09-04-2020, 03:54 PM
Visual angle typically refers to the angle a visual stimulus subtends on the eye. In the illustration below, angle V is defined as the angle formed by rays extending from the eye O to points A and B, which could represent the two edges of an object, or two points on a visual display.
(diagram by author Ojosepa; used under Creative Commons license https://en.wikipedia.org/wiki/File:EyeOpticsV400y.jpg)
Visual angle is often used in vision science rather than absolute physical units such as cm, since it reflects the eye movement and / or stimulus size irrespective of the distance to the stimuli. For instance, a reported stimulus size of 5 cm is meaningless without knowing the viewing distance, whereas a reported stimulus size of 5° allows for easy replication. Visual angle is thus a less ambiguous measure. The EyeLink systems use degrees of visual angle to calculate velocity and measure saccade amplitudes.
There is a longer and more detailed discussion of visual angle in this blog on our website, along with a handy online visual angle calculator which can be used to convert pixels to degrees of visual angle and vice-versa.
(diagram by author Ojosepa; used under Creative Commons license https://en.wikipedia.org/wiki/File:EyeOpticsV400y.jpg)
Visual angle is often used in vision science rather than absolute physical units such as cm, since it reflects the eye movement and / or stimulus size irrespective of the distance to the stimuli. For instance, a reported stimulus size of 5 cm is meaningless without knowing the viewing distance, whereas a reported stimulus size of 5° allows for easy replication. Visual angle is thus a less ambiguous measure. The EyeLink systems use degrees of visual angle to calculate velocity and measure saccade amplitudes.
There is a longer and more detailed discussion of visual angle in this blog on our website, along with a handy online visual angle calculator which can be used to convert pixels to degrees of visual angle and vice-versa.