An optical element including a first lens, a second lens, and an optical resin arranged between the lenses. The opposing surfaces of the first lens and the optical resin, and/or the opposing surfaces of the optical resin and the second lens are joined to each other with an adhesive resin having a specific elastic modulus. The first lens is a convex lens and the second lens is a concave lens. The thickness of the center of the optical resin is thicker than the thickness of the outer edge of the optical resin. The second lens has, on its outer periphery, a step structure thinner than the maximum thickness of the second lens. The thickness of the step structure and the thickness of the center of the optical resin satisfy a specific relationship.

A propagation optical system includes: a first optical system; an intermediate optical element; and a second optical system. The first optical system, the intermediate optical element, and the second optical system being sequentially arranged in a direction from the image display element toward the light guide member along an optical axis. The intermediate optical element has a non-rotationally symmetric curved surface, of which shape is non-rotationally symmetric about the optical axis. A cross-sectional shape, of the non-rotationally symmetric curved surface, in a first plane including the optical axis is non-arc shape. The first plane is a plane in which the non-rotationally symmetric surface has a strongest positive power among planes including the optical axis. The propagation optical system is configured to form, between the first optical system and the second optical system, an intermediate image corresponding to an image displayed on the image display element.

A propagation optical system includes: a first optical system; an intermediate optical element; and a second optical system. The first optical system, the intermediate optical element, and the second optical system being sequentially arranged in a direction from the image display element toward the light guide member along an optical axis. The intermediate optical element has a non-rotationally symmetric curved surface, of which shape is non-rotationally symmetric about the optical axis. A cross-sectional shape, of the non-rotationally symmetric curved surface, in a first plane including the optical axis is non-arc shape. The first plane is a plane in which the non-rotationally symmetric surface has a strongest positive power among planes including the optical axis. The propagation optical system is configured to form, between the first optical system and the second optical system, an intermediate image corresponding to an image displayed on the image display element.

Spectrometer systems are provided including a detector array; an imaging lens assembly coupled to the detector array, the imaging lens assembly including a first element of positive optical power followed by a second element of negative optical power and a positive optical power element split into two opposing identical singlets; a dispersive element coupled to the imaging lens assembly; and a fixed focus collimator assembly coupled to the dispersive element. Related imaging lens assemblies and collimator assemblies are also provided.

Spectrometer systems are provided including a detector array; an imaging lens assembly coupled to the detector array, the imaging lens assembly including a first element of positive optical power followed by a second element of negative optical power and a positive optical power element split into two opposing identical singlets; a dispersive element coupled to the imaging lens assembly; and a fixed focus collimator assembly coupled to the dispersive element. Related imaging lens assemblies and collimator assemblies are also provided.

An optical system includes, in order from an object side to an image side, a front unit having a positive refractive power and including one or more lens units configured to move during focusing, and a negative lens unit having a negative refractive power. The one or more lens units included in the front unit moves to the object side during focusing from infinity to a short distance so as to widen a distance between the front unit and the negative lens unit. The front unit includes a first subunit having a negative refractive power and including a lens disposed on the object side of a first positive lens that is one of positive lenses included in the optical system, which is the closest to an object. A predetermined condition is satisfied.

An optical system includes, in order from an object side to an image side, a front unit having a positive refractive power and including one or more lens units configured to move during focusing, and a negative lens unit having a negative refractive power. The one or more lens units included in the front unit moves to the object side during focusing from infinity to a short distance so as to widen a distance between the front unit and the negative lens unit. The front unit includes a first subunit having a negative refractive power and including a lens disposed on the object side of a first positive lens that is one of positive lenses included in the optical system, which is the closest to an object. A predetermined condition is satisfied.

An imaging lens includes first and second lens groups at object and image sides, respectively, and an aperture between the first and second lens groups. The first lens group includes first and second lenses from the object side to the image side. The second lens group includes third, fourth, fifth, and sixth lenses from the object side to the image side. The fifth lens includes a concave object-side surface and a convex image-side surface, each of which has at least one inflection point. The sixth lens includes a convex object-side surface and a concave image-side surface, each of which has at least one inflection point. The imaging lens satisfies 0.56<f/TTL<0.67 and f/EPD≤2.0. f is an effective focal length of the imaging lens, TTL is a distance from an object-side surface of the first lens to an image plane, and EPD is an entrance pupil diameter of the imaging lens.

An imaging lens includes first and second lens groups at object and image sides, respectively, and an aperture between the first and second lens groups. The first lens group includes first and second lenses from the object side to the image side. The second lens group includes third, fourth, fifth, and sixth lenses from the object side to the image side. The fifth lens includes a concave object-side surface and a convex image-side surface, each of which has at least one inflection point. The sixth lens includes a convex object-side surface and a concave image-side surface, each of which has at least one inflection point. The imaging lens satisfies 0.56<f/TTL<0.67 and f/EPD≤2.0. f is an effective focal length of the imaging lens, TTL is a distance from an object-side surface of the first lens to an image plane, and EPD is an entrance pupil diameter of the imaging lens.

An observation optical system is configured to observe an image displayed on an image display plane and includes a negative lens and a positive lens. At least one of the negative lens and the positive lens has a lens surface having a Fresnel shape. A predetermined condition is satisfied.