Greetings Everyone, let me share my findings here.

TLDR: Found that Benq monitor with supposed DC dimming is actually pulsating with 47 Hz bright flashes which are not detectable on Iphone slo-mo.

Let me preface that I started having issues with strain caused by monitors not so long ago - the first monitor that caused it with 100 percent is HP z24i g2
I have myopia (-6 dioptries) and little astigmatism. The list of issues and usable electronics might depend on your specific eye disorders, so read them with caution.

The symptoms it caused -

  • blood vessels popping, in terms of direction there was a visible path from the center of the iris to the nose but it also was everywhere on the eye, just generally eyes were horribly red.
  • feeling of tension in the eye muscles.
  • feeling of soreness, dry eyes, like I had sand in the eyes.
  • Just general eye fatigue which was preventing me from working

Symptoms stayed for 2 to 3 days until I recuperated by limiting eye fatigue and moving to the countryside.
My list of useable electronics from the easiest to use to hardest
Iphone XR/11 & CRT monitor with 85 Hz > MBP 16 2019 > Steamdeck IPS > ps vita > New Nintendo 3ds XL > Dell P2421D > LG C2 (on the lowest brightness setting) > Benq BL2420PT > HP z24i G2(the absolute worst)

Now, I started noticing the same issues and same feeling in the eyes right when I got this HP. It was a trigger point of sorts, from which I started researching this question. I have tried to find myself "good" monitor after that, and spent quite a lot of time and effort digging the internet until I found good things regarding Benq BL2420PT. Considering that out of ALL the options it was the only monitor that I could buy as new I stopped my search on it. After getting it I noticed some strain, but it appeared better than other screens so I tried to push forward with it - it also was relatively gentle in the sense that it did not cause immediate discomfort. The effect of its work is cumulative as a lot of folks on this forum know.

Fast forward few months of usage and I decided to use it for prolonged periods of time few days in a row and got a horrible symptoms compared to those that HP monitor gave me. It caused me to investigate this issue further, until I found a definitive answer for the cause, and I want to laid it out for everyone to see.

When I bought the monitor I attempted to weed out a good model using multiple tests that would eliminate possible causes for eyestrain. You can find test methodology here:
https://ledstrain.org/d/2503-personal-compilation-of-information-from-this-forum-and-some-others/3

This Benq BL2420PT passed most of these things, which lulled into false sense of security about it, especially considering that folks around internet were positive about this specific model.

My initial search for the causes led me to check the panel used in the monitor. According to this website
https://www.displayspecifications.com/en/model/140a738

this was the panel that was supposed to be used (and which was praised by the people)
AOU M238DAN01.3
https://www.panelook.com/M238DAN01.3_AUO_23.8_LCM_overview_24260.html

And I found that my monitor used other panel instead going to the service menu:
BENQ-BL2420PT-that-i-bought

LG LM238WQ1-SLA1
https://www.panelook.com/LM238WQ1-SLA1_LG%20Display_23.8_LCM_overview_27444.html

So I already established that the panel that monitor used is different from what it should have been.

Now to the juicy part. Since I started to reinvestigate the causes I bought myself an OPPLE Light Master Pro/ III tester. Using it inside a black box with the screen shining at it I was able to measure a lot of screens used daily, as well as light bulbs (but thats another question). Long story short, here are the results:

As you can see I measured strong brightness pulses roughly every 22 milliseconds. If you divide 1000 / 21,27 = 47,01 Pulses per second = 47 Hz.
These pulses supposedly have stroboscopic effect on humans which are harmful as per multiple research papers. This bright pulse persists on all the modes, and all the levels of brightness, but the dip is less severe on a higher brightness.
Also, my assumption is that a bright environment lowers the effect of these pulses compared to dark room, therefore people recommend to use screens at well lit environment.

Now I need a help from the community to understand that deeper, but using Iphone slo-mo camera I cannot see the flickering, and my suggestion is that Iphone slo-mo shutterspeed of 240 hz does not cover 47 hz splashes as it is a prime number and out of all the 240 "snapshots" that camera takes it doesn't see the dip in brightness. Its also possible that iphone camera adjusts brightness sensitivity rapidly enough that you cannot see the flickering - the monitor backlight does not shut down completely, it merely lowers the brightness to roughly 25 lux from the highest point of 200 lux. Again, my assumption is that this trick is good enough to fool most of slo-mo cameras that people have on hand (iphones duh), and it might be used in multiple new monitors out there, mimicking flicker free expirience.

To close the topic - I think that lot of you who had the eye strain issues could now potentially target very specific cause. I was very relieved when I realized that the possible causes for my issues are not hypochondriacal in nature, and could be measured by the equipment.

    you bought used monitor, right?

    Look, your monitor ON time differ to backlight, it looks someone exchanged backlight and panel inside it and then sell. Also, those peak, I dont think panel + controllers + backlight work sync in your case.

    On my BL2420Z with 1080p AOU M238 I have no issues, no peaks in Opple graphics at all

      Not sure if this is the issue but reduce any response time settings or other black frame insertion options as they strobe the panel

        simplex The backlight ON time differs because this monitor has a sensor named ECO Sensor which disables the screen if human is absent from it for 15 seconds. Acutally very useful feature, I would love it on the monitor which doesn't burn my eyes out

        jordan I have tested on all modes of Response times, there are no BFI possible at all with this model as far as I know.

            simplex The benq monitor is bought new from reputable seller. The suggestion regarding exchanged backlight might hold some truth to it.

              Yvetox Benq BL2420PT

              Lol, I have a BL2420PT too.

              I can agree that it is NOT a good monitor, the main issue I have with mine is that it 100% has FRC temporal dithering.

              When connecting any input, even a super old Windows computer, by moving my head left to right I can make out an alternating intense red and blue checkerboard pattern that keeps alternating left and right for the first few minutes after turning on the monitor. This happens on all inputs, HDMI, DVI, and DP.

              Then, the more aggressive checkerboard pattern begins to """stabilize""" (but not actually truly stabilize…) into another dithering pattern that looks more like random moving static noise.

              If I put a dark gray rectangle on the screen, I can literally see moving static in the rectangle with my own eyes while looking close up at the screen. This is especially easy to spot when using max brightness and messing around with color settings to make dark colors look brighter.

              My BL2420PT is definitely 6-bit + FRC despite no one stating this online anywhere, because if I connect it to an M1 Mac, I can visibly see Apple's temporal dithering pattern clashing with the monitor's own. If I then activate Stillcolor, Apple's dithering stops, but I can still see static coming from the monitor itself especially on darker grays.

              The amount and pattern of moving static will actually differ between each unique shade of dark shade of gray that is on screen.

              The color settings on the monitor are also really confusing, it's not clear what "Gamma" mode is true linear gamma.

              In addition, there's this really weird quirk where the monitor always applies a warm tint to it after booting up no matter what color settings you use.

              If you have "Custom" color temperature set with R, G, and B all set to 255 (the only way to get anything close to native, as according to the Factory menu ALL preset color modes including "sRGB" do not actually use full or equal RGB color primaries…)

              You actually have to enter that menu, do nothing, then press back and then it will suddenly switch to a more cold-looking image. I have to do this every single time I turn on the monitor to actually get natural colors.

              In my case, watching videos or playing games on the monitor (AKA, using it as a TV) is totally fine and that's when it actually looks pretty crisp, but the moment I'm actually using it for computer use it just immediately makes me tired to work on. It's just really hard to focus on text without suddenly feeling really fatigued. Background colors also look really unstable.

              I don't have it at the current location I'm living at right now, but for some reason I also kind of remember seeing "SLA1" when checking out the service page a few months ago? Might be misremembering though.

              I wonder if I also have the worse panel that you have. I bought mine all the way back in 2016. My main issue with mine seems to be FRC and not PWM though.

              The size + resolution combo is also just bad in general, especially on macOS you only get "extremely small 2K" and "extremely oversized 720p" as your only pixel-perfect options.

              Due to this, years before I learned about PWM or temporal dithering I was already having strain with this monitor but always had (mostly incorrectly) chalked it up to size+resolution being the reason.

              Since 2017 it just never felt like I could get any actual work done on this monitor, I remember trying to code on it a bunch of times and then either just getting really tired or distracted (pretty much due to not being able to read) or switching to my laptop instead only minutes into using it.

              I would always end up constantly adjusting the brightness to try to make out text, and was switching back and forth from the BL2420PT to older TN monitors constantly (which weren't much better in retrospect, as they also all have FRC) and at that point I had no idea why.

              I didn't truly realize my reading comprehension issues were literally coming from the screens I used until 2022, and didn't even fully understand what temporal dithering was until 2023…

              I unfortunately currently own no external monitor without FRC, so for now I switched to an older 1080p Samsung TN monitor that also has some FRC but a lot more subtle, no PWM at max brightness (but REALLY agressive PWM anywhere below that… LOL) and most importantly a much more comfortable feeling backlight spectrum for my eyes.

              I just end up using my laptops for now though, possibly due to lacking an actual FRC-free monitor, but mostly just because I hate sitting in one place at a desk forever anyway (external keyboards just don't work well ergonomically for me).

                  DisplaysShouldNotBeTVs When connecting any input, even a super old Windows computer, by moving my head left to right I can make out an alternating intense red and blue checkerboard pattern that keeps alternating left and right for the first few minutes after turning on the monitor. This happens on all inputs, HDMI, DVI, and DP.

                  On my zephyris g16 with a 16" 165hz 250nit 8bit FHD panel, I experienced a similar effect: when turned laptop on, a “black vertical grid” was noticeable, quite low-frequency, which disappeared after literally 3...5 minutes. I tried different timings and screen refresh rates, but this had no effect. As a result, the screen put a lot of agressive pressure on temples

                    9 days later

                    DisplaysShouldNotBeTVs

                    @Yvetox BTW, I checked and my BL2420PT actually does have the "original" AOU M238DAN01.3 panel that yours doesn't have, but of course I still get strain from using it.

                    Even with the AOU panel, I can totally see obvious FRC dithering when looking up close and moving my head from left to right, regardless of which computer is connected or which port is used.

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