Optical detector

An optical detector comprising:

• a light emitting unit that emits a laser light having temperature dependence on a peak wavelength to shift to a longer wavelength side as a temperature rises;
• and
• a light receiving unit that receives a reflected light which is the laser light reflected by a measurement object, wherein the light receiving unit has
• a detection element that detects the reflected light, and
• a condenser lens system that collects the reflected light to the detection element, the condenser lens system having a plurality of lenses made of optical material and having a positive optical power as a whole, the condenser lens system having
• a temperature change factor that increases the optical power as a whole of the condenser lens system at a high temperature than at a low temperature lower than the high temperature, in the optical material with respect to the reflected light, and
• a chromatic aberration factor that decreases the optical power as a whole of the condenser lens system at a long wavelength than at a short wavelength shorter than the long wavelength, and
• 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 wavelength range where a shift of the peak wavelength is assumed,
• wherein
• the condenser lens system has
• a first lens group including at least one positive lens which is a part of the plurality of lenses, and
• a second lens group including at least one negative lens which is a part of the plurality of lenses excluding the lens belonging to the first lens group,
• the optical material forming the negative lens has a dispersion characteristic that is higher in dispersion than the optical material forming the positive lens, and
• the optical material forming the negative lens has temperature dependence in a refractive index, in which differentiating value of the refractive index with respect to temperature is higher in the optical material forming the negative lens than in the optical material forming the positive lens.