[willis936]

P22 phosphors are the ones used in all trinitron PC monitors. I get conflicting numbers on the actual decay time of these phosphors, but with a 240 fps camera the decay time is difficult to judge by flipping through frames. I’d have to make a proper set up to try and even give an estimate.

I can safely say the P22 decay time is below 1 ms. Here [0] it is stated to be 100 us for G & B and up to 1 ms for red. A commonly referenced list of phosphor decay times [1] lists it as “medium”. In general, CRT phosphors seem to have sub 1-ms decay times unless you want a long persistence.

The limiting factors will be in a monitor’s max horizontal scan rate (typically 130 kHz or lower) or the DAC’s maximum pixel clock (it used to be 400 MHz but most VGA adapters these days struggle to keep 200+ MHz stable).

0. https://www.epanorama.net/documents/video/phosphor_decay.htm...

1. http://www.bunkerofdoom.com/tubes/crt/crt_phosphor_research....

[ChuckMcM]

Excellent data! Now you need to add the time to saturation (TTS) which gives you how long the beam has to sit on the phosphor to achieve the maximum amount of light. Different tubes do this differently from moderating the energy in the beam itself to modulating "time on spot".

What you end up with at the end of all this research is a really nice ideas of the "hard bits" around engineering a CRT for a particular application. When we looked at CRTs in depth in the EE program one of the things that came across was how so many of the things you had to vary were interconnected. The professor suggested that was why there were so many different models to choose from, even when they were all the same form factor.

Thanks for the links too, I've added the bunkerofdoom one to my collection.

[ChuckMcM]

And to be clear, the time it takes for a phosphor dot to go from "off" to "full bright" to "off" again is its frequency response. And every dot of phosphor has to be "visited" by the electron beam on every frame. With those two numbers you can get the absolute fastest you can scan through the dots with full dynamic range.

In the CRT designs we looked at in college they typically used a fixed "dwell time" of the beam, so it was on each pixel for a fixed amount of time, and modulated drive power to get different intensities. (some phosphors are non-linear in their response so you can correct that in the drive table).

And as I was reminded in email, the beam has to visit every pixel AND get back to the top of the screen for the next frame. So on a 480i screen that is 640 x 240 per frame at 30Hz, that is 30 frames per second / 640 * 240 pixes per frame so you have just 195 uS to spend on each pixel. (less than that actually since with overscan you have 720 pixels per line but it is a very short time).