Film Image Qualities
Paul Hein
Image qualities in the film screen system consist of two major areas, photographic and geometric. I will start with the photographic qualities; first off they are split into two different types of photographic qualities. The first one is density, or the amount of blackening on the film. Optical density is measured by a device called a densitometer whose useful range should be between .25 and 2.5 OD. The formula to figure out the optical density is log10 (incident light/transmitted light). Density is indirectly controlled by a number of factors including: SID, OID, grid ratio, patient thickness, collimation, CR angulations and compensating filters. Density is affected by a couple of factors directly such as scatter and fog. Fog comes from background radiation, safelight fog and the processing chemicals; it increases density and decreases contrast. Scatter main source is from Compton interactions with matter and it too adds a layer of unwanted density. Contrast is the difference in adjacent densities and is controlled by KVP. An increase in KVP causes a lower/longer scale of contrast while a decrease causes a shorter higher scale of contrast. The higher the contrast the more shades of gray are present and the lower the contrast the more black and white the image produced is. Contrast is determined by grid ratio, OID, SID and film emulsion. The film emulsion is the silver halide crystals imbedded into the film and the size and distribution of these crystals has a direct affect on contrast.
The geometric qualities of film images include recorded detail and distortion. The distortion factor is broken down into two different categories, size and shape. The size factor takes into consideration the magnification of the image from its normal size due to an increase in OID. The shape factor contains two different types, elongation and foreshortening. Elongation is caused by an increase in tube angulations and the foreshortening is caused by increased angulations of the part itself. Primary affecting factors for distortion in film/screen imaging are, motion, FSS, and spatial resolution. Motion has two causes and decreases spatial resolution. The first cause is voluntary and is caused by a lack of communication or the proper use of immobilization devices. The other cause in involuntary motion and is motion that cannot be stopped on tried to control. To control involuntary motion the most important thing is to use a short exposure time to try and minimize the motion.
Spatial resolution also known as recorded detail is also a factor affecting the geometric qualities of film. Spatial resolution is considered the geometric sharpness of the film. Spatial resolution is measured in line pairs per mm and an increase in the number in showing an increase in spatial resolution and thus better detail. The primary factors for spatial resolution are the same as for distortion, FSS the smaller the better, motion the least amount for better resolution.