Be careful of LED backlights (Score 1)

I just bought a laptop online. It looked great on paper, had a great keyboard, and within 10 minutes it was driving me crazy. I hadn't bought a laptop since LED backlighting took over, and it turns out that there's now Yet Another Hidden Specification you have to check for.

Most manufacturers (not Apple) use pulse-width modulation (PWM) to control the backlight brightness. When you set the brightness to less than 100%, the backlight LEDs will be pulsing at some fixed frequency (perhaps 200-700 Hz) and a duty cycle determined by your brightness setting. (Some laptops may pulse the backlight even at 100% brightness, depending on the backlight LEDs.)

The problem is the laptops with a PWM frequency closer to 200 Hz than 700. This is above your eye's flicker-fusion threshold, so if you're staring directly at the screen you won't see any blinking. But when your eyes are moving over text, instead of getting a nice clean motion blur, they'll get a few distinct copies of the letters. This makes the screen look blurry in a weird indefinable way that can drive you crazy. (It gives me headaches.) At higher PWM frequencies, your eye sees something closer to a true motion blur, and the problem decreases.

The problem is much worse with LED backlights than CCFLs, because the CCFLs respond more slowly to the PWM pulses, so the brightness ramps up and down more evenly. LEDs respond quickly, producing clean, sharp pulses.

Unfortunately, no manufacturer seems to list the backlight PWM frequency in their specifications. Luckily, it's not too difficult to test if you have the laptop in front of you. Go to a store and look at the demo laptops. Bring up a mostly-white screen (e.g. a web browser displaying about:blank), then turn down the backlight to 50%. Wave your hand back and forth in front of the screen as fast as you can. You probably won't see a nice clean motion blur unless you're looking at a Mac. Instead you'll see many distinct outlines of your fingers. If you see a lot of outlines, spaced close together, the backlight is decent. If you see a few, spaced far apart, beware! I tried this earlier this week at my local Best Buy, and only a few laptops had a PWM frequency higher than the one that was giving me headaches.

This article explains the problem in more detail, including instructions for using a digital camera to find the actual PWM frequency of your monitor.