There are provided a pair of upper surface electrodes 21, 31 at both end sections, which are opposed to each other, of the insulating substrate 1 made of alumina. The ring collar is adapted to be fastened to the leading edge of a lens, and at least one cushion is disposed at a bottom area of the ring collar, respectively. Each of the first lens unit, the second lens unit, and the third lens unit consists of a single positive lens and a single negative lens, and the fourth lens unit consists of a single positive lens. Within a group, the weighted samples are summed to produce a weighted total amount of energy detected within a sector. The rotatable means is provided such that it in its release position can follow the piston means in the mixing space in the direction towards said opening.