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How does a screen work?

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572 points chkhd | 3 comments | 13 Jul 25 14:09 UTC | HN request time: 0.764s | source
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retrac ◴[13 Jul 25 16:39 UTC] No.44551618[source]▶
CRTs are still slightly magical to me. The image doesn't really exist. It's an illusion. If your eyes operated at electronic speeds, you would see a single incredibly bright dot-point drawing the raster pattern over and over. This YouTube video by "The Slow Mo Guys" shows this in action: https://youtu.be/3BJU2drrtCM?t=190
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hinterlands ◴[13 Jul 25 16:59 UTC] No.44551773[source]▶
That slo-mo video is somewhat misleading, though. The phosphor glows for a good while, so there is a reasonable chunk of the image that's visible at any given time.

The problem in that video is that the exact location the beam is hitting is momentarily very bright, so they calibrated the exposure to that and everything else looks really dark.

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layer8 ◴[13 Jul 25 18:31 UTC] No.44552411[source]▶
The phosphor still drops off very quickly [0][1][2], roughly within a millisecond. That’s why you would need a 1000 Hz LCD/OLED screen with really high brightness (and strobing logic) to approximate CRT motion clarity. On a traditional NTSC/PAL CRT, 1 ms is just under 16 lines, but the latest line is already much brighter than the rest. The slow-motion recording showing roughly one line at a time therefore seems accurate.

[0] https://blurbusters.com/wp-content/uploads/2018/01/crt-phosp...

[1] https://www.researchgate.net/figure/Phosphor-persistence-of-...

[2] https://www.researchgate.net/figure/Stimulus-succession-on-C...

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bgnn ◴[13 Jul 25 20:00 UTC] No.44553180[source]▶
I'm not sure about this calculation though. Phosphor decays exponentially with a time constant of roughly 5ms (according to HP [1]). This means when a new frame comes at 60Hz refresh rate there is still 10-15% of the previous frame related excitation is present. This means there is considerable amount of nonlinearity, hence the performance is even worse than 10ms LCD/OLED displays.

Genuine question: why do you think CRTs are better?

[1] https://hpmemoryproject.org/an/pdf/an_115.pdf

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1. layer8 ◴[13 Jul 25 20:42 UTC] No.44553546[source]▶
That HP reference is from 1970; CRTs did improve over time. The references I gave show that the intensity drops to below 10-15% within about a millisecond. The difference with LCD/OLED displays is that the latter are sample-and-hold, meaning that they show the image at full brightness for the duration of the whole frame. Their pixel response time may be faster than CRT phosphor persistence, but that is less relevant. The problem with LCD/OLED is that they hold the picture for the duration of the frame, which means that a depicted moving object that is supposed to move smoothly during the duration of a frame, is shown as not moving for that duration, which the eye perceives as motion blur. That motion blur is significantly reduced on CRTs, because they show the object only for a fraction of the frame duration at high brightness, as if under a stroboscope, which makes it easier for the eye (or brain) to interpolate the intervening positions of the object.

> Genuine question: why do you think CRTs are better?

CRTs are worse in most aspects than modern displays, but they are better in motion clarity. As to why I think that: I used both in parallel for many years. The experience for moving objects is very different. It is a well-known drawback of sample-and-hold display technologies. And it is supported by the more systematic analyses done by the likes of Blur Busters.

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2. charcircuit ◴[14 Jul 25 05:17 UTC] No.44556712[source]▶
>The problem with LCD/OLED is that they hold the picture for the duration of the frame

Not necessarily. For example on VR headsets the LCD/OLED will only hold the picture for 10% of the frame.

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3. cubefox ◴[14 Jul 25 10:23 UTC] No.44558347[source]▶
Yeah, they do backlight strobing (LCD) or black frame insertion (OLED), to reduce blurring during smooth eye movements, at the cost of overall screen brightness. I actually think small CRTs would be perfect for VR headsets in this regard, as they are naturally have very short frame persistence.

One likely problem for battery powered headsets is the (I believe) relatively high CRT power draw. Another is probably the fact that they aren't used for anything else anymore, meaning CRT development has stopped a long time ago. There were quite small CRTs in the past for special applications, but probably not as small as is optimal for modern VR headsets. Both for optics and weight and space reasons.