An exposure device includes a substrate on which a light emitting element array is provided, a focusing lens that focuses light emitted by the light emitting element array, a supporting member that supports the substrate and the focusing lens. A bottom surface of the housing forms a mounting surface of the circuit board while a top surface forms a cable layout surface for laying out the connection cable in the surface direction of the circuit board. A system for measuring a transmission performance characteristic, comprises a demodulator for demodulating a received signal, a signal generator for producing an ideal signal from the demodulated signal and a processor for estimating the performance characteristic from a difference between the ideal signal and the received signal.