What is the throw ratio on projection TVs?

Answer 1

Most modern video projectors have a variable and completely adjustable throw ratio.

Answer 2

Simplistically throw ratio is the distance from lamp (or front of lens) divided by the width of the picture on the viewing surface (glass screen of a RPTV or screen for a front projector. Some manufacturers use the diagonal size of the projected image, but this is a function of the aspect ratio, so width is preferable.

I see little practical use to knowing the throw ratio of a RPTV, since it is set by the manufacturer. That said, a practical (although inaccurate) estimate of throw ratio would be the width of the viewing surface divided by the depth of the TV.

Throw ratio is important for front projectors since it determines the distance the projector must be placed from the screen. Note that for a zoom lens throw ratio varies and can be calculated at each extreme of the lens.

For example, for the Canon SX-60, with a 1.6:1 zoom lens the calculator available at the link below shows that for a 100" width screen, the projector can be placed as close as 12.2' (146") or as far as 20.2' (242") from the screen. The throw ratio therefore varies from 1.46 to 2.42 (based on width. The number is different if the diagonal is used, and varies further for a 4:3 or 16:9 or 2.35:1 aspect ratio screen.

Practically speaking you need to know the throw range for a projector (for a defined screen size) ... in the above case 12.2' to 20.2'for a 100" wide screen ... or the range of screen sizes that can be accommodated with a projector set a certain distance from the screen.

A further important factor is the "offset", that is where does the lense have to be located vertically relative to the screen. Few projectors project an image so the top and bottom of the image are equidistant above and below the centre of the lense. Typically, those with a fixed offset project the image upwards so that a projector sitting on a table will illuminate a screen with the top well above the level of the table. Some have a "lens shift" feature that allows the image to be moved up or down relative to the projector. The latter allow more flexibility in mounting.