# Radiochromic Films for the Two-Dimensional Dose Distribution Assessment
## 3 Instruments for Dose Reading
### Densitometers and Spectrometers  
Densitometers provide direct measurement of the optical density (OD) of transparent films at fixed wavelengths. The OD calibration is usually verified with step tablets of known OD. Digital RCF readers are compact and easy-to-use instruments. In some cases, densitometers allow measurements for two wavelengths in order to provide two dynamic ranges. As an example, the Digital Radiochromic Reader Model FWT-92D, suited for the reading of high dose FWT-60 films, features incident light of two wavelengths (510 and 600 nm) to cover the entire range of sensitivity [92]. RCF spectral response is an important piece of information for the analysis. To this aim, spectrophotometers have been used for the characterization of several RCF types to different radiation fields [93; 94; 95]. RCF spectral characterization has also been carried out by means of broadband spectrometers in combination with optical fibers [88; 89; 96; 97]. As an example of this, Figure 6 shows the spectral response of EBT-3 Gafchromic(tm) film to several doses in the range 0-20 Gy.  
The spectra of Figure 6 have been acquired by means of an opto-electronic system, of which a full description can be found in [96; 97]. For the sake of clarity, its basic operation is summarized here. A light source sends light into an optical fiber transmission probe. The RCF is positioned between the end of the probe and a light-diffusive element, in this case, a Teflon cylinder. The incident light interacts with the RCF, and then it is backscattered by the Teflon cylinder and collected by an optical fiber which sends it to the spectrometer. Spectra of Figure 6 can be easily converted into optical density, according to Equation (1), as shown in Figure 7.  
Spectra of Figures 6 and 7 show that the range of sensitivity of the RCF analyzed, in this case EBT3 Gafchromic(tm) film, is 500-700 nm. The RCF which has not interacted with radiation has the highest intensity and optical density zero. The strengths of the approaches providing RCF spectral response rely on the possibility of defining optimal wavelength ranges for maximize the sensitivity or for extending the dynamic range in the RCF calibration. The evaluation of the intensity (or optical density) at fixed wavelength, as well as of the integral of the intensity (or that of the optical density) in a defined wavelength range, allows great versatility in the data analysis. The aforementioned opto-electronic system has been developed and optimized to operate in real time [97]. Indeed, standard techniques for the reading of the dose, such as flatbed scanners and densitometers, do not allow the dosimetry in real-time. On the contrary, optical fiber technology has proven to be effective to this purpose. On the same topic, real-time dosimetry systems with RCFs and optical fibers have been performed by [PERSON] et al. and [PERSON] et al. [98; 99].  
Figure 6: Spectral response of EBT3 (EBT = External Beam Therapy) GafchromicTM films exposed to doses in the range 0–20 Gy. Adapted from Reference [86].  
Figure 7: Spectral response (in optical density) of EBT3 GafchromicTM films exposed to doses in the range 0–20 Gy.