Abstract: A cross dichroic prism comprises four triangular prisms having isosceles right triangular cross sections identical in shape, opposing side faces thereof being bonded and secured together with rectangular portions thereof facing each other; and a dichroic film having an X-shaped cross section formed between the bonding faces of the triangular prisms; wherein, of the four triangular prisms, a given prism and at least one of the prisms adjacent thereto have a refractive index difference therebetween of 0.00005 or less, and the remaining two prisms opposed thereto have a refractive index difference therebetween of 0.00005 or less.

Abstract: A cross dichroic prism comprises four triangular prisms having isosceles right triangular cross sections identical in shape, opposing side faces thereof being bonded and secured together with rectangular portions thereof facing each other; and a dichroic film having an X-shaped cross section formed between the bonding faces of the triangular prisms; wherein, of the four triangular prisms, a given prism and at least one of the prisms adjacent thereto have a refractive index difference therebetween of 0.00005 or less, and the remaining two prisms opposed thereto have a refractive index difference therebetween of 0.00005 or less.

Abstract: The present invention relates to a projection type display apparatus which utilizes a polarizing beam splitter and/or a color separating/combining optical system, each being constituted by an optically transparent material block having a specific characteristic, thereby being capable of securing optically stable performances against influences of various thermal and external stresses and suppressing image quality deterioration to a low level. In particular, in this projection type display apparatus, the polarizing beam splitter used for a polarized light separating optical system and analyzing optical system, and/or an optical member servicing as a color separating/combining optical system are constituted by an optically transparent material block in which a wavelength where the absolute value of a photoelastic constant becomes a minimum level exists in a wavelength range of a blue light component.

Abstract: In order to obtain an image without a registration shift, there is provided a color composing optical system which comprises a liquid crystal light valve having a plurality of pixels and a cross dichroic prism for color-composing the light beams emitted from the liquid crystal light valve, characterized in that the following equation is satisfied:L.times..vertline.tan.THETA..vertline..ltoreq.W/2,where the length of a pixel pitch of the liquid crystal light valve is W, the air-converted length from the liquid crystal light valve emission surface to the emission surface of the cross dichroic prism is L, and a value of a relative angular deviation when two light rays emitted from the liquid crystal light valve in parallel with the optical axis enter the cross dichroic prism and then are emitted from the cross dichroic prism is .THETA..

Abstract: An optical glass composition having high refractive index and low dispersion is disclosed which is characterized in that it contains no harmful thorium compound and instead contains WO.sub.3 in a content of 10 to 35 percent by weight introduced into the system of B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -ZrO.sub.2 -Ta.sub.2 O.sub.5. The optical glass herein disclosed has a high refractive index (nd) ranging from 1.85 to 1.97 and a low Abbe's number (.nu.d) ranging from 28 to 38. Also, the optical glass is stable against devitrification and excellent in chemical durability.

Abstract: A glass for eyeglass lenses has a refractive index between 1.68 and 1.71, Abbe number between 41.6 and 46.5 and specific gravity between 2.88 and 3.14 and consists essentially of B.sub.2 O.sub.3, CaO and at least one of TiO.sub.2 and Nb.sub.2 O.sub.5, the CaO being included in substantial amount.

Abstract: Optical glass contains 11 to 32% by weight of P.sub.2 O.sub.5, 34 to 60% by weight of PbO and 22 to 50% by weight of Nb.sub.2 O.sub.5.

Abstract: A high refractive, low dispersive optical glass composition based on a B.sub.2 O.sub.3 -La.sub.2 O.sub.3 -Y.sub.2 O.sub.3 -TiO.sub.2 quaternary system which is free of thorium oxide. The glass made of the composition has a refractive index of 1.79 to 1.93 and an Abbe number of 30 to 41.

Abstract: In thorium-free optical glass containing oxygen ions and fluorine ions as anions, cations in the glass calculated as oxides are in the following ranges of percentage by weight:______________________________________ B.sub.2 O.sub.3 10-44, La.sub.2 O.sub.3 30-66, Y.sub.2 O.sub.3 4-31, ZrO.sub.2 0-11, Ta.sub.2 O.sub.5 0-21, Gd.sub.2 O.sub.3 0-12, Nb.sub.2 O.sub.5 0-19, RO 0- 9, PbO 0-10, TiO.sub.2 0- 8, and GeO.sub.2 0-26, ______________________________________where RO is an ingredient or a combination of two or more ingredients of MgO, CaO, SrO, BaO and ZnO, and part of the oxygen ions is substituted for 0.2 to 10% by weight of fluorine ions relative to the glass. The thorium-free optical glass has a refractive index of 1.71 to 1.85 and Abbe number of 40 to 57.

Abstract: A composition for optical glass having a refractive index from 1.57 to 1.98 and Abbe number from 18 to 46 comprises, by weight, 1 to 25% B.sub.2 O.sub.3, 20 to 50% P.sub.2 O.sub.5 and 22 to 70% Nb.sub.2 O.sub.5.

Abstract: An optical glass having a refractive index (nd) in the range of 1.71 to 1.74 and Abbe number (v) in the range of from 49 to 54, the glass consisting essentially of, by weight, 1 to 5% SiO.sub.2, 30 to 41% B.sub.2 O.sub.3, 2 to 6% Al.sub.2 O.sub.3, 42 to 50% La.sub.2 O.sub.3, 3 to 7% ZrO.sub.2, and 1 to 12% Ta.sub.2 O.sub.5.