| 摘要 |
PROBLEM TO BE SOLVED: To easily increase a focus depth of ultrasonic beams and eliminate minute control of an ink liquid face, by constituting an ultrasonic focus means of a Fresnel lens having a predetermined groove, and forming a concaved curved face of a specific curvature at a bottom part of the Fresnel lens. SOLUTION: When a groove of a Fresenel lens 18 has a depth (d), a concaved curved face is formed at a bottom part with a curvature R holding 2μm<R<d. The Fresnel lens 18 has grooves 18a-18f (indicated totally by a reference 18n), vertical side walls 31, 32 and a horizontal bottom face 33. The curvature R is provided at corner parts 33a, 33b constituted of the side walls 31, 32 and bottom face 33. Since the concaved curved face part of the curvature R is formed at the bottom face of the Fresnel lens 18, a depth of the obtained lens is increased without lowering ultrasonic focus efficiency. |
