Editing Overhead projector (section)

===Condenser===
Because the focusing lens (typically less than {{convert|10|cm|disp=sqbr|abbr=on}} in diameter) is much smaller than the transparency, a crucial role is played by the optical [[Condenser (optics)|condenser]] which illuminates the transparency.<ref name="Young">{{cite book |url=https://books.google.com/books?id=l8AZCQAAQBAJ&pg=PA36 |pages=36–37 |title=Optics and Lasers: Including Fibers and Optical Waveguides |last=Young |first=Matt |edition=4th |publisher=Springer Science & Business Media |year=2013 |isbn=978-3-662-02699-1}}</ref> Since this requires a large optical lens (at least the size of the transparency) but may be of poor optical quality (since the sharpness of the image does not depend on it), a [[Fresnel lens]] is employed. The Fresnel lens is located at (or is part of) the glass plate on which the transparency is placed, and serves to redirect most of the light hitting it into a converging cone toward the focusing lens.<ref name="Gayeski">{{cite book |url=https://books.google.com/books?id=_3jsj-DuBLEC&pg=PA86 |title=Designing Communication and Learning Environments |editor-last=Gayeski |editor-first=Diane |publisher=Educational Technology |year=1995 |page=86 |isbn=0-87778-285-7}}</ref> Without such a [[Condenser (optics)|condenser]] at that point, most of the light would miss the focusing lens (or else the focusing lens would have to be very large and prohibitively expensive). Additionally, mirrors or other condensing elements below the Fresnel lens serve to increase the portion of the light bulb's output which reaches the Fresnel lens in the first place. In order to provide sufficient light on the screen, a high intensity bulb is used which often requires [[Forced convection|fan cooling]].