Throw distance definition

I am trying to figure out how to get the image size for a projector for a given throw distance. The problem I have is finding a definition of what throw distance is.

Some web sites state that it is the ratio of throw distance against the image diagonal

http://www.proxima.com/support/FAQthrowratio.asp
http://www.evertech.net/Main/Sales/Communications/Projection/ProjectionTerms.asp#throwratio
http://faculty.goucher.edu/leharris/lcdprojectors.htm#throwratio

Other web sites state that the throw ratio is the throw distance against the width of the image.

http://www.sanyo.com/business/presentation/techtips_15.html
http://www.audiogeneral.com/Products/Glossary.html

What is the correct answer?



Also, if you look at the specification for an InFocus LP530

Min Throw distance: 1.5m
Throw Ratio 1.2:1

The table of projection distance /image sizes states that a throw of 1.5m will give a maximum image diagonal of 1.1m and a minimum image diagonal of 0.9m.

Using the equation

Diagonal = distance /throw ration

Gives us a diagonal of 1.25.

How do they calculate their figures?

Thanks in advance

 

Have a look at projector central's projection calculator - that may help!

R