# Radiochromic Films for the Two-Dimensional Dose Distribution Assessment
## 2 Basics of Radiochromic Films
### Physical Quantities for Measuring the Radiochromic Film Darkening  
The exposure of RCFs to a defined radiation dose provokes darkening which can be related to the absorbed dose; thus, the darkening needs to be quantitatively measured. The physical quantity used for this purpose can be defined on the basis of the characteristics of either the RCF type or of the reading instrument. The interaction of the light with a thin film, such as an RCF, is a complex physical phenomenon which has been explained by [PERSON] et al. [61]. Referring to the schematics of Figure 2b, the light intensity interacting with the RCF has three main components:  
1. transmitted (\(I\)) through the RCF and collected by the detector,
2. scattered (\(I_{s}\)) by the RCF and not collected by the detector,
3. absorbed (\(I_{abs}\)) by the RCF.  
Here, the basic definition of transmittance \(T\), reflectance \(R\), and optical density \(OD\) (or absorbance \(A\)) are recalled. Let \(I_{0}\) be the light intensity detected from the light detector in absence of the RCF (see Figure 2a); thus, the transmittance is defined as the ratio  
\[T=I_{0}/I,\]  
the reflectance as the inverse of the transmittance  
\[R=I/I_{0},\]  
and the optical density (or absorbance) as the logarithm of the transmittance, in formula:  
\[OD\equiv A=\log_{10}T=\log_{10}(I_{0}/I).\]  
Figure 2: Schematics Please, note that I’ve uploaded a new version of figure 2 of improved quality (in particular some unwanted lines have been removed). showing the principle of measurement of the RCF darkening. \(I_{0}\) denotes the light intensity detected without RCF (**a**); \(I_{s}\), \(I_{abs}\) and \(I\) denote the components of light scattered, absorbed and transmitted respectively, by the RCF (**b**).  
These physical quantities are often not directly used for the reading of the RCF darkening. Firstly, most of the RCF reading instruments do not measure directly the component of light transmitted by the RCF but a combination of light transmitted and scattered. In the second place, as the aforementioned quantities are functions of the wavelength, or of specific wavelength range, care should be taken when these are considered. Furthermore, the RCF grade of darkening has to be put in relation to the change of light intensity of the RCF after the exposure to radiation and before it. The net optical density (netOD) is defined as:  
\[netOD=\log_{10}\frac{I_{unexp}-I_{bckg}}{I_{exp}-I_{bckg}}, \tag{1}\]  
where \(I_{exp}\) is the light intensity of the film exposed to radiation, \(I_{unexp}\) is that of the film not exposed to radiation, and \(I_{bckg}\) is the light intensity collected by the detector in absence of light from the source. The net optical density considers all these aspects and is commonly used for the RCF reading. The net reflectance and net transmittance are defined in the same way. The net reflectance is used for the reading of RCFs having an opaque layer (e.g., Gafchromic(tm) XRQA-2 films) by setting the reading instrument, typically a flatbed scanner, in reflectance mode. Other physical quantities can be used on the basis of the reading instrument. Today, most widespread instruments for RCF reading are flatbed scanners. The digitized image is analyzed and the Pixel Value (PV) of a defined region of interest (ROI) is evaluated. The PV of the exposed film can be then normalized by the PV of the unexposed film and converted to OD or netOD. It has been proved by [PERSON] et al. that the reflection scanning mode leads to higher sensitivity even for RCFs featuring transparent layers, such as EBT-3 Gafchromic(tm), at the cost of limiting the dose range and of a higher uncertainty in comparison to transmission scan-based response functions [62]. The same authors reported also that the normalized PV of RCFs scanned in transmission mode leads to linear response up to 11 Gy. The reading of RCFs with flatbed scanners is overall a delicate topic, discussed in Section 3.1.