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Comment by sathackr | Hacker News Reader

Most LEDs have their brightness controlled via PWM, which essentially switches the LED on and off many thousand times per second.

You can see this in most LED car taillights if you move your eyes while paying attention to the light -- it will create a 'strobe' effect, a trail of images along the path that it traveled in your field of view.

Some people are sensitive to this flicker. A similar thing happens with most DLP projectors.

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I hate PWM car taillights, but I doubt that LED screens will have the same problems. PWM is not the only way to control LED brightness and even PWM can be OK if the frequency is high enough. Also, projectors already flicker.

What other methods are there to control LED brightness?

PWM is just one (very inexpensive) way to implement a DAC (digital to analog converter). Other methods actually set the output voltage to the target voltage, such as a R-2R ladder. I don't think R-2R ladders are used very much in practice anymore but I can't remember any other implementations off the top of my head.

The other main way would be using a constant current driver[1]. By adjusting the amount of current to the LED, you can control the brightness.

[1] https://electronics.stackexchange.com/questions/256336/does-...

LEDs are current devices; any mechanism for controlling current precisely can be used to control LED brightness. there are a lot of them.

running a smaller, continuous, current through them.

PWM is done because it's cheap to implement given a fixed power rail and a digital control : need a single/couple of power switches that can even be integrated on die for low-ish power targets. continuous things usually need some kind of discrete filters, that means more components etc

Eizo has screens that use DC dimming:

http://www.eizoglobal.com/library/basics/eyestrain/index.htm...

I never tried them though.

I have some kind of "flickering sensitivity" (if this is a thing). I bought a retina MBP about 4-5 years ago and I had to return it because it was killing me. After 30 minutes watching the screen, I had terrible headaches and my vision was becoming blury for the next hours.

Now I have a new (2017) MBP and I don't have any problem anymore, so I guess that they improved the technology somehow.

Plasma TVs do it to me also. It used to drive me nuts to go into a Best Buy and look at a wall of plasma TVs.

Looking up the retina displays, I see they are either IPS or OLED. The IPS likely has an LED backlight, and OLED is...LED. So a flicker is possible if they used PWM to limit the current to the LEDs.

Did you notice any change in the effect at different brightness levels?

The only Apple device with an OLED screen is the iPhone X; the built-in displays on the 2012-2015 retina MacBook Pros as well as the ones made after the 2016 redesign are IPS with a 60hz refresh rate. There were issues with the graphics on the mid 2012 to early 2013 retina MacBook Pros - perhaps the parent got one of the defective models.

> The only Apple device with an OLED screen is the iPhone X

I think the iWatch also has an OLED panel.

All three of them do! I'm not sure how it slipped my mind. Thankfully, Cunningham's Law [1].

[1]https://meta.wikimedia.org/wiki/Cunningham%27s_Law

Most people could barely perceive the flicker of 60Hz CRT televisions. The kHz refresh of LEDs is not perceptible.

There are plenty of people[1] that can perceive whatever PWM frequency is typically used in vehicle LED taillights. Like a few the people in the linked forum, the switching in the vertical Cadillac tail lights are particularly visible to me.

I agree that when viewed directly, it's quite unlikely anyone can perceive a switching frequency in the thousands of hertz. But if you sit in one of the closer seats, your eyes will be darting back and forth across the screen, which would be enough for that strobe effect to bother me if the frequency is anywhere near what is used in the taillights.

[1] https://www.thenakedscientists.com/forum/index.php?topic=451...

I’m able to reliably detect flicker up to ~ 80Hz. Made for expensive monitors back in the CRT days

Crts don't switch on an off. The phospher decays much more slowly then a switched diode.

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