is believed that all direct light incidents a from the leds are reflected at the outer curved prism sheet 8 when the source width d is small compared to r as illustrated in fig1 from this it can be derived that as an example for a refractive index n of 1 50 pmma d r lt 0 168 that is if the led source has a width of 1 mm the diameter 2 r of the prism tube should be 12 mm or larger according to one example embodiment of the present invention the inner concave surface 24 is provided with a plurality of scattering areas 50 not shown typically the scattering areas 50 cover a surface fraction of 10 50 of the inner concave surface 24 however other surface fractions are conceivable as is evident for the skilled person the scattering areas 50 here are formed by a plurality of dots as an example the scattering areas 50 can be obtained by a paint pattern using a screen printing process the plurality of dots can e g be printed in a hexagonal arrangement and can have typical sizes from 0 1 mm in diameter up to 1 mm in diameter the function of the scattering areas 50 is to improve the efficiency of light extraction from the light emitting module i e the optical system in this manner the light incidents from the light unit leds escape via scattering at the side reflector regions and via scattering at the scattering areas 50 according to another example embodiment of the present invention the side reflector region 4 4 u2032 here consists of a specular reflecting material for instance each side wall 5 5 u2032 may include a specular reflecting material without being.