An optical detector includes a light emitting unit and a light receiving unit that receives a reflected light reflected by a measurement object. The light receiving unit has a detection element and a condenser lens system that collects the reflected light to the detection element. The condenser lens system has a plurality of lenses. The condenser lens system has a temperature change factor that increases the optical power at a high temperature than at a low temperature, and a chromatic aberration factor that decreases the optical power at a long wavelength than at a short wavelength. The optical power of each of the plurality of lenses is adjusted based on a correspondence between a change in temperature and a shift amount of the peak wavelength, so that the chromatic aberration factor balances with the temperature change factor within a predetermined wavelength range.

An optical detector includes a light emitting unit and a light receiving unit that receives a reflected light reflected by a measurement object. The light receiving unit has a detection element and a condenser lens system that collects the reflected light to the detection element. The condenser lens system has a plurality of lenses. The condenser lens system has a temperature change factor that increases the optical power at a high temperature than at a low temperature, and a chromatic aberration factor that decreases the optical power at a long wavelength than at a short wavelength. The optical power of each of the plurality of lenses is adjusted based on a correspondence between a change in temperature and a shift amount of the peak wavelength, so that the chromatic aberration factor balances with the temperature change factor within a predetermined wavelength range.

The imaging lens consists of, in order from the object side, a first positive lens group, a stop, a second positive lens group, and a third negative lens group. The first lens group includes a negative lens which has a concave surface on the image side, and two positive lenses which are disposed to be closer to the image side than the negative lens and which have respective convex surfaces on the object side, convex surfaces on the object side. The second lens group includes an aspheric lens, and two sets of cemented lenses disposed to be closer to the image side than the aspheric lens. The third lens group consists of one lens component.

An imaging optical system includes three lens elements. The three lens elements, in order from an object side to an image side, are a first lens element having an object-side surface facing the object side and an image-side surface facing the image side, a second lens element having an object-side surface facing the object side and an image-side surface facing the image side, and a third lens element having an object-side surface facing the object side and an image-side surface facing the image side. The third lens element has positive refractive power, the imaging optical system has a total of three lens elements, and there are air gaps between paraxial regions of the first lens element, the second lens element and the third lens element.

An imaging optical system includes three lens elements. The three lens elements, in order from an object side to an image side, are a first lens element having an object-side surface facing the object side and an image-side surface facing the image side, a second lens element having an object-side surface facing the object side and an image-side surface facing the image side, and a third lens element having an object-side surface facing the object side and an image-side surface facing the image side. The third lens element has positive refractive power, the imaging optical system has a total of three lens elements, and there are air gaps between paraxial regions of the first lens element, the second lens element and the third lens element.

An endoscope objective optical system consists of in order from an object side, a front group having a negative refractive power, an aperture stop, and a rear group having a positive refractive power, wherein either the front group or the rear group includes one or more than one cemented lens, and the cemented lens includes a lens having a positive refractive power and a lens having a negative refractive power, and the rear group includes a positive lens which is a single lens, on the object side, and the following conditional expressions (1), (2), (3), and (4) are satisfied.
1.1<Ih/ft<1.8(1)
ff/ft<0.9(2)
45<d1(3)
LOs/Ih<1.5(4) where, Ih denotes the maximum image height, ft denotes a focal length of an overall endoscope objective optical system, ff denotes a focal length of the front group, d1 denotes Abbe's number for a glass material of the lens having a positive refractive power, on the object side of the rear group, and LOs denotes a distance from a first surface on the object side of the endoscope objective optical system up to the aperture stop, and here the first surface on the object side of the endoscope objective optical system is a lens surface positioned nearest to object in the endoscope objective optical system.

An endoscope objective optical system consists of in order from an object side, a front group having a negative refractive power, an aperture stop, and a rear group having a positive refractive power, wherein either the front group or the rear group includes one or more than one cemented lens, and the cemented lens includes a lens having a positive refractive power and a lens having a negative refractive power, and the rear group includes a positive lens which is a single lens, on the object side, and the following conditional expressions (1), (2), (3), and (4) are satisfied.
1.1<Ih/ft<1.8(1)
ff/ft<0.9(2)
45<d1(3)
LOs/Ih<1.5(4) where, Ih denotes the maximum image height, ft denotes a focal length of an overall endoscope objective optical system, ff denotes a focal length of the front group, d1 denotes Abbe's number for a glass material of the lens having a positive refractive power, on the object side of the rear group, and LOs denotes a distance from a first surface on the object side of the endoscope objective optical system up to the aperture stop, and here the first surface on the object side of the endoscope objective optical system is a lens surface positioned nearest to object in the endoscope objective optical system.

The imaging lens consists of, in order from the object side, a first positive lens group, a stop, a second positive lens group, and a third negative lens group. The first lens group includes a negative lens which has a concave surface on the image side, and two positive lenses which are disposed to be closer to the image side than the negative lens and which have respective convex surfaces on the object side, convex surfaces on the object side. The second lens group includes an aspheric lens, and two sets of cemented lenses disposed to be closer to the image side than the aspheric lens. The third lens group consists of one lens component.

A lens system is provided. The lens system includes a positive first lens group, an aperture diaphragm, a positive second lens group, and a third lens group, disposed sequentially from a subject side. The first lens group includes at least three positive lenses and at least one negative lens. The second lens group includes four or more lenses including at least one cemented lens. A negative lens with a concave surface toward the subject side is disposed at the subject side. The third lens group includes three or more lenses including at least one positive lens and at least one negative lens. When focusing is moving from an infinity-distance subject to a short-distance subject, the first lens group, the aperture diaphragm, and the second lens group move integrally toward the subject side, while the third lens group is fixed with respect to an image surface.

An objective includes a first lens group that has a positive refractive power and a second lens group that has a negative refractive power. The objective includes a first negative lens and satisfies
0.005d.sub.o1/h.sub.o10.1(1)
0.005h.sub.min/h.sub.l0.72(2)
10 mmL100 mm(3)
0.1L/f31(4)
when d.sub.o1 is a thickness of the first negative lens, h.sub.o1 is an axial marginal ray height on a lens surface on the object side of the first negative lens, h.sub.min is a minimum value of an axial marginal ray height in the second lens group, h.sub.l is a height of an axial marginal ray on a lens surface of the objective that is situated closest to the image side, L is a length of the objective, and f is a focal length of the objective.