Projection device for use in copying machine

Claim 1

In a photocopying machine capable of copying an original document of a type selected from either one of a thin original document and a thick original document depending on necessity, in which the original document is moved past an exposure station by transport means during an electrophotographic copying process when the original document is a thin one, and said thick original document is carried on a transparent carrier member moving past said exposure station during an electrophotographic copying process when the original document is a thick one, said thick original document being spaced from said exposure station by a distance equal to the thickness of said transparent carrier member, a projection device comprising; a light source including reflector means for emitting light rays along a first optical path and for illuminating an original document moving past said exposure station, said light rays reflected from the original document being transmitted along a second optical path towards a projection station where a latent image of the original document is formed; and a transparent plate member having a predetermined thickness and movable between a first position where the transparent plate member intrudes into said first optical path when said thin original is placed over said exposure station and a second position where the transparent plate member intrudes into said second optical path when said thick original is placed over said exposure station on said transparent carrier member a predetermined amount of light rays being absorbed and refected by said transparent plate member when said transparent plate member is positioned in said first position so as to approximately equalize the intensity of the light projected onto said projection station which is absorbed and reflected by said transparent plate member and transparent carrier member when said transparent plate member is positioned in said second position, the difference in the lengths of the optical path from the exposure station to the projection station caused by said thickness of the transparent carrier member being compensated by the refraction index of said transparent plate member, whereby the projected image of the thick original document on said projection station has brightness and sharpness substantially equal to that of the thin original document.