but with a different geometry in fig2 the input face 12 is vertical 0 u00b0 and is positively curved the output face 14 is flat and is tilted to illuminate at the desired distance between u2212 20 u00b0 and u2212 32 u00b0 with u2212 22 u00b0 being shown in the example here again the light is deviated to the desired location without being subject to a reflection in general terms the desired result may be achieved with a prism in which the input face has an angle from u2212 10 u00b0 to 60 u00b0 and the output face has an angle from u2212 60 u00b0 to 10 u00b0 with one of the two being substantially vertical e g within u00b1 5 u00b0 fig1 and 2 show the prism assembly as rotationally symmetrical however a non symmetrical form may be preferred depending on the area to be illuminated turning now to fig3 one specific form of an optical mouse is illustrated using a prism assembly as in fig1 the mouse has a base 20 including ribs 22 upon which it can be slid across a surface such as a mouse pad 24 the base 20 defines an aperture 26 an led 28 is mounted horizontally i e parallel to the mouse pad 24 to provide light 32 via a prism 30 to impinge on the mouse pad 24 via the aperture 26 a lens 34 focuses an image of the illuminated surface via an optical aperture 36 onto the image surface 38 of an image sensor package 40 the led 28 is mounted on an extension 42 of the prism 30 to collect the maximum amount of light the led 28 is placed close to.