Computer Screens Flicker On Camera: Why Does This Happen?

Computer screens may flicker on camera due to a mismatch between the camera's frame rate and the monitor's refresh rate. This issue is more likely to occur with CRT monitors, where the refresh rate is often variable, typically ranging from 60Hz to 85Hz. LCD screens, on the other hand, do not usually exhibit the same banding problems as CRTs due to their different image-creation process. However, LED-backlit LCD monitors may still exhibit some flicker due to the use of pulse width modulation to regulate brightness. To resolve the issue of flickering screens, it is recommended to match the camera's frame rate to the monitor's refresh rate.

The frame rate of the video recording does not match the hertz rate of the location

The frame rate of the video recording and the hertz rate of the location being shot need to match to avoid screen flicker. The hertz (Hz) is the unit of frequency, often described as one event or cycle per second. In the context of screens, the hertz rate refers to the refresh rate, or the number of times the display's image is refreshed per second.

The frame rate of a video refers to the number of individual images or frames that are captured per second. If the frame rate of the video recording does not match the hertz rate of the location, it can result in screen flicker when recording a computer screen or television set. This is because the camera is capturing multiple images or frames during the time it takes for the screen to refresh, resulting in a flickering effect.

To eliminate screen flicker, the frame rate of the video recording needs to be adjusted to match the hertz rate of the location. For example, if the hertz rate of the location is 60 Hz, the video recording should be set to a frame rate of 30 fps (frames per second) or a multiple of 30. Similarly, for a location with a hertz rate of 50 Hz, the video recording frame rate should be set to 25 fps or a multiple thereof.

By matching the frame rate of the video recording to the hertz rate of the location, the camera can capture images in sync with the screen's refresh rate, resulting in a stable image without flicker. This technique is commonly used by professionals in film and video production to ensure high-quality recordings of computer screens and displays.

The shutter speed of the camera is too fast

If your camera's shutter speed is too fast, it can cause a computer screen to appear to flicker when viewed through the camera lens. This is due to the difference in scanning frequencies between the computer monitor and the camera. The scanning frequency of a monitor is typically measured in hertz (Hz), and it refers to the number of times the screen is refreshed or updated in one second. For example, in regions with a 50Hz electricity supply, the power source utilises 100 pulses of light each second.

In contrast, a camera's shutter speed refers to the amount of time that the shutter remains open to allow light to enter and expose the image sensor. A fast shutter speed means that the shutter is open for a very short period, capturing a brief snapshot of the light emitted by the monitor.

If the shutter speed is not synchronised with the monitor's refresh rate, the camera may capture moments where the screen is not fully illuminated, resulting in a flickering effect. This issue is more likely to occur with faster shutter speeds because there is a reduced chance of alignment between the shutter and the screen's illumination.

To mitigate this problem, it is recommended to adjust the shutter speed of the camera. By setting the shutter speed as a multiple of the power source's frequency (50 or 60, depending on the region), you can reduce the likelihood of capturing the screen during its off-cycle. For example, if you are in a region with a 60Hz power source, you can set the shutter speed to intervals divisible by 60, such as 1/60 or 1/120.

Additionally, decreasing the frames per second (FPS) on your camera can also help reduce the flickering effect. A lower FPS means fewer frames are captured per second, reducing the chances of capturing the screen during its brief off-cycle.

The scanning frequency between the TV and the camera is different

The scanning frequency between the TV and the camera being different is one of the two reasons why computer screens flicker on camera.

The scanning frequency of a TV or computer screen refers to the rate at which the electron beam completes its cycles of vertical and horizontal motion, scanning horizontal lines of pixels across the screen. This is also known as the refresh rate. The refresh rate of a typical TV is 60Hz, meaning the screen refreshes every 60th of a second.

The issue of flickering occurs when the scanning frequency of the TV or computer screen is different from the frame rate of the camera. The frame rate of the camera refers to the number of frames or images it captures per second. If the frame rate of the camera does not match the refresh rate of the screen, the camera will pick up the screen at different stages of its refresh cycle, resulting in a flickering effect.

To resolve this issue, one must ensure that the refresh rate of the screen and the frame rate of the camera are synchronized. This can be achieved by adjusting the settings on the camera or screen, or by using specialized equipment such as display cards for personal computers, which allow for customizable refresh rates.

It is worth noting that flat-screen LCD displays do not typically suffer from this issue due to their different display technology.

The way phosphor dots are perceived between the human eye and the camera's image sensor is different

The human eye and a camera have many similarities. Both have a diaphragm to control the amount of light that gets through to the lens, a lens to focus the light and create an image, and a method of sensing the image. However, there are also significant differences in the way they perceive and process images.

The human eye relies on refraction and lenses to form images. The image is then focused on the retina, and a system of rods and cones converts the image to electrical impulses, which are sent along the optic nerve to the brain for interpretation. The eye can adjust extremely quickly when focusing on objects at different distances, and its lens shape changes to create a sharp image.

In contrast, a camera's lens has a fixed focal length. The image is captured on film or a digital sensor, which is made up of light-capturing units called pixels. These pixels are organised into columns and overlaid with a filter, allowing them to convert light into colour. While cameras can make accurate guesses about colour, this level of processing is less advanced than what the human eye and brain can achieve.

When it comes to phosphor dots, the human eye perceives them differently from a camera's image sensor. The phosphor dots on a screen glow for about 1/30th of a second, which is long enough for our eyes to see a steady image. However, a camera is much less sensitive to persistence, and the dots appear to have a shorter glow duration when captured by a camera. This difference in persistence perception between the human eye and the camera's image sensor results in the flickering effect observed when recording screens with a camera.

The pulse width modulation used to regulate the brightness of LED-backlit displays can cause flickering

Pulse Width Modulation (PWM) is a technique used to adjust the brightness of LED-backlit displays. It is a cost-effective way to control the brightness of a display, but it has some drawbacks, such as flickering, which may cause eye strain and headaches.

PWM works by cycling the backlight on and off very rapidly at a frequency that is typically imperceptible to the human eye. However, some people are more sensitive to flicker and may experience discomfort, headaches, or other health issues when exposed to PWM displays. The flickering is more visible in LED-backlit displays compared to older CCFL backlights because LEDs can switch on and off much faster and do not continue to "glow" after the power is cut off, resulting in sharper transitions between on and off states.

To achieve a lower brightness on a display using PWM, the backlight is turned on and off in a very high frequency. For example, to achieve 50% brightness, the backlight is run in a 50% duty cycle, where it is on for half the time and off for the other half. This will be perceived as half as bright as a 100% duty cycle.

The frequency of PWM can vary depending on the type of backlight used. CCFL-based backlights typically cycle at around 175Hz, while LED backlights have been reported to run from 180Hz to 420Hz, with some even faster frequencies above 2000Hz. The higher frequencies are less likely to cause visible flicker.

While PWM is a simple and cost-effective way to control display brightness, its side effects, such as flickering, eye strain, and headaches, have led some manufacturers to adopt alternative methods, such as using direct current (DC) methods of backlight dimming to provide "flicker-free" monitors.

Frequently asked questions