iMac and MacBook Pro Flicker at All Brightness Levels

AshX

I’ve been having issues with a 2019 21.5” 4K Retina iMac. Really bad headaches, dizziness, and eye pain. I was given a 2015 MacBook Pro to try and while it’s a little better, I do get similar symptoms after a while.

I recorded videos of both at 240 fps and there seems to be a flicker at all brightness levels. I booted the iMac into Safe Mode and the flicker was still there, reset the NVRAM, etc. No help. I also recorded videos at 120 fps and the flicker is also visible.

Is this a normal sort of thing? I don’t think this is PWM since I thought the 2017 iMac Pro was the only one with PWM? I’ve used SwitchResX to change to “millions of colors” on the iMac, and while it helps a little with focusing on text (it was very easy to focus on text in Safe Mode), it doesn’t resolve this flicker. I assumed this might have been wear and tear, the GPU going bad, etc…but now I’m not so sure? I’ve linked videos of the 240 fps videos below. Would love to get some feedback.

iMac: https://streamable.com/nx4vd5

MBP: https://streamable.com/sxkrio

Clokwork

There will be a constant flicker at 60Hz or maybe even 120Hz for these devices due to frame rate and or PWM. I wish I could measure your devices with my meter. I'm tempted to look like a crazy person and waltz into Best Buy with my laptop and meter to measure the flicker precisely.

AshX

Clokwork

Is there a way to measure this without buying an Opple Light Master? I’ve searched and searched and can’t find any data on these models having PWM, other than a few forum posts claiming both iMacs and MBP’s from the 2015-2019 era having obvious flicker and PWM.

I wondered if this was just the normal 60Hz refresh rate…but then why do the MacBook Airs from the last few years not seem to have this flicker yet are also IPS LCD screens? It seems to be constant regardless of brightness. Am I mistaking something normal for these displays as being an issue…or is this some sort of constant LCD PWM flicker?

Dithering was traditionally what caused me issues with the iMac after a COVID infection, but SwitchResX seemed to largely resolve that. I haven’t used the computer in nearly 9 months, so I wonder if this is a new sensitivity. I start getting dizzy just even looking at the iMac for a few seconds. Quite concerning.

DisplaysShouldNotBeTVs

AshX Did you update the same iMac in recent times to a newer macOS? OS version is way more likely than you think change screen output (for example how Win10 1511 and Win10 1607 feel very different to me, with all versions 1607 and later feeling worse)

Unfortunately for the Intel Macs, the system EFI firmware which is where screen changes might have happened is also upgraded with macOS updates, and actually can't be downgraded (even if the OS is downgraded, the firmware stays on the new version — e.g. back when Yosemite changed the boot screen to black background, the background would stay black even after downgrading to Mavericks)

Only M series Macs are possible to downgrade the firmware, using the DFU restore mode

For actual proof that firmware and VBIOS updates can affect the screen on Intel integrated graphics for example, see this header file for VBIOS parameters on Intel Whiskey Lake architecture (same parameters can be observed on Linux with intel_vbt_decode)

https://github.com/andreiw/lampone-edk2-platforms/blob/master/Platform/Intel/WhiskeylakeOpenBoardPkg/Include/GopConfigLib.h

As you can see about 75% through this file, "pixel dithering" is a parameter that can be set in the VBIOS, in addition to some suspicious ones such as "Smooth Vision", "Panel Fitting Graphics mode - enhanced", "Enable Gamma feature", and "LACE enabled status" (referring to Local Adaptive Contrast Enhnacement)

I don't think these parameters have ever been researched here before

AshX

DisplaysShouldNotBeTVs

No, I didn’t update the OS which is what makes this even stranger. It has been on Mojave since release and I haven’t installed a single update. It remains unchanged from when I last used it in June 2024.

I was reinfected with COVID in July 2024, and already had long COVID and other health issues previous. My best guess if the machine is normal is that my tolerance has evaporated completely. I’m wondering if certain long COVID symptoms were actually neurological symptoms from working on this screen.

My iPhone 13 is tolerable despite having PWM. I’ve been using it every day throughout all these illnesses so I wonder if it allowed me to neurologically handle this particular phone, and the difference between the two is too great. Or maybe I just got lucky with this phone screen and the iMac is deteriorating with age. Your post gives a lot of food for thought.

JTL

DisplaysShouldNotBeTVs Unfortunately for the Intel Macs, the system EFI firmware which is where screen changes might have happened is also upgraded with macOS updates, and actually can't be downgraded (even if the OS is downgraded, the firmware stays on the new version

Can't is a strong word. It may still be possible to downgrade it.

a) If an update package for an older firmware is found, attempting to force the update externally (from what I remember, the update packages have external programs that handle EFI/VBIOS updating)

b) Attempting to use a hardware programmer to overwrite the existing EFI/VBIOS with what's in the update package (may miss certain per unit specific data)

c) This requires a backup taken before an upgrade, but I've been aware of others who've explicitly made backups of firmware before certain upgrades.

AshX

JTL

I read up on that before I posted this. I also read the posts trying to disable dithering on AMD GPUs on intel Macs and no one seemed to have had any luck. Since the Apple Silicon M-series was released any interest in the intel Macs seems to have evaporated.

I have not updated this computer since I updated into that version of Mojave when it was released. So there’s no way any firmware was updated. I had a family member look at the screen and they said it’s fine, so clearly this is a “me problem.”

My question then becomes: what is this flicker? Is this the 60Hz refresh rate? Or is it dithering at half that rate? Or is this some sort of LED backlight PWM from the mid-2010s screens? I’ve read conflicting reports that the MBP 15” 2015 has PWM. It wouldn’t surprise me if the iMac did as well since the 2017 iMac Pro 5K 27” had PWM at a very low frequency.

My guess is this is PWM or a dithering flicker. I don’t know why PWM would be present with auto brightness turned off, but I don’t presume to be an expert. Dithering makes more sense since SwitchResX doesn’t seem capable of fully disabling it, but switching from billions to millions of colors does allow me to look at the screen longer than without. I could decipher what this is I think I’d at least know what to avoid in the future.

I suppose I could try buying an external monitor and seeing if the issues persist?

macsforme

JTL Can't is a strong word. It may still be possible to downgrade it.

I can confirm that downgrading firmware is possible in certain scenarios. I believe I read that at least one person was able to manually copy an older firmware package into the appropriate location in the EFI partition and “bless” it, resulting in a successful downgrade (I am unsure whether this method still works with more modern firmware versions).

Another method is via reconstruction using a raw firmware update package from Apple (“.fd” file) and copying certain information into it from a dump of your previous firmware (so your machine knows its serial number and can authenticate to Apple’s services). This method is available for models that stored their firmware on EEPROM chips that can be read and flashed either using OS tools or with an external reader (so generally 2015 and earlier, prior to T2 (not sure about T1) and prior to Apple silicon). I used this method several times on a MacBookPro11,4/11,5, and once on a MacPro5,1.

At least in my case, after a modern macOS and/or the corresponding firmware update caused eye strain, downgrading the firmware and downgrading macOS did not perceptibly improve my symptoms. This led me to conclude that in the case of my eye strain, my body is what changed versus the device or its firmware/software. However, this does not preclude that someone else could find an OS or firmware update to be a trigger for their own eye strain condition.

AshX

Just wanted to let you guys know I ended up selling the iMac. I had other folks look at the screen and they had no problems with it. So either it’s been here all along or when I had the RAM and SSD upgraded by an independent shop in 2020, they knocked a cable loose and some transistor leakage developed as a result causing dips. Or maybe just from moving it in the car its time was up.

It seems virtually identical to what @DisplaysShouldNotBeTVs posted about with the MacBook Air M2 models a while back. I’d really love to know for sure what this flicker is so as to avoid it. Doesn’t seem like it’s dithering because a slow motion camera at 240 fps and 120 fps shouldn’t be able to pick it up. I found components on eBay for the iMac - a PWM controller - so I’m leaning toward it being a badly designed PWM backlight flicker.