This is not enterely true i think as i specify a window of 1024x768 and yet specify 640x480 in glortho a pixel would not be equals to an opengl unit in that case.

on the delphi forum i got the answer that 1pt = 1/72 inch. Together with some more researche i made up the following:

e.g if i specyify 640x480 in glortho mode and my real screen size is 1024x768. How do i get the unit size? Posible Solution: divide real screensize with glortho screensize? Does that give pixels per opengl unit?

but a third size comes into play EM . e.g. a font is stored as units per em and 1 em being the current font size ?
a ttf font is supposed to have 2048 units per em. http://www.w3.org/TR/SVG/fonts.html

Using google i found the following table:
http://sureshjain.wordpress.com/2007/07/06/53/
stating that:
12pt = 16px = 1em = 100%
and 12/72 gives about 0,16666 so that's 16px.

Now how do get from the 2048 unit per em to a font that is drawn at the correct size.

768/480 give 1,6 so an opengl unit would be 1,6 pixel?
assuming i want to draw a font at 12pt i would need to draw something at 16px heigh.
16 / 1,6 states to me i should draw something at 10 opengl units heigh.
i now assume that 10 opengl units is 1em and thus equals the 2048 units per em. So 2048 = 10.
Now i can calculate from a font point to opengl point.

I should try to write some code for this. To see if my theory works in practice.

Let me know if the above makes sense or not. Thanks for your answers in advance.