This document provides an overview of the physics of film screens used in radiography. It discusses the structure and components of radiographic film, intensifying screens, and cassettes. Intensifying screens absorb x-rays and convert them to light, which is more easily detected by the film. This increases the film's sensitivity and reduces the necessary radiation dose. The characteristic curve describes the film-screen combination's response to radiation by relating optical density to exposure. It is used to determine the film's speed, gamma, latitude, and fog level. Intensifying screens allow lower radiation doses to produce diagnostic images compared to film alone.

1. Physics overview of Film Screen
Radiography
Dr Muhammad Wasim Awan
Head Of Radiology Department
Diagnostic Rad Dept: KRL Hospital

2. Objectives
• Film and intensifying screen structure
• Image Formation
• Characteristic curve
• Film-screen sensitivity

3. Radiographic Film
Film
The radiographic images are recorded on a sheet of X-ray film.
Composition of Film:
Film consists of polyester base which is coated on both side by
emulsion.
Emulsion is suspension in gelatine of silver iodobromide crystals.
Bromide (90%) Iodide (10%)

4. Film is affected by storage and handling
implications as
• Mechanical pressure
• Creasing
• Static electricity
• Chemical Liquids
• Vapours
To Protect against curling and abrasions films are
coated with antistatic super coat.

5. Polyester Base
Silver iodobromide
Silver iodobromide

6. Intensifying Screen
A photographic emulsion is more
sensitive to ultraviolet and visible light
than X-rays
X-ray films are relatively insensitive to
x-rays and if used alone require
unacceptably high dose of radiation to
produce satisfactory image.
To avoid this intensifying screens are
used.

7. Structure of Intensifying Screen
An intensifying screen consists of 4 layers:
1 A Base “for support” polyester “plastic” 0.25mm thick.
2 A Phosphor layer:
0.1-0.5mm thick
Dense layer of fine phosphor crystal (3-10 micrometer)
bound by a transparent resin.

8. 3 A Reflective layer “not in all screens”
Thin white reflective layer- b/w the base and
phosphor
Some of the light is directed towards the back of the
screen reflective layer reflect that light again towards
the film.
4 A Protective layer (outer most) Physical
protection to phosphor layer

9. Polyester base
Reflective Layer
Phosphor
Protective Layer
Intensifying Screen

10. • Old traditional phosphor used was Calcium Tungstate.
• Most commonly use phosphor is Lanthanum
oxybromide/oxysulphide because of increased
efficiency.
• Phosphor contain impurities (activator) which trap
energy.
• The intensity of light emitted by a screen depends of
phosphor and color of it depend on the activator.
• Films that are coated with emulsion on both sides
needs screens on both side.

11. INTENSIFYING SCREENS
Function:
Intensifying screens absorb the energy in the x-ray
beam that penetrated the patient
Convert it into light pattern

12. Advantages of intensifying screen
Film expose directly to X-rays
• More exposure is required if no screen are used
• At same density , contrast is always lower for a film
expose to x rays only rather than for the same film
exposed to light from intensifying screen
How this actually work --- as intensifying screens are
less sensitive than film to lower energy x-rays--- effect
of lower energy (scattered radaition----- contrast

13. Film exposed with screens to X-rays
Use of screens
• Reduce the Dose to the patient
• Reduce the Loading of tube and generator
• Allow the use of Short exposure time-- motion blur
• Allow the use of Small focal spot-- geometrical blur

14. SCREEN SLECTION
• Screen materials should not emit delayed
fluorescence because these would retain “memory”
of previous images .
• On reloading the cassette the pervious image would
be recorded on the new film and superimposed on
the image

15. It is flat light tight box which consists of pressure pads
to keep film in uniform contact with intensifying
screen.
Front of the cassette is near to the tube is made up of
low atomic number (carbon fiber) to increase
attenuation of beam so reduce dose for patient.
The back cassette is made up of thin sheet of lead to
minimize back scattered.

16. Pressure pads
Pressure pads

18. Two features of silver iodobromide crystals accounts
for photographic process:
• Small proportion of iodide as compare to bromide
distort the lattice and allows the silver ions to move
through the lattice.
• Silver iodobromide crystals posses sensitivity specks
on their surfaces

19. A crystal absorbs a light photon an electron liberated
which reach the sensitivity speck and its potential
energy is reduced.
When hundreds of photons have been absorbed and
enough electrons accumulated at sensitivity speck ,
these electrons attract mobile silver ions to join them
and be neutralized.
Now this forms a submicroscopic speck of silver metal
on the surface of crystal. Distribution of silver speck in
the emulsion form a latent image in the film which is
now awaited for development process

20. SILVER IODOBROMIDE CRYSTAL
Sensitivity
Speck
Silver ions
PHOTON
E
Changes occur with Exposure

21. The invisible pattern of latent images made
visible by processing.
Processing is of two types :
Manual (Old Technique)
Automatic ( Recent widely used now a days)

22. There are 4 steps of processing
1. DEVLOPMENT
2. FIXING
3. WASHING
4. DRYING

23. Step 1 Development :
Film is developed by immersion in alkaline solution
of an reducing agent which enter into the crystal and
reduce the positive silver ions to silver atoms latent
image grows into grains of metallic silver.
Crystals which carry no latent image remain
unaffected initially as bromide prevent entry of
developer. But when enough time given developer
penetrate the crystals and results in background fog
in the image.

24. Step 2 Fixing :
Film is now fixed with acidic solution
(Thiosulfate) which dissolve out unaffected
silver ion , so that image remain stable and
unaffected by further light
Step 3 Washing:
The film is now washed and dried under
hot air . Washing is important so that acid
doesn't turn film yellow brown as time pass
by.

25. Automatic processor use roller feed
system to transport film through
different solutions

28. • Film now caries a pattern of silver grains
corresponding to X-rays leaving the patient.
• It is a negative image
• Portion of the film where X-rays are intense
appear darkest and vice versa
• Image is viewed on illuminator

29. The term optic density (D) means BLACKENING of an area
depends on the number of silver grains per unit area
The degree of film blackening is :
 Directly related to intensity of radiation
 Directly related to silver grains/unit area
 Inversely related to light transmision
The measurement of film BLACKNESS is called optic Density

30. It is a defined as log of the ratio of incident and
transmitted light
D = log10 (incident light/transmitted light)
It is measured by Densitometer.
Why we use LOG to express density?
Because Logarithms express large differences in
number on small scale
Physiological response of human eye to different
light intensities is logarithmic

31. If an unexposed x-ray film is processed , it will
demonstrate the density of about 0.12 . This
density is refered as Base plus Fog Level
Base density (0.07) caused by plastic material and
dye used to make film base
Fog (0.05) : the density result form developed
unexposed silver grains

33. The response of a film and screen combination to X-ray
is described by characteristic curve.
It is measured using instrument sensitometer
Characteristic curve shows the relationship between
optic density and relative exposure
Primary function of characteristic curve is to describe
the contrast characteristics of the film throughout a wide
exposure range.

35. The area of correct exposure:
• Straight line portion
• Steepest
• Densities within the area of diagnostic interest should lie
within this range
• Density is proportional to log exposure.
Shoulder Region
• Shallow curve
• High exposure region
• Dense image
• Dark image
Toe Region
• Shallow curve
• Low exposure region
• Low density
• Bright image

36. Area of high exposure in which slope is
decreased is referred as SHOULDER.
The two significant characteristics of
image areas receiving exposure within
this range are that the film is quite
dark (dense) and contrast is LOW.
The part of the curve associated with
relatively low exposures is designated
the TOE, and also corresponds to the
light or low-density portions of an
image . It contribute to LOW contrast.
The highest level of contrast is
produced within a range of exposures
falling between the toe and the
shoulder. This portion of the curve is
characterized by a relatively straight
and very steep slope in comparison to
the toe and shoulder regions. In most
imaging applications, it is desirable to
expose the film within this range so as
to obtain MAXIMUMcontrast.

37. Latitude The exposure range over
which a film produces useful
contrast is designated the
LATITUDE.
Exposure values above the
latitude range also produce
areas with very little contrast
and have the added
disadvantage of being very
dark or dense.
An underexposed film area
contains little or no image
contrast.
Over
Exposure
Under
Exposure

38. Four Properties of film screen combination derived
from Characteristic Curve
1.Film speed
2.Gamma
3.Film Latitude
4 Fog Level

39. Speed
Speed here is loosely defined as the reciprocal of
exposure required to produce net density 1.
It is said to be High speed film when low exposure
required to produce useful density .
Factors Speed
• Increase grain size
• Modern emulsions with flat crystals
• X-ray photon energy 30-40 keV

40. Film Gamma (Y): Film gamma is average slope
of characteristic curve .
• It influences the amount of contrast on the
radiographic image.
• Steeper the curve higher the gamma therefore display
more contrast.
• Slope of curve increased with increased exposure and
density.
• Generally average slope is between densities 0.25-2,
• Average Film Gamma value is in range 2-3

41. Film Latitude: It is range of exposure that produce
net useful densities ranging from 0.25-2.
Below 0.25 gradient is too low.
Above 2 too dark to visualize on illuminator

42. Developer temperature cause increase rate of
chemical reaction:
• Speed
• Fog level
• Gamma
Increasing developer concentration and developing
time cause similar effects

43. Film Screen
Sensitivity

44. Film exposed alone without screen is relatively
insensitive to X-rays. In such case only 2% of
incident photon will interact with film emulsion
and form single latent image.
While using Film with intensifying screen about
30% of the photon interact which is greater
than the film alone.
During interaction X-ray photon generate 1000
light photon (intensification factor) and only
half of them reach the film.

45. Film and Screen Film
X-ray absorbed by phosphor, 30
Light photon produced, 18000
Light photon reaching film, 9000
Latent images produced, 90
Absorbed by film, 2
Latent images produced , 2