GB1299647A - Projection system - Google Patents

Abstract

1299647 Projection system BRUNSWICK CORP 21 May 1970 24750/70 Heading G2J A projection system, for example for displaying bowling scores, comprises a light source 13, a prism 11, means for directing light into the prism and means including a relay lens whose principal axis is inclined to the optical axis for projecting an image therefrom on to a viewing screen 25. To one surface 11a of the prism is secured a plate 12, which may carry on its surface 12a a permanent image of a scoresheet. There is provided means for impressing on this surface the score at a given time, so that light directed into the prism by paraboloidal mirror 14 and condenser lens 15 is reflected by the surface 12a so as to carry the image of the score but not of the impressing means. The plate 12 also serves to protect the prism against damage by such means. It is known that for the prism 11 to produce full depth of field correction with the image i' normal to the exiting rays, the angle B' is related to the angle A' by where X is the refractive index, but that this causes astigmatism in the final image on the screen 25. The angle B' of the prism 11 is therefore made substantially less than the value given by this formula, so that the image is at i, and the projection means is arranged to complete the depth of field correction without introducing astigmatism. Light from the prism is reflected by plane mirrors 16 and 17 and then the relay lens 18, whose principal axis is inclined to the optical axis of the system at this point by an angle equal to half the angle between the image i and the rays leaving the prism, renders the image normal to the system axis. The rays then travel through aperture 27 in lower housing 26, through a field lens 21 in aperture 29 in upper housing 28, and after reflection by mirrors 22 and 23 the image is projected on to the screen 25 by lens 24. The measurements of the system may be chosen so that condenser lens 15 focuses the filament of bulb 13 at the centre of lens 18, and field lens 21 focuses it at the centre of projection lens 24 so that an image of the filament does not appear on the screen.