Measurement of the Optical Density of X

Chapter 7: Non-destructive Testing
Reference procedure
Measurement of the Optical Density of X-ray Films
optical density, X-ray film with graded optical densities, film step tablet
Quantities and items tested
optical density, film step tablet, radiographs
Testing range
Uncertainty of results
0,2 up to 4,8
Fields of application
Verification and calibration of densitometers for measurement of the optical density on radiographs (X-ray films) in
industrial radiographic testing
Methodology and instrumentation
Traceable measurement of optical densities of X-ray films with a calibrated densitometer according
to ISO 5-2 (diffuse optical transmission density)
Qualification and quality assurance
Traceability to a reference standard of the National Institute of Standards and Technology (NIST) and a calibrated
step tablet by the Physikalisch -Technische Bundesanstalt (PTB)
The densitometer is checked and calibrated periodically by the above mentioned reference standards.
Dr. Uwe Zscherpel
+49 30 8104 3677
[email protected]
+49 30 8104 1837
Division 8.3: Radiological Methods
Date: Feb. 2014
back to Catalogue
of Reference Procedures
Reference procedure
Chapter 7: Non-destructive Testing
Further information
Flaws in technical installations or devices can be detected by differences in optical density of X-ray films/
It is therefore important for quality assurance of film evaluation, that the optical density is defined and traceable to
The optical density is measured by transmission of light through the film according to ISO 5-2 and ISO 5-3.
The optical density is defined by:
D  lg
D = diffuse optical density
Lo = Luminance in cd/m2 (on the side of the light source before the film, diffuse)
LF = Luminance in cd/m2 (on the side of the observer, behind the film, specular)
The optical densities measured with a densitometer are traced back onto a NIST-standard respectively a density
step tablet calibrated by the PTB.
The density step tablets measured with the reference method “Measurement of the optical density” are used in
industrial radiography to check the densitometers and therefore for the quality assurance of the exposed x-ray
films, i.e. for a proof of the achieved density.
The measured density step tablets have to be recalibrated in certain intervals depending on national directives
(remeasurement of the optical density). The measurement results are summarized in a certificate.
Example for a measured density step tablet with certificate:
Date: Feb. 2014