Abstract: An ink jet printer serves to solve a problem that ink is not normally discharged when bubbles are present in ink contained in a ink chamber of a print head. The ink jet printer performs normal discharge of ink by detecting bubbles contained in ink and carrying out an optimum head recovery operation, so that stable and high-quality printed matter can be obtained. According to a bubble detecting circuit and method employed in this ink jet printer, the impedance of the piezoelectric element of the head is first measured at given frequencies. Then, impedance versus frequency characteristics are obtained. Subsequently, it is determined, based on the impedance versus frequency characteristics, whether or not a bubble adheres to the piezoelectric element.

Abstract: When bubbles affecting the performance of spitting ink are generated in an ink reservoir, the bubbles are completely removed to attain a state full of ink without bubbles of the ink reservoir. An inkjet head comprising an ink tank, an ink reservoir having a nozzle array, and an ink supply path for introducing ink from the ink tank into the ink reservoir is characterized in that, with the ink supply path being closed and with a vent hole for making the ink reservoir in communication with the external being open, an ink suction device introduces the outside air via the vent hole into the ink reservoir to which negative pressure is applied by the suction of ink, while ink in the ink reservoir is sucked out to the external via a suction path.

Abstract: The particle diameter of droplets is controlled without improvements on a nozzle plate. An ultrasonic wave causes a radiation pressure to be intermittently applied to an opening in a cycle having a period shorter than the fundamental vibration period of a liquid surface in the opening. A high-order standing wave is then generated at the liquid surface in the opening to cause a plurality of droplets to be emitted simultaneously. Since the plurality of droplets are simultaneously emitted from a plurality of mounds of the high-order standing wave, the droplets have a diameter smaller than the diameter of the opening and are emitted vertically upwardly. The diameter of the droplets is controlled by the order of the high-order standing wave to be generated. The order of the standing wave is increased by shortening the period for which the radiation pressure is applied to the opening.

Abstract: A retro-focus type wide-angle lens having an aperture ratio of 1:2, an angle of view of about .+-.42.degree., and a back focus about 1.5 times the focal length comprises only nine component lenses. The first lens disposed nearest to the object side is a positive meniscus. The second and third lenses are each a negative meniscus. The fourth lens is a positive lens. One of the fifth and sixth lens is a double-convex lens, the other being a positive meniscus. The seventh, eighth, and ninth lenses are a double-concave lens, a positive meniscus, and a double-convex lens, respectively. The sixth and seventh lenses are cemented together. The wide-angle lens satisfies the following conditions:(1) 1.0f<.vertline.f.sub.1,2,3 .vertline.<1.3f; f.sub.1,2,3 <0(2) 1.1f<r.sub.6 /(n.sub.3 -1)<1.5f(3) 0.4f<d.sub.6 +d.sub.7 <0.7f(4) (n.sub.4 +n.sub.5)/2>1.65(5) n.sub.6 <n.sub.7(6) 0.6f<.vertline.r.sub.12 .vertline.<0.9f; r.sub.12 <0where f is the focal length of the whole system, f.sub.

Abstract: A photographic lens system such as a telescopic lens system adapted for use with a rear stop diaphragm in which the backfocus is about 0.3F, the F number is about F/2.8, the system is small in size and aberrations are well compensated for. Front and rear lens groups are provided with the front group including in order from the object positive, negative, positive and positive lenses while the rear group includes negative and positive lenses. Preferred focal lengths, radii of curvature, refractive indices, and Abbe numbers for each of the lenses are disclosed.