum exposure Low enough not to produce a measurable Low enough not to produce a measurable densitydensityMaximum exposureMaximum exposureHigh enough to produce a density around 3.0High enough to produce a density around 3.0IncrementIncrement Wedge factorWedge factor (Ratio between two exposures) (Ratio between two exposures) of 2 is adequate. (2of 2 is adequate. (2½½ can be used to get more can be used to get more levels)levels)

Alternatively the time steps available in the Alternatively the time steps available in the machine may be used to get more points on machine may be used to get more points on the graphthe graph

Intensity scale method 1Intensity scale method 1

Making a single exposure Making a single exposure using using

a a calibratedcalibrated step step wedgewedge

X-ray machineX-ray machine

a loaded cassettea loaded cassette

(The intensity of x-rays (The intensity of x-rays passing through the passing through the steps are different & steps are different & the duration of the duration of exposure is the same)exposure is the same)

X-ray exposure

Intensity scale method 2Intensity scale method 2

The film is exposed to a series of different The film is exposed to a series of different intensities of light for the same duration intensities of light for the same duration using an instrument called the using an instrument called the ““SensitometerSensitometer”. ”.

The wedge factor is usually 2The wedge factor is usually 2½½

The colour of light should match the The colour of light should match the spectral sensitivity of the filmspectral sensitivity of the film

Film obtained using time scale method

Plotting the curvePlotting the curve

AreaArea AA BB CC DD

Exposure (Time)Exposure (Time) 0.010.01 0.020.02 0.040.04 0.080.08

Relative exposureRelative exposure 11 22 44 88

Log relative Log relative exposureexposure

00 0.30.3 0.60.6 0.90.9

DensityDensity 0.250.25 0.30.3 0.40.4 0.90.9

Plotting the curvePlotting the curve

AreaArea EE FF GG HH II

Exposure (Time)Exposure (Time) 0.160.16 0.320.32 0.640.64 1.281.28 2.562.56

Relative Relative exposureexposure

1616 3232 6464 128128 256256

Log relative Log relative exposureexposure

1.21.2 1.51.5 1.81.8 2.12.1 2.42.4

DensityDensity 1.451.45 2.12.1 2.52.5 2.72.7 2.92.9

1

2

3

4

1 2 3 4Log relative exposure

Den

sit

y

Shoulder

Toe

Characteristic curve

GF

GF = Gross Fog

DMax

Straight line portion

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0.00

0.60

0.88

1.10

1.40

1.70

1.88

2.10

2.30

2.48

2.60

2.88

3.10

3.24

3.35

3.48

Log Relative exposure

Den

sity

Characteristic curve

Shoulder

Straight line portion

Toe

D-Max

GF

(Features) Information obtainable(Features) Information obtainable

Gross fog (Basic fog)Gross fog (Basic fog)

ThresholdThreshold

ContrastContrast

Latitude (film latitude & exposure Latitude (film latitude & exposure latitude)latitude)

Speed & SensitivitySpeed & Sensitivity

Maximum densityMaximum density

ReversalReversal

Gross fog (Basic fog)Gross fog (Basic fog) & & net densitynet density This is the density of the horizontal part of the This is the density of the horizontal part of the

curve at the minimum exposure levelcurve at the minimum exposure level

Gross fogGross fog = = Base DensityBase Density + + FogFog

Base DensityBase Density :- Density produced by the base :- Density produced by the base material material

FogFog :- Density produced by the development of :- Density produced by the development of silver halide crystals which have not silver halide crystals which have not

received an intentional received an intentional exposureexposure

Net densityNet density = Gross density – Gross fog = Gross density – Gross fog

1

2

3

4

1 2 3 4Log relative exposure

Net

Den

sit

y Shoulder

Toe

Characteristic curve (with net density)

Net density = gross density – gross fog

0

ThresholdThresholdThe region where the film emulsion begins to The region where the film emulsion begins to respond to the exposurerespond to the exposure

ContrastContrastThe rate of change of density for a given change of The rate of change of density for a given change of log relative exposurelog relative exposure

Contrast = Contrast = ΔΔD / D / ΔΔEE

It is given by the slope (gradient) of the straight line It is given by the slope (gradient) of the straight line portion of the curve. If it is a true straight line then portion of the curve. If it is a true straight line then the contrast is called the contrast is called Gamma.Gamma.

Since, in practice, the curve is not an exact straight Since, in practice, the curve is not an exact straight line, the line, the average gradientaverage gradient is taken as the contrast. is taken as the contrast.

Characteristic curve

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0.00

0.60

0.88

1.10

1.40

1.70

1.88

2.10

2.30

2.48

2.60

2.88

ΔD

ΔE

Contrast = ΔD/ ΔE

1

2

3

4

1 2 3 4Log relative exposure

Densi

ty

Average gradient

DX

DY

Log EX Log EY

A

B

C

Average gradient = BC/AC DY - DX

= ----------- log EY – log EX

How to select points A and B ?

Point A ? Point B ?Point A ? Point B ?

A : DA : DX X = 0.25 above Basic fog= 0.25 above Basic fog

B : DB : DYY = 2.0 above Basic fog = 2.0 above Basic fog

The densities from 0.25 to 2.0 is called The densities from 0.25 to 2.0 is called the the Useful Density range.Useful Density range.

Useful density range is the density range Useful density range is the density range in which the differences can be identified in which the differences can be identified by the human eyeby the human eye..

The densities which represents different The densities which represents different structures on a radiograph should lie structures on a radiograph should lie within this range of densities.within this range of densities.

Average Gradient

Useful density range

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

LatitudeLatitudeLatitude is an expression of the tolerance of a Latitude is an expression of the tolerance of a system to extreme conditions of exposure.system to extreme conditions of exposure.

It refers to the ability of a film or film-screen It refers to the ability of a film or film-screen system to record successfully a wide range of system to record successfully a wide range of exposure . exposure .

(considered in two parts)(considered in two parts)

Film latitudeFilm latitude && Exposure latitudeExposure latitude

Film LatitudeFilm Latitude

The difference between the upper and The difference between the upper and lower limits of log relative exposure which lower limits of log relative exposure which produce densities within the produce densities within the useful rangeuseful range

Significance of film latitudeSignificance of film latitude

The range of x-ray intensities transmitted The range of x-ray intensities transmitted through the body part should lie within the through the body part should lie within the film latitude, if they are to be viewed as film latitude, if they are to be viewed as useful densities on the radiograph.useful densities on the radiograph.

Any x-ray intensity that falls out side the Any x-ray intensity that falls out side the film latitude will not reveal any information film latitude will not reveal any information & a useful piece of information might be & a useful piece of information might be lostlost

Film latitude & Average gradientFilm latitude & Average gradient

1

2

3

4

1 2 3 4Log relative exposure

Densi

ty

DX

DY

Log EX Log EY

A

B

C

When Dx = 0.25+BF & Dy = 2.0 +BF

DY - DX

Average gradient = ----------- becomes log EY – log EX

1.75

= ------------------

Film latitude

Film latitude

Exposure latitudeExposure latitudeThis refers to the freedom of the This refers to the freedom of the radiographer to select slightly different radiographer to select slightly different exposures (to make room for errors) for a exposures (to make room for errors) for a particular examination so that the resulted particular examination so that the resulted densities remains within the useful density densities remains within the useful density range.range.

Exposure latitude = film latitude – Exposure latitude = film latitude – subject subject contrast (contrast (log relative exposurelog relative exposure range range transmitted from a particular body part)transmitted from a particular body part)

Subject contrastSubject contrast

E

E1 E2 E3E4

E5E6

E7

kV mA Sec FFD

BB Air

Transmitted Transmitted intensitiesintensities

EE11 EE22 EE33 EE44 EE55 EE66 EE77

Relative Relative IntensityIntensity

RERE11 RERE22 RERE33 RERE44 RERE55 RERE66 RERE77

Log relative Log relative intensityintensity

ee11 ee22 ee33 ee44 ee55 ee66 ee77

Subject contrast

ee11ee77

Image contrastImage contrast

ee11 ee77

D1 D2 D3 D4 D5 D6 D7

Characteristic curve

0.00

0.50

1.00

1.50

2.00

2.50

3.00

0.0

0

0.6

0

0.8

8

1.1

0

1.4

0

1.7

0

1.8

8

2.1

0

2.3

0

2.4

8

2.6

0

2.8

8

Film latitude

Subject contrast

Exposure latitude

X Y

e1 ≥ X ; e7 ≤ Y

Exposure latitude = film latitude – Subject contrastExposure latitude = film latitude – Subject contrast

Sensitivity refers to the exposure required Sensitivity refers to the exposure required by a film or film-screen system to produce by a film or film-screen system to produce a net density of 1. a net density of 1.

Sensitivity is expressed in miliroentgens Sensitivity is expressed in miliroentgens (mR) (mR)

A A high sensitivehigh sensitive (have (have low mRlow mR value for value for sensitivity) or high speed system requires sensitivity) or high speed system requires less exposure than that of a low sensitive less exposure than that of a low sensitive or low speed system.or low speed system.

Speed & SensitivitySpeed & Sensitivity

Numerically the Speed Numerically the Speed is proportional to the is proportional to the reciprocal of the reciprocal of the sensitivity (mR) and sensitivity (mR) and is expressed as is expressed as

128128Speed = --------------Speed = --------------

Sensitivity Sensitivity (mR)(mR)

SpeedSpeed Sensitivity Sensitivity (mR)(mR)

12001200 0.10.1

800800 0.160.16

400400 0.320.32

200200 0.640.64

100100 1.281.28

5050 2.562.56

2525 5.05.0

1212 10.010.0

1

2

3

4

1 2 3 4Log relative exposure

Densi

ty

1+BF

Log ESA

Log ESB

A B

Comparison of Speeds of two films (film-screen systems)

Speed A > Speed B

Speed A α 1/ ESA

Speed B α 1/ ESB

Speed A ESB

------------- = ------

Speed B ESABF

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 = b – aThen Then Speed A / Speed B = antilog (b-a)

Maximum Density (DMaximum Density (DMaxMax))

The density produced when all the silver The density produced when all the silver bromide crystals in the emulsion is bromide crystals in the emulsion is exposed and developedexposed and developed

ReversalReversal This is the region where the density This is the region where the density reduces with the increasing exposure reduces with the increasing exposure greater than that produce Dgreater than that produce DMaxMax

1

2

3

4

1 2 3 4Log relative exposure

Densi

ty

D Max & Reversal

BF

DMax

Reversal

Spectral SensitivitySpectral Sensitivity

This is the term This is the term used to describe used to describe the response of the response of the film to the the film to the range of range of wavelengths wavelengths (spectrum of (spectrum of colours) of lightcolours) of light

300

400

500

600

700

0.5

1

Rela

tive

resp

on

se

Wavelength (nm)

Monochromatic film

Orthochromatic film

Uses of the characteristic curveUses of the characteristic curveInformationInformation

Gross fog (Basic fog)Gross fog (Basic fog)ThresholdThresholdContrastContrastLatitude (film latitude & Latitude (film latitude & exposure latitude)exposure latitude)Speed & SensitivitySpeed & SensitivityMaximum densityMaximum densityReversalReversal

UsesUsesSelection of filmsSelection of filmsAssessing processor Assessing processor performance (Quality performance (Quality control)control)Selection of exposure Selection of exposure factorsfactorsComparison of film-Comparison of film-screen systemsscreen systemsDuplication of Duplication of radiographsradiographs

SummarySummary

Definitions of termsDefinitions of terms

Methods of producing a sensitometric Methods of producing a sensitometric stripstrip

Plotting the Characteristic curvePlotting the Characteristic curve

Features of the characteristic curveFeatures of the characteristic curve

The practical applicationsThe practical applications

Thank You

V.G.Wimalasena, Principal, School of Radiography