Patent ID: 6879395

Claim:
A spectrum measuring instrument including a dispersing element and a detector for detecting spectrum light coming out from said dispersing element, wherein: said detector has a two-dimensional detecting surface having a first portion and a second portion; and the spectrum light coming out from said dispersing element is irradiated obliquely to said first portion on said detecting surface; and further including: a signal processing unit responsive to the output of the detector for obtaining a spectrum signal excluding adverse effects of stray light in which signal intensity at the second portion on said detecting surface is subtracted from signal intensity on said first portion; a light transmitting window covering said detecting surface, and having a thickness and an inner surface; and said oblique irradiation of the spectrum light by the dispersing element being at an angle with respect to the normal of the detecting surface in a plane defined by the normal and a direction intersecting at right angles with a direction along which the one-dimensional spectrum extends so that spectrum light reflected by the detecting surface is reflected by the inner surface of the light transmitting window back to the detecting surface.