Two rare earth cations, thulium (Tm) and terbium (Tb) have been incorporated into a yttrium aluminum garnet (YAG) host material to obtain a blue phosphor. Thulium concentrations of up to 5% yield a saturated dark blue phosphor which exhibits a low efficiency. The highest efficiency for YAG:Tm occurs at a Tm concentration of 2%. A 0.5% concentration of terbium yields an unsaturated blue phosphor with an efficiency of approximately a factor of 15 times greater than that of Tm. The cathodoluminescence spectrum of YAG:Tm, Tb depicts features identifiable with YAG:Tb even at low Tb concentrations (0.5%). The light emitted by a Tb, Tm coactivated phosphor exhibits a clear shift toward the green region of the spectrum. There appears to be a resonant energy transfer from the 1D2 Tm3+ state to the 5D4 Tb3+ state. In the case of small concentrations of Tb in YAG, thulium behaves as a sensitizer for Tb cathodoluminescence.

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When an off-axis paraboloidal mirror focuses a parallel beam, the image is formed on one side of the optical axis. For a tilted beam focused by an off-axis paraboloidal mirror, the focus is no longer pointlike (not considering the diffraction effect); rather, it is a distorted spot. This is due to the inherent aberrations of the surface. In addition, there is a change in the focus position. We calculate by exact ray-trace equations the modified wave-front aberration and express it in power series. Our formulation uses the optical path variation along a defined principal ray that we relate to the parameter that describe the surface and the beam angle of incidence. We designate this ray as that reflected by the center of the entrance pupil and field of view. We employ the direction cosines of the principal ray to compute the wave-front aberration function of a beam reflected by an off-axis paraboloid.