Distribution of light produced by a light source can be important or even critical in some applications. The light source can be, for example but not necessarily, a light emitting diode “LED”, a filament lamp, or a gas-discharge lamp. FIG. 1 shows a view of a section taken from an exemplifying light guide 101 according to the prior art for modifying a light distribution pattern of a light source 102. Some of the light beams radiated by the light source 102 are depicted with dashed line arrows in FIG. 1. The light guide 101 can be rotationally symmetric with respect to a geometric line 120 shown in FIG. 1. The geometric line 120 is parallel with the z-axis of a coordinate system 199. The light guide 101 is made of transparent material having refractive index greater than unity. The light guide comprises a first surface 103 for acting as a light-ingress surface for receiving light emitted by the light source 102, and a second surface 104 for acting as a light-egress surface where the light exits the light guide. Both the first and second surfaces of the light guide comprise prism ridges for collimating the light emitted by the light source 102. In FIG. 1, one of the prism ridges of the first surface 103 is denoted with the reference number 111 and one of the prism ridges of the second surface 104 is denoted with the reference number 112. In a case where the light guide is rotationally symmetric, the prism ridges are advantageously formed concentrically around the geometric line 120. A light guide of the kind shown in FIG. 1 is presented for example in publication US20110286221.
An inconvenience related to light guides of the kind described above is that it may be quite difficult to see whether the light source 102 is on or off when the light guide 101 is seen so that the viewing angle α shown in FIG. 1 is small. For example, in conjunction with kitchen and other domestic lightning applications it would be advantageous if one could see whether the light is on or off, also when the viewing angle is small.