Hi,
Contrast, definition, detail etc. are qualitative(can not be measured) term , whereas, Density is a quantitative (can be measured) term.
Following is the extraction from www.ndt-ed.org , which may be helpful to you.
Radiographic contrast describes the differences in photographic density in a radiograph. The contrast between different parts of the image is what forms the image and the greater the contrast, the more visible features become. Radiographic contrast has two main contributors: subject contrast and detector or film contrast.
Subject contrast is determined by the following variables:
- Absorption differences in the specimen
- Wavelength of the primary radiation
- Scatter or secondary radiation
- Absorption differences in the specimen
- Wavelength of the primary radiation
- Scatter or secondary radiation
Film contrast is determined by the following:
- Grain size or type of film
- Chemistry of film processing chemicals
- Concentrations of film processing chemicals
- Time of development
- Temperature of development
- Degree of mechanical agitation (physical motion)
- Chemistry of film processing chemicals
- Concentrations of film processing chemicals
- Time of development
- Temperature of development
- Degree of mechanical agitation (physical motion)
Exposing the film to produce higher film densities will generally increase contrast. In other words, darker areas will increase in density faster than lighter areas because in any given period of time more x-rays are reaching the darker areas. Lead screens in the thickness range of 0.004 to 0.015 inch typically reduce scatter radiation at energy levels below 150, 000 volts. Above this point they will emit electrons to provide more exposure of the film to ionizing radiation thus increasing the density of the radiograph. Fluorescent screens produce visible light when exposed to radiation and this light further exposes the film.
Radiographic definition is the abruptness of change in going from one density to another. There are a number of geometric factors of the X-ray equipment and the radiographic setup that have an effect on definition. These geometric factors include:
- Focal spot size, which is the area of origin of the radiation. The focal spot size should be as close to a point source as possible to produce the most definition.
- Source to film distance, which is the distance from the source to the part. Definition increases as the source to film distance increase.
- Specimen to detector (film) distance, which is the distance between the specimen and the detector. For optimal definition, the specimen and detector should be as close together as possible. .
- Abrupt changes in specimen thickness may cause distortion on the radiograph.
- Movement of the specimen during the exposure will produce distortion on the radiograph.
- Film graininess, and screen mottling will decrease definition. The grain size of the film will affect the definition of the radiograph. Wavelength of the radiation will influence apparent graininess. As the wavelength shortens and penetration increases, the apparent graininess of the film will increase. Also, increased development of the film will increase the apparent graininess of the radiograph.
- Focal spot size, which is the area of origin of the radiation. The focal spot size should be as close to a point source as possible to produce the most definition.
- Source to film distance, which is the distance from the source to the part. Definition increases as the source to film distance increase.
- Specimen to detector (film) distance, which is the distance between the specimen and the detector. For optimal definition, the specimen and detector should be as close together as possible. .
- Abrupt changes in specimen thickness may cause distortion on the radiograph.
- Movement of the specimen during the exposure will produce distortion on the radiograph.
- Film graininess, and screen mottling will decrease definition. The grain size of the film will affect the definition of the radiograph. Wavelength of the radiation will influence apparent graininess. As the wavelength shortens and penetration increases, the apparent graininess of the film will increase. Also, increased development of the film will increase the apparent graininess of the radiograph.
Nandeesh
From: Mohamed Motawea <eng.motawea@gmail.com>
To: Materials & Welding <materials-welding@googlegroups.com>
Sent: Tuesday, 29 November 2011 3:12 PM
Subject: [MW:13095] Re: contrast of film radiographic
Dear sir;
There is differnec between the film contrast and film density.
Regards;
Mohamed
On Nov 29, 10:43 am, yogeshkumar Kumbhar <kumbharyoge...@gmail.com>
wrote:
> Dear Mehdi,
>
> Measure radiography film contrast ( Density ) by densitometer.
>
> Regard's
>
> Yogesh V Kumbhar
>
>
>
> On Tue, Nov 29, 2011 at 10:21 AM, Mehdi Emami <mehdi.em...@yahoo.com> wrote:
> > Dear experts
> > Can anyone tell me how to measure contrast of radiographic films?in fact,
> > I want Definition & contrast calculation method
> > Thanks
>
> > --
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> > w.r.t. applicable code/standard/contract documents.- Hide quoted text -
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> - Show quoted text -
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The views expressed/exchnaged in this group are members personel views and meant for educational purposes only, Users must take their own decisions w.r.t. applicable code/standard/contract documents.
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