The document discusses photographic performance assessment methods, particularly the use of characteristic curves in sensitometry to evaluate film response to exposure. It details the process of creating and analyzing these curves, including factors like optical density, contrast, latitude, speed, and sensitivity of films. Additionally, it explains the regions of the characteristic curve, the significance of density measurement, and the impact of exposure conditions on film performance.

2. Describing photographic
perfomance
 To compare and to evaluate one film or film screen
system with another.
 Method - Characteristic curve
 Essential tool in sensitometry , scientific study of
response to exposure of photosensitive materials.

3. Optical density
 Opacity = I0/ It
 Optical density – expressing opacity in logarithmic
terms.
 Optical density= log 10 opacity
= log10 (Io / I t )
D=Density
I0=Light incident on film
It=Light transmitted by the film

4. Log relative exposure
 Typical radiographic image – range of exposure at
different areas of film.
 Expressed as logarithmic values.
 Range of 1 – 1000 becomes a range of 0 – 3.

5. Characteristic curve
 It is a graph which shows way in which a film or film
screen system responds to different levels of exposure.
 Plots optical density (D)against log relative exposure
(log E ).
 D log E curve
 Hurter and Driffield / H and D curve

6. STAGES OF CURVE
 1. Exposing and processing the film.
 2. Measuring the densities produced.
 3. Plotting the curve

7. 1.Exposing and processing film
 Irradiate film with a series of exposures.
 Exposure series can be achieved in 2 ways :
 Time scale sensitometry– each area on film is exposed to
same intensity but duration of exposure is varied.
 Intensity scale sensitometry_Each area on film is exposd for
same duration of time but intensity varied
 X- ray exposure : Al step wedge calibration done to find
wedge factor.
 Wedge factor :- relationship with one exposure with next
exposure.
 Visible light exposure : Sensitometer

8. 2.Measuring the densities
produced
 Densitometer – Optical density of each exposed step
on film strip
 Density is read out from a digital display or from an
analogue display using pointer & calibrated scale.

9. 3. Plotting curve


10. Features of curve
 Curve is divided into 3 regions :
1. Region to left of toe
Base
density Fog Threshold

11. Region to left of toe
 Region of underexposure
 Base density
Due to absorption of light as it is transmitted through
base
 Fog : density produced by development of silver halide grains which
have received no intentional exposure.
Types : Storage fog, Safelight fog
 Threshold : As exposure increases , film emulsion begins to respond to
exposure and density begins to rise above gross fog.

12. Region between toe and shoulder

Contrast
Latitude
Speed &
Sensitivity

13. Contrast

1. Density difference and
image contrast
maximum where
curve is steepest.
2. Slope/ Gradient ( G)
of curve - measure
of contrast .

14. Gradient ( G) at a point on curve –
slope of the tangent to curve.
It angle of slope is A ,
G = tan A.
Gamma - Value of G is constant
along straightline .
Point of Inflection : Point
where curve changes from
concave to convex
Max gamma – tangent of angle
of slope of curve at its point of
inflection.

15. Average gradient
G̅ = DY – DX
log EY-log EX
FACTORS :
1. Film emulsion
characteristics
2. Film processing
conditions
3. Characteristics of
intensifying screen
For some direct exposure films, Point of inflection
occurs at density above midrange value used in
diagnostic imaging.

16. Latitude
 It refers way in which a film/ film screen system is able
to record successfully wide range of exposures.
FILM
LATITUDE
EXPOSURE
LATITUDE

17. Film latitude =log EY- log EX
Average gradient = DY - DX = Useful density range
log EY - logEx Film latitude

18. Exposure latitude
 Represents tolerance of a film / film screen system to
errors in selection of exposure factors when
radiographic exposure is made.
 Exp latitude = Film latitude – log exposure range

21. Speed & Sensitivity
 It is an expression of xray exposure required to
produce a given image density.
 Higher speed of system, less exposure to produce
given density. ( curve will be further to left )
 Speed of system A = mAs for system B
Speed of system B mAs for system A

22. How is relative speed assesed from
curve ?
Log exposure values corresponding to a net
density of 1 is 1.4 and 1.8
Log EB – log EA = .4 EB /EA = antilog ( .4) = 2.5
System A is 2 .5 times speed of system B

23. Region to right of shoulder

Maximum
Density
Reversal

24. Maximum density
 As film/ film screen is subjected to greater exposure ,a
point is reached where all of silver halide grains in
emulsion are reduced to silver.

25. 1. Silver coating weight
2. Processing conditions

26. Reversal
 Reduction in image density due to increase in
exposure.

27. Testing for reversal property
 Film removed from cassette
 Exposed to bright light
 Film processed
 Normal films – black but film which exibits reversal –
almost transparent.

28. References
 Chesney’s Radiographic Imaging – John Ball and Tony
Price