  It is important to study absorption spectrum in film dosimetry because the
spectral absorbance of the film relates to the film's total absorption dose. We
investigated the absorption spectra of Gafchromic EBT2 film with various
rotational angles in a visible wavelength band. The film was irradiated with 6
MV photon beams and a total dose of 300 cGy. Absorption spectra were taken
under different rotational angles after 24 h after irradiation and we fitted
the spectra using Lorentzian functions. There were two dominant absorption
peaks at approximately 586 nm (green) and 634 nm (red). The measured spectrum
was decomposed 542 nm, 558 nm, 578 nm, 586 nm, 626 nm, 634 nm, and 641 nm. The
maximum total area of the red band absorption spectrum was at
45{\deg}(225{\deg}) and the minimum at 90{\deg}(270{\deg}). As the angle of
rotation changed, the intensity and integrated area of the blue and green peaks
also changed with 180{\deg} period, with minima at 90{\deg} and 270{\deg}, and
maxima at 0{\deg} and 180{\deg}, although the overall absorbance is very low.
The spectral peak wavelengths remained constant within 2.4 nm for all angles.
There was no hysteresis of absorption spectrum of the film; spectra taken at
0{\deg} and 360{\deg}were substantially the same and showed similar behavior
for all rotational angles. The change of absorbance with rotational angle of
the film affected the dosimetric properties, resulting in rotationalvariations
of film dosimetry in each red-green-blue channel.
