Abstract: Generally, in an optical system for recording and reproducing an optical information medium, a semi-conductor laser is used as a light source, an approximately parallel luminous flux is obtained by a collimator lens, and thereafter it is converged on a recording surface by an objective lens. If the semi-conductor laser causes a mode hop to abruptly change an oscillating wavelength, an error results when the objective lens has a chromatic aberration. A collimator lens with a reverse chromatic abberation can be used so that various conventional objective lens may be utilized as they are without any error as described.

Abstract: In an objective lens for an optical disk of the type in which focussing is carried out by moving a unit of the entire optical system including a light source and an objective lens, variation of the numerical apertures and deterioration in performance for the focussing are not present, but it is important for reducing the weight of the unit as light as possible to minimize the distance from a light source to an information recording surface while securing an operating distance as required. The lens for that purpose is an aspherical single lens, but it is necessary, for an aspherical shape less affected by a high-order term to be easily fabricated, to make a surface on the light source side an aspherical surface, of which conical coefficient is within a predetermined range.

Abstract: A projection lens for a three-tube television projector of the type in which three CRT's of blue, green and red are projected on the screen. The projection lens comprises a plastic first lens having an extremely small refracting power, a glass second lens, a plastic third lens and a plastic fourth lens disposed immediately before a face plate of CRT. There are large spaces between the first lens and the second lens and between the third lens and the fourth lens so that reflecting mirrors may respectively be arranged therebetween. For reducing the cost, the fourth lens can be formed from a liquid lens filled with a liquid between a thin-wall shell and a face plate.

Abstract: A five unit projection lens for a projector composed of, in order from the screen side, a first positive lens unit having at least one aspherical surface, a second negative lens unit comprising a double concave lens, a third positive lens unit, a fourth lens unit, and a fifth negative lens unit with a concave surface having a great curvature directed toward the screen.

Abstract: A projection lens in which F is approximately 1.0 to 1.3 used for a 3-tube television projector. The projection lens comprises a first lens of extremely low refractive power, two positive lenses arranged spaced apart from the first lens through a large air space and a negative fourth lens arranged spaced apart therefrom through a large air space, the first lens L.sub.1 having at least one surface in the form of an aspherical surface.It is possible to arrange a reflecting mirror in the large air space to thereby form an optical system into a compact form. In addition, if the second lens L.sub.2 and the third lens L.sub.3 are made of glass, even if the first lens L.sub.1 and the fourth lens L.sub.4 are formed into plastic lenses, movement of focal point due to variation in temperature hardly occurs, and a lens of low cost may be obtained.

Abstract: This invention relates to an objective lens for regenerating a video disc or the like. A semiconductor laser is used as a light source to directly drive an objective lens for carrying out tracking, and thus the lens must be small and light-weight, but the aberration on the optical axis could well be corrected. However, because of small-size of lens, a manufacture error such as eccentricity is greatly affected. A positive meniscus lens on the image side is approximated to a concentric configuration by an objective lens comprising two lenses whereby the influence resulting from eccentricity may be minimized.

Abstract: A large-aperture single lens with aspherical surfaces to be used as a pickup lens for video disks, especially for tracking the video disk by directly moving the pickup lens. It is preferable that the single lens is made of plastic material in order to make it compact and light in weight. Both refractive surfaces of the single lens are arranged to have positive refractive powers in order to make the working distance of the lens long. Spherical aberration of the single lens is corrected to the degree that the diameter of the circle of confusion thereof is decided approximately by diffraction of light. Both refractive surfaces of the single lens are formed as aspherical surfaces and the shapes of the refractive surfaces are determined so as to correct aberrations including the sine condition to the required range, by taking the error to be caused at the time of manufacture into consideration.

Abstract: A large-aperture single lens with aspherical surfaces to be used as a pickup lens for video disks, especially for tracking the video disk by directly moving the pickup lens. It is preferable that the single lens is made of plastic material in order to make it compact and light in weight. Both refractive surfaces of the single lens are arranged to have positive refractive powers in order to make the working distance of the lens long. Spherical aberration of the single lens is corrected to the degree that the diameter of the circle of confusion thereof is decided approximately by diffraction of light. Both refractive surfaces of the single lens are formed as aspherical surfaces and the shapes of the refractive surfaces are determined so as to correct aberrations including the sine condition to the required range, by taking the error to be caused at the time of manufacture into consideration.