Originally Posted by jvincent
The two terms are often used interchangeably. For a digital display, they are equivalent.
They are often used interchangabley, and thus incorrectly. They are not the same except under a special (not normal) circumstance. Whether or not the HCFR authors know the difference I can't say. Surely they do.
The issue is generally about what the measuring hardware/software is expecting. If the software is expecting IRE, then one must know if the software means IRE with pedestal or not. No pedestal; IRE=%stim. But almost all software is expecting the pedestal so for example, 10 IRE is not = 10%stim As you get closer to 100%, then they get closer (100IRE=100%). But except at 100%, they are not equal.
This generally only affects gamma calculations. The software may say to display and measure a "10 IRE" pattern. It is expecting a certain luminance level for that reading. It will assume what you displayed is what it expected. If you are using a source pattern than is putting out %stim, but the software is truly expecting IRE, then the luminance level will be off, and thus the software will calculate the gamma incorrectly.
I don't use HCFR so I don't know if they built in the ability to switch what it is expecting (i.e. IRE, IRE with pedestal, % stim) or not.
In any case %stim is NOT equal to IRE for digitals. For example the 10 IRE pattern from Avia will not be the display the same luminance/brightness on screen as the 10% stim pattern from my DVD which has %stim level patterns. FOr digitals, the digital values for 10IRE are not equal to the digital values for 10% stim.
You can see a chart of the relationship between IRE and % stim, including digital values for each in the appendix of my manual avail at www.calibrate.tv