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On This Page:  • Non-Temporal Parameters Influence Flashing   • Parameters of Temporal Modulation 
• Related Psychophysical Results  • Demonstrations  

Temporal modulation (blinking, flashing) is a very powerful tool for drawing attention. This, along with auditory alerts, is the heavy artillery of attention manipulation and must be used sparingly, with care. Ad designers of commercial websites know that it's almost impossible to ignore large flashing graphic elements in peripheral vision, even to the point that it's difficult to read the non-flashing main content. The most powerful types of temporal modulation should therefore be used only for hazards, i.e., for data that require immediate user action.

Blinking and flashing are not all-or-nothing, though. At the powerful end of the scale they interfere with attention to all other data elements. At the weak end of the scale they can be barely noticeable. The auditory analogues are sirens and klaxons at the strong end, and confirmatory sounds (such as the clicking of an auto turn signal) at the weak.

Non-Temporal Parameters Influence Flashing Back to the top of the page.

Non-temporal design parameters such as size, shape, color, luminance, and position in the field of view have an influence on the salience of flashing, but they are frequently dictated by other design considerations. Once those variables are fixed, the salience of flashing will be governed by the parameters of the temporal modulation.

Parameters of Temporal Modulation Back to the top of the page.

Graph showing the luminance maximum and minimum of a blinking stimulus over time.
The temporal frequency, f, and the contrast, C, have strong influences on the attention demand. The temporal frequency is the number of flashes per unit of time. The contrast is the difference between the luminance of the bright and dark phases of the flashing (relative to twice the average luminance).

Temporal Frequency. The rate of flashing has a powerful influence on the salience of flashing elements. The human eye is most sensitive to frequencies of 4-8 Hz (cycles/second). Very slow and very fast blinking are less attention-demanding than rates near that peak.

Graph that shows the luminance of 3 different blinking stimuli over time. The stimuli vary in luminance variance and in temporal modulation.
The stimulus graphed in b flashes faster (higher f) than the one in a. The stimulus in c has a lower depth of modulation (lower C) than the one in a.
Duty Cycle . All of the above waveforms have a 50% duty-cycle, i.e., the on-state occupies 50% of each cycle. Increasing the duty-cycle can reduce the salience of the blinking.
Graph displays luminance changes of  2 blinking stimli. A higher duty cycle is displayed as a stimulus with greater time periods of higher luminance. In a the duty-cycle is 50%. In b the duty-cycle has been increased to 88%.

Contrast, Depth-of-Modulation. The change in luminance between the peak and trough of the modulation also has a strong influence on the demand on the users' attention. The graphic element will have maximum salience if it goes from maximum luminance to minimum luminance in a cycle, at a frequency of 4-8 Hz. On the other hand, if the luminance contrast of the data element is low the blinking may be barely noticeable.

Related Psychophysical Results Back to the top of the page.

The vast majority of studies of perception of temporally modulated lights involve measurement of contrast thresholds, the minimum contrast that can be seen to be flickering. Generally speaking, for stimulus lights of the luminances that occur in electronic displays, a plot of threshold C as a function of f shows a minimum (i.e., maximum sensitivity to the temporal modulation) at 10-20 Hz. Even maximum-contrast modulation is undetectable at frequencies above about 60 Hz.

These threshold experiments are not directly relevant to our primary concern, manipulation of salience, which is about attention rather than detection and suprathreshold flicker rather than threshold flicker. Much less research has been devoted to temporal modulation for salience and most of that is not easily generalized beyond the exact type of stimulus used in the study.

Demonstrations Back to the top of the page.

** Caution: The demonstrations below display blinking symbols.
Blinking symbols may cause seizures in users with photosensitive epilepsy. **

View Flashing Demonstrations go to this page

Link to the blinking demos. Temporal Frequency. The 4-Hz flashing symbol and text (left panel) draw attention more insistently than the 1-Hz flashing symbol and text (right panel). At frequencies below 1-Hz it may be best to avoid total disappearance during the off-interval by turning the graphics only partially off (see next demonstration). Link to the blinking demos.
Contrast, Depth-of-Modulation. The salience of the flashing can be reduced by reducing the contrast of the modulation. In this panel the parameters are all identical to those in the right panel above, except the luminance in the off-interval is only slightly reduced rather than turned completely off. Link to the blinking demos.

Link to the blinking demos. Duty Cycle . All of the above demonstrations have a 50% duty-cycle. In the left panel the duty-cycle has been increased to 88%.
Legibility of Flashing Text.
Temporal modulation of alphanumeric information requires careful attention to legibility, color discrimination and identification, and impact on the overall salience hierarchy of the display.
Link to the blinking demos. Large Text Blocks. Temporal modulation of large text blocks can reduce legibility. The example at left flashes at 2 Hz, with 100% modulation.
Link to the blinking demos. Reduced Modulation Depth. Reducing the depth of modulation leaves the text visible in the "off" state.
Link to the blinking demos. Flashing Symbol. Another way to draw attention to a large block of text that must be read is to flash an adjacent symbol and leave the text on continuously.

Related Topics:
go to this page Applied Color Science
go to this page Color Discrimination and Identification

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