by a factor up to 2 0 the mirror 47 has been axially displaced to an exaggerated extent for the sake of clarity in fig5 alternatively a hemispherical shell not shown may be used at the end of each tube furthest from the coupling means 46 each hemispherical shell is of the same initial cross section as the tube it is connected to and may be separate from or integral with the end of the tube spaced apart from and beneath the collector 30 is a specular or diffuse reflector 48 such as aluminium foil a metalised plastic sheet or a white painted surface fig5 and 6 show the thickness of the tube walls and the gaps between the tube walls in exaggerated proportion for the sake of clarity typical dimensions might be an external diameter of the outer tube 40 of 40 mm a total wall thickness includes all tubes plus the spaces between them of 2 mm a channelling means 46 of 18 mm diameter and a tube length of 1 meter to 2 meters the total cross sectional area of the light transmitting part of the tubes 40 41 and 42 plus the spaces between them is approximately the same as the cross sectional area of the channelling means 46 the channelling means 46 is in this embodiment a flexible optical cable having a uniform cross section.