The extrinsic (absolute) efficiency of a phosphor is expressed as the ratio of light energy emitted per unit area at the phosphor surface to incident x-ray energy fluence. A model described in earlier work has shown that by knowing the intrinsic efficiency, the particle size, the thickness and the light extinction factor ξ, it is possible to deduce the extrinsic efficiency for an extended range of particle sizes and layer thicknesses for a given design. The model has been tested on Gd 2 O 2 S:Tb and ZnS:Cu fluorescent layers utilized in two quality assurance devices, respectively, aimed for the assessment of light field and radiation field congruence in diagnostic radiology. The first unit is an established device based on both fluorescence and phosphorescence containing an x-ray sensitive phosphor (ZnS:Cu) screen comprising a long afterglow. Uncertainty in field edge position is estimated to 0.8 mm (k=2). The second unit is under development and based on a linear CCD sensor which is sensitized to x-rays by applying a Gd 2 O 2 S:Tb scintillator. The field profiles and the corresponding edge location are then obtained and compared. Uncertainty in field edge location is estimated to 0.1 mm (k=2).The properties of the radioluminescent layers are essential for the functionality of the devices and have been optimized utilizing the previously developed and verified model. A theoretical description of the maximization of phosphorescence is also briefly discussed as well as an interesting finding encountered during the development processes: focal spot wandering. The oversimplistic physical assumptions made in the radioluminescence model have not been found to lead the optimizing process astray. The obtained functionality is believed to be adequate within their respective limitations for both devices.

Purpose: Existing methods for checking the light field radiation field congruence on x-ray equipment either do not fully meet the conditions of various quality control standards regarding inherent uncertainty requirements or contain subjective steps, further increasing the uncertainty of the end result. The aim of this work was to develop a method to check the light field radiation field congruence on all x-ray equipment. The result should have a low uncertainty which is accomplished by eliminating most subjective user steps in the method. A secondary aim was to maintain the same level of usability as of comparable methods but still able to store the result.Approach: A new device has been developed where the light field and corresponding radiation field are monitored through measurements of the field edge locations (in total: 2 x 4 edges). The maximum field size location deviation between light field and radiation field in the new method, is constrained by the physical limitations of the sensors used in various versions of the prototype: Linear Image Sensors (LISs) of 25-29 mm active sensor length. The LISs were sensitized to x-rays by applying a phosphor strip of Gd2O2S:Tb covering the light sensor input area. Later prototypes of the completed LIS device also have the option of a Bluetooth (100 m range standard) connection, thus increasing the mobility. Results:The developed device has a special feature of localization a field edge without any prior, subjective, alignment procedure of the user, i.e. the signals produced were processed by software storing the associated field edge profiles, localizing the edges in them, and finally displaying the calculated deviation. The uncertainty in field edge location difference was estimated to be less than 0.1 mm (k=2). The calculated uncertainty is lower than for other, commercially available, methods for light field radiation field congruence also presented in this work. Conclusions:A new method to check the light field radiation field congruence of x-ray systems was developed to improve on the limitations found in existing methods, such as device detector resolution, subjective operator steps or the lack of storing results for later analysis. The development work overcame several challenges including mathematically describing real life edges of light and radiation fields, noise reduction of radiation edges and mapping/quantification of the rarely observed phenomenon of focal spot wandering. The assessment of the new method showed that the listed limitations were overcome, and the aims were accomplished. It is therefore believed that the device can improve the work in quality controls of x-ray systems.