Flame-detecting apparatus including a field-limiting device

摘要

In a flame-detecting apparatus for developing an electrical signal responsive to light, impinging on a light-responsive surface from a predetermined flame area, for example, in a combustion chamber, a convex lens member is disposed at a distance from the surface longer than the focal length of the lens member such that light impinging substantially perpendicularly to the lens member is converged then slightly diverged onto the light-receiving surface. An aperture-defining member is positioned approximately at a focal point of the lens member between the lens member and the light-receiving surface for only giving a light passage around a light axis of the lens member, the cross-sectional area of the light passage being smaller than that of the light-receiving surface so that extraneous light impinging obliquely on the lens member is prevented from passing to the light-receiving surface.