A Winston cone is a non-imaging light collector in the shape of an off-axis parabola of revolution[1][2] with a reflective inner surface.
[3] The collection of incoming rays is maximized by allowing off-axis rays to make multiple reflections before reaching the exit aperture.
Winston cones are used to concentrate light from a large area onto a smaller photodetector or photomultiplier.
They are widely used for measurements in the far infrared portion of the electromagnetic spectrum in part because there are no suitable materials to form lenses in the range.
It is commercialized by companies such as Winston Cone Optics[5]