9″ FPCRT Red vs Clear C-Element Comparison
The other day I felt like comparing a red verses clear C-Element in a G90 projector. What got me interested in this experiment was the fact that the red primary on CRT front projectors using red c-elements is over saturated. This results in red push in the primary and I wanted to fix this.
All the readings I have ever taken show that the red tube, with a red c-element, is too low on blue. On the other hand, a red tube with a red c-element does have the correct amount of green for Rec709. I was hoping that removing the red c-element and replacing it with a clear c-element would make the primary closer to REC709 by removing red and essentially pushing the primary towards blue while holding the green correct.
As it turns out, swapping a red element for a clear one does not affect the amount of blue at all. Unfortunately the clear c-element only pushes the primary towards green which is the opposite of what I wanted. Because of this, a clear c-element on a red tube makes the primary worse than when using a red c-element (this was really why red c-elements were implemented in the first place).
This testing has been done before, but I just wanted to confirm it for myself by using the same projector and trying both c-elements. I used my PR-650 pointed into the lens to take the measurements on the red primary. Below are the charts showing the data for both red and clear elements using CIE1931 charts and CIE1976 charts. This confirms that a red c-element should be used for the closest matching of red to REC709.