An illuminator for a viewing unit of an optical inspection machine for the quality control of parts, in particular gaskets, comprises a diffusion chamber, a diffuse illumination source, a low-angle illumination source, a direct illumination source placed in the diffusion chamber, and an annular reflective element placed in the diffusion chamber. The annular reflective element is integral to the direct illumination source so that together they form a chamber closure assembly delimiting the upper end of the diffusion chamber. The chamber closure assembly can move axially by translational movement in the diffusion chamber both toward and away from the diffuse illumination source.