Your film, 1994 Computer Lab, is in post-production. While reviewing the dailies, you’ve noticed a big problem. The CRT monitors in every shot have fat, dark lines scrolling vertically on their screens. No worries: You have a billion-dollar budget to work with, so you’ll simply rename the film 2014 Computer Lab and reshoot all those scenes with LCD monitors.
But what’s this? There are no lines on the screens anymore, but the LCD monitors look like they’re flickering in your footage. Will you need to rename the movie 2034 Computer Lab and develop futuristic monitor technology to use in the film? And why don’t any of these monitors look right when you point a camera at them?
Different Monitors, Similar Problems
Although the technologies inside CRT and LCD monitors are quite different, what’s happening in each scenario has a similar underlying cause: Your camera’s frame rate doesn’t match up with the monitor’s refresh rate. What adds to the problem is that human beings process moving images in a very different way than video cameras do.
Unless you also use a kerosene-powered camcorder, you probably don’t use a CRT monitor anymore. That’s a shame, because the underlying technology is cool. An electron gun in the back of the monitor blasts electrons toward the screen. Before they get there, they are precision-routed by magnets, line by line, in a sweeping motion from the top to the bottom of the screen. The final stage of the electron beam’s journey involves hitting a phosphor coating on the inside of the screen that lights up and emits color.
That whole top-to-bottom, line-by-line process happens 60 times a second with a CRT TV, equivalent to a refresh rate of 60Hz to match up with TV broadcast standards. But with a CRT computer monitor, the refresh rate is often variable, and it’s usually between 60Hz and 85Hz.
Unless your camera’s frame rate matches up with a CRT monitor’s refresh rate, you’ll see those dark bands scrolling vertically on the screen. That’s because the camera is capturing the electron gun in mid-sweep, when parts of the screen are still fresh with brightly glowing phosphors but others aren’t. Our eyes don’t pick up those fading phosphors on certain portions of the screen, but the camera does.
A lot of that has to do with the differences between human and machine vision. A camera simply takes rapid-fire shots of specific moments in time and stitches them together. Our eyes and brains don’t work like that. There’s a constant flow of communication between our eyes and our visual cortex, crunching data, providing context, and making split-second adjustments. When we’re looking directly at a source of light—a monitor, for example—an afterimage hangs around on our retina due to our eyes’ sensitivity to light. This afterimage can bridge the gap between individual screen refreshes, making on-screen motion look fluid and preventing us from seeing a strobe or striping effect. Cameras aren’t so easily fooled.
LCDs (including LED-backlit LCD monitors) don’t have the same banding problems as CRTs, because they create images in an entirely different way. There’s a constant source of backlight behind the screen, and the entire image doesn’t have to be “redrawn,” line by line, at a cyclical rate.
However, you may still see a bit of flicker when an LCD monitor is recorded on video. Much of this has to do with the pulse width modulation used to regulate the brightness of many LED-backlit displays. Pulse width modulation is essentially like flicking a power switch on and off at a very fast rate: It pulses the amount of power supplied to the backlight system based on the selected brightness level. Even though it’s rarely visible to the naked eye, some people report getting headaches from it.
How to Fix It
In each of these cases, fixing the odd effects of shooting a monitor on video is easy to fix. Simply match the frame rate of your video camera to the refresh rate of the monitor. In Windows, right-click the desktop, select “Screen Resolution,” click “Advanced Settings,” and select a refresh rate from the Monitor settings menu. In Mac OS X, click the Apple icon, click “System Preferences,” click “Displays,” and select a refresh rate (the default for all Macs will be 60Hz).
Sixty frames per second for a 60Hz monitor is ideal, and 30fps for a 60Hz monitor should also work. If you’re shooting any monitor at 24fps, you may see some weirdness—it’ll just be far more subtle with an LCD display. If you’ve already synced up the frame rate and the refresh rate but you’re still seeing flickering on an LED-backlit monitor, try turning its brightness all the way up. That should mitigate any pulse width modulation madness.