An aircraft system includes a heat source and a passage near the heat source for carrying fluid having a cooling capacity to cool the heat source. In some embodiments, optical signals from two or more optical emitters are directed at different regions of a photo detector. Examples of such elements include optical fiber bundles; optical light guides; optical gratings; an integrating sphere; and an acousto-optic modulator. A position indicator moves within the cache bar and tells the user visually where his current position is within the program material. A moving-image difficulty-level calculating unit calculates a moving-image difficulty level indicating a complexity of the moving image according to the first and second difficulty levels and the combination ratio changed along the time axis. A third reflector can be disposed in front of the LED light source and downward with respect to the optical axis.