replacing a condenser lens?

[invisiview]

I have a cracked condenser lens in an ask impression 970 projector that I want to replace. The diameter is 78.6 mm and the thickness of the lens at the center is 12 mm. How do I calculate the lens focal length to find a replacement. Any idea where I can find replacements?

 

[dave123]

measure the mm roughly from the lens to the first fresnel but even a little less will work.