Look on the bottom of the iMac stand for this information.

UPDATE

To find out the LCD model (and manufacturer) of the display in your Mac , open a terminal window and paste the following code followed by enter:

ioreg -lw0 | grep IODisplayEDID | sed "/[^<]*</s///" | xxd -p -r | strings -6

This gives you e.g. the following information:

B133EW01 V0

Color LCD

The first line is the LCD model number, the second is the active color profile.

If you google the LCD Model number, you can find out more about your screen (contrast ratio, brightness).

For my screen (build into a 1st generation Macbook build in 8/2006) I get the following information:

Info about B133EW01 Screen Model