Digital displays are increasing in ubiquity and complexity. Traditional movie theaters and televisions presented a sequence of closely related images at 48–60 Hz. Stereo 3D television presents a coded sequence of frames intended for the left and right eyes at a total of 120 Hz. DLP projection technology creates full color images from a temporal sequence of dozens of very brief monochrome sub-frames, each lasting only microseconds1. Rather than a simple sequence of frames, each of which is a natural image, display designers and researchers now think in terms of coding the temporal output of a display. Applications include using three or more sub-frames to allow simultaneous 3D and 2D television2, embedding imperceptible codes into normal images3,4 and making private displays viewable only with the appropriate glasses5,6.

The light output of modern displays may at no point of time actually resemble a natural scene. Instead, the codes rely on the fact that at a high enough frame rate human perception integrates the incoming light, such that an image and its negative in rapid succession are perceived as a grey field. This paper explores these new coded displays, as opposed to the traditional sort which show only a sequence of nearly identical images.

The key question that must be answered to build these devices is “What framerate is necessary to provide the illusion of a stable picture?” This question has been the subject of research for more than 50 years and nearly all articles and textbooks on the subject contain a statement similar to the following, The critical flicker fusion rate is defined as the rate at which human perception cannot distinguish modulated light from a stable field. This rate varies with intensity and contrast, with the fastest variation in luminance one can detect at 50–90 Hz5,6,7,8,9,10,11,12,13.

These primary perceptual findings have been incorporated into international standards for display ergonomics14 and a belief that “…a frame rate of 72 Hz for computer displays is sufficient to avoid flicker completely.”15.

In the present study, we find that viewers can distinguish between modulated light and a stable field at up to 500 Hz, much higher than the widely reported rate. We hypothesize that unconscious rapid eye movements across high frequency edges in the displayed image are responsible.

Most existing studies have been carried out using a spatially uniform light source16, however digital displays provide a spatially varying image. Spatio-temporal sensitivity has also been explicitly measured, with reports of a maximum perceivable rate approximately equivalent to spatially uniform lighting17,18. These studies have sometimes attempted to measure retinal stabilized response, often with special equipment to insure that eye movement does not affect the measurements19,20. The relationship between eye movement and flicker perception has received mixed reports. Some researchers have reported that eye movement can enhance threshold perception21,22, while others have concluded that eye movement do not substantially affect the visibility of motion artifacts or flicker on spatial displays8,23.

The effect measured in this paper is likely due to saccades and thus related to the phantom array, a repeated pattern observed with high frequency modulation of a bright point or bright line24,25,26. Unfortunately, none of the existing work on phantom arrays addresses the question of the maximum perceivable modulation rate on displays with 2D spatial extent, the information required by display designers.

The work presented here attempts to clarify “the rate at which human perception cannot distinguish between modulated light and a stable field.” We allow for a spatially varying light source and do not attempt to constrain natural eye movements of our subjects. We follow prior work by measuring the viewer's contrast sensitivity, the ratio between background illumination and modulated illumination which is perceivable. However, rather than varying the brightness of the modulated signal as is done in most perceptual research, we mimic real world situations in which displays have constant brightness and the surrounding ambient light level varies. In our tests the display modulates between an image and its inverse at a constant brightness level, while the subject adjusts the level of ambient illumination until flickering artifacts are just noticeable.