PURPOSE: To realize observation under good conditions using an irradiation light conversion camera corresponding to the fringe of a droplet by reflecting a light from a scattering/reflective light receiving face on a reflective face provided on the inside thereof and then collecting the reflected light by means of the camera.

CONSTITUTION: A region 7 between the fringe 8 of a droplet 9 and an undercoat 11 including the interface is irradiated with a vertical parallel light beam 6 radiated from a laser 1, passed through an optical system 3 and reflected on a reflective prism 5a. The spreading range of irradiation light conversion, represented by a beam 14 generated as a result of interaction of the droplet 9 and the undercoat 11, is measured on one light receiving plane intercepting the beam 14. The beam 14 is reflected on the reflective surface 13b of a truncated conical element 13a having conical top angle, of a scattering/reflective light receiving plane, corresponding to the angle of field of a camera C to become beam 16, then reflected on a mirror 17 mounted on the inner surface of the reflective surface 13b to produce a light beam 18. When the small truncated conical bottom face of the element 13a surrounds the camera C and the large bottom face surrounds the mirror 17, the majority thereof is captured by the camera C.