Practical insights

Practical insights 3 min

Flicker 3 min

Perceived flicker in visual displays can be uncomfortable for viewers. It can also reduce the sense of realism and, therefore, immersion in virtual environments. People can typically detect flicker at frequencies up to around 60-77 Hz⁸⁴.

Light and dark adaptation 2 min

Our eyes adapt to the dark over time, with most adjustment happening in the first 5 to 10 minutes, and full adaptation taking up to 30 minutes. As our eyes adapt, we see less colour and rely more on differences in brightness and contrast. Slightly higher light levels or careful lighting can preserve some colour without sacrificing the immersive feel of darkness.

Visual illusions 2 min

Our visual system can be tricked by a range of illusions, some of which offer interesting creative opportunities.

• Forced perspective: Our brains interpret visual size and distance based on context. For example, we know from experience that objects at a distance appear smaller. The forced perspective illusion tricks this aspect of visual perception through the strategic placement of objects and the viewing angle to make items appear larger or smaller, or at different distances, than they actually are. In The Lord of the Rings films, director Peter Jackson offset actors in depth but hid this difference with carefully duplicated props to make hobbits appear half-sized. The hobbit character would sit further away from the camera than the non-hobbit, with props such as benches and tables merging flawlessly in camera to hide the cues that would normally help us notice these differences in depth. This way, the actors' scale was changed via optics, not in post-production.

• Hollow-face illusion: An inverted (concave) face mask, viewed from the front, is interpreted as a normal face that eerily appears to track the observer as they move. The illusion works under modest lighting and when viewed from 1.3m⁸⁵ and beyond, making it a low-cost way to add an unsettling presence to galleries or stage backdrops.

• Pepper's ghost: An angled sheet of glass can be positioned to reflect a hidden, brightly lit performer or projection, creating the illusion of a lifelike ghost appearing among live on-stage actors. From Victorian parlour tricks to the present day, this technique has been key to hologram-style stagecraft⁸⁶.

• Illusory contours: The brain often fills in visual lines or edges where these are implied, even if they are not fully depicted. For example, in the figure below, most people perceive a white triangle sitting on top of another triangle and three black circles, even though this white triangle is not drawn.

Illusory triangle: Three pac-man discs and three corner bars induce the perception of a bright white triangle, with crisp edges and apparent occlusion, despite no triangle being drawn. © Fibonacci, 2007, CC BY-SA 3.0.