Created on 14 June 1997.
Last modified on 5 August 2000.

Equiluminance: The Nulling of Apparent Motion


Two properties of the visual system combine to produce the effect illustrated here: its tendency to make automatic associations between similar objects, and, at a somewhat higher level, its ability to choose the most "natural" of a set of possible interpretations.

The illusion is created by cycling repeatedly through a set of four images. The first and third show alternating white and black squares while the second and fourth show alternating green and red squares shifted by half a square width with respect to the intervening black and white patterns. When the particular shades of red and green differ sufficiently in their luminance component, the visual system naturally associates the darker colour with black and the lighter one with white. This induces a perception of motion toward the right because the most natural interpretation is that of a light/dark pattern that shifts half a block width to the right in each successive frame (the less naturally interpretation being a shift of three half block widths to the left).

When the red and green colours are adjusted so as to be equiluminous the association between light and dark squares becomes ambiguous, so it is unclear whether the coloured pattern represents a shift to the right or left. In this circumstance, the illusion of motion is destroyed. This simple effect has a number of both important and useful consequences, discussed below.

The interactive demonstration

The applet window for this demonstration contains a horizontal row of squares of alternating colours and a number of controls for adjusting its appearance, as described below. The order in which the controls are listed here has proven to be a useful one in which to use them when showing the demonstration to an audience.

The pattern of squares will advance to the next frame each time this button is pressed. The fourth frame is always followed by the first. This can be used to show how, if the red squares are perceived as darker and therefore naturally associated with black, the pattern of light and dark shifts half the width of a square to the right (or three halves to the left) in each successive frame. When the frames are cycled quickly, this will be interpreted as motion towards the right.
Start, Stop
The Start button will start the pattern cycling automatically with a pre-defined period. This will substantiate the claim of the previous step. As long as the pattern is cycling, the Stop button will be enabled; pressing it will of course stop the automatic cycling.
This slider adjusts the size of the pause between successive frames when the pattern is automatically cycling. Although the default value is usually suitable, you may find that you can strengthen the illusion of motion -- both at this and later points in the demo -- by adjusting it slightly.
Red luminance, Green luminance
According to the explanation given above, the illusion of motion is dependent on the automatic association of the darker colour (usually red in the default configuration) with black and the lighter colour with white. These sliders can be used to reverse the relative luminance of the red and green squares. Doing so should therefore cause a reversal in the direction of apparent motion. Try it. (You may find it more convenient to use the arrow buttons on the sliders rather than the slider knob.)
What happens when there is no detectable difference in luminance between the red and green squares (i.e., when they are equiluminous)? You should be able to achieve a balance in which there is no longer a clear sense of motion (if you are having trouble achieving this, make sure you are pointing a relaxed gaze at the center of the pattern as you make the adjustment -- it is usually possible to detect motion, regardless of the relative luminance, if you focus on one end of the pattern).
Block size
You can use this slider to alter the size of the squares in the pattern. The number of blocks drawn is always selected to completely fill the width of the window, given a reasonable border on each size. If you make the squares quite large, then, you will probably also want to make the window wider using whatever control your window system provides. The original author of this page generally ignores this control when showing the demo to an audience, but there are undoubtedly interesting variations that can be achieved with suitable sizes.
As for all demo applets, the Dismiss button at the bottom of the applet window will remove the window from the screen. Selecting the APPLET link at the top of this page will then return it to the screen just as you left it, with the exception that automatic cycling is turned off when the window is dismissed and not restored when it is returned.


This is usually a convincing demonstration of the visual system's ability to make automatic associations and to interpret changes in position as motion. It also illustrates what happens when the visual system is provided with insufficient information to make a reasonable selection from a set of alternatives (in this case, choosing between rightward and leftward motion when the red and green squares are equiluminous), and it allows you to explore the limits of motion interpolation by lengthening the pause between successive frames until the illusion of motion breaks down. These are all factors that must be considered when choosing parameters for an animated display.

Unlike many of the other illusions in this collection, this demonstration can also be put to practical use. (Actually, you would need a slightly more flexible implementation that allowed you to replace green or red with any particular hue and that displayed the numeric value of the luminance ratio; furthermore, better ways of achieving the same goals are usually available.)

For example, you can use it to calibrate your CRT display if you know the relative intensities at which its phosphors should be equiluminous. You would set the relative intensities of the red and green squares to this value, then adjust the red or green electron guns until the illusion of motion was destroyed, indicating perceptual equiluminance.

Similarly, if you wanted to know the relative intensity at which two hues are equiluminous, you would replace red and green with those colours and adjust their intensities until the illusion of motion was again destroyed.

Scott Flinn (