What is frame rate?

To explain frame rate, it’s important to understand how video works. Simply put, it’s a series of still images that are rapidly visualized one after the other, to create an illusion of movement. Compare it to the little flipbooks that has an image on every page, and when flipping through the pages quickly the image appears to be moving.

If you call every image a frame, then the frame rate is the speed at which those images are shown. In other words, the faster you flip through the booklet, the higher the frame rate.

Frame rate is usually expressed in frames per second (fps). A simple enough concept: it expresses how many images (or frames) are shown in a second’s time. 30 fps then means that 30 images appear every second.

Returning to the flipbook example: when flipping through the pages faster, the movement becomes smoother. In other words, the higher the frame rate, the more fluently the video appears to be.

Some typical examples of frame rates are 24 fps for Hollywood movies, and 30 fps or standard television in the US. The 24 fps for Hollywood movies is rather peculiar, given that most equipment can deliver higher frame rates. It is only slightly higher than our brain’s capacity to distinguish separate images, so if it were a little lower, we would actually see the image stutter.

The 24 fps standard is more a matter of taste: many people feel that 24 fps delivers a more cinematic experience. There have been experiments with 48 fps movies (The Hobbit: An Unexpected Journey, for example), which many people thought was ‘too realistic’. But then again, this could be a matter of adaptation and maybe in a few years 48 fps will be the new standard. Most contemporary projectors can handle this frame rate anyway.

A market where high frame rates already found their entrance, is Training & Simulation. For rapidly moving video, you need the additional frames to follow objects correctly. If not, an image could pass the screen in only 2 frames or so, which will make it barely visible – creating a very unnatural effect. 60 fps, 120 fps and even 240 fps are an undeniable plus in this market. Another application where high frame rates matter is gaming.

Fps (frames per second) is a parameter that refers to the video itself. For projectors and displays, the standard is the refresh rate, measured in Hz: the number of times the screen can be refreshed per second. Although related, that is an important difference. Ideally, the fps of the source should match the Hz or the display or projector. If the frame rate is higher than the refresh rate, some frames will be skipped resulting in a stuttering image. If the fps is lower, then the same frame will be shown multiple times, giving a choppy look. Ideally, the display or projector should be able to handle a wide range of refresh rates, making it more versatile.

Smearing and ghosting effects are important elements in visualization environments. These are ‘bleeding’ effects that appear on the spot where an object was situated one frame ago. This results in unsharp images (smear) or even in the perception of objects that aren’t really there (ghosting).

This not only causes distraction but can also lead to dizziness and motion sickness. The very smooth result of the high framerate projectors and displays helps to minimize eye fatigue and allows trainees to practice for longer periods of time.