Luminescence Characteristics 2.2.1 Luminescence spectrum and photostimulated excitation spectrum As shown in Figure 2.3, the photostimulable phosphor employed by the IP emits bluish-purple light. This luminescence is invoked by the bivalent europium ions contained in a trace amount in the phosphor. The intensity of this luminescence varies with the wavelength of the light to which the IP is exposed after an x-ray exposure. This dependence of photostimulated luminescence on the wavelength is represented by a photostimulated excitation spectrum. Briefly, photostimulated luminescence is invoked most effectively by red light having a wavelength of about 600 nm. The photostimulated excitation spectrum is fairly consistent with the light absorption pectrum of the color s center formed within the phosphor upon exposure to X-rays. This is one of the main reasons for stating that photostimulated luminescence is caused by the color center. To enhance the image S/N ratio in the lP-based FCR system, it is necessary that the wavelength of photostimulated luminescence that has X-ray information be sufficiently different and optically separable from the wavelength of the photostimulated excitation light, and that the peak of the photostimulated luminescence spectrum be close to the 400 nm point, at which the photomultiplier (hereinafter referred to as the PMT) detects luminescence with high efficiency. The two spectra shown in Figure 2.3 have desirable characteristics that meet the above conditions.