Data packet transmission involves transmitting a plurality of data packets from said first station to a second station at a first rate of transmission. The current-voltage conversion circuit includes a range-switching circuit that switches a current-voltage conversion range. Anchored on the margin of each filter lens is at least one clip having a ledge at one end which supports the margin of the lens, and an arm at the other end that curves over the frame at the corresponding optical lens to retain the clip-on.