Research study about the stroboscope effect and perception of movement.


Phi phenomenon, or strobe effect, is an impression of illuminated alternating movement between two, equally strong light sources in a dark room, which results in back-and-forth pulses of light.

Goal and intent

My intention was to investigate the onset of this phenomenon’s effects. Specifically, I aimed to determine at what point in time someone perceives the flashing light source as moving or blinking.


In the preliminary investigation, the first step was to optimize two parameters in which motion of the object was very well perceived, the size of the objects and their distance from each other.

The main study included the investigation of the parameter rate, which refers to the duration of the object’s visibility in relation to its invisibility, and the frame rate within this relationship. In addition, the effect is tested by additional, permanently visible objects.


It was expected that when the object was visible for a long duration (i.e. a non-existent or short break between pulses) a higher frame rate was possible, in which the object didn’t appear to blink. Conversely, it was probable that in a short duration of object visibility (a long pause) a lower frame rate existed, in which movement was detected and eventually perceived as blinking.

In short it is expected that:
1. When the object is visible for a long duration, the viewer will not perceive a movement, even with high frame rate.
2. When the object lights up briefly, the viewer will perceive a blinking with low frame rate.

Overall, the study predicted that with a smaller frame rate, movement is detected more than when the object was viewed at a higher frame rate.

By adding a permanently visible object to the test, it was anticipated that one’s perception would alter. Depending on the objects’ form and size, the flashing points would connect, disturb, or eliminate movement, or ultimately rearrange the movement’s path.

Research results

The results of ten test participants:



For the exclusive study of velocity within the frequencies


The investigation in the selected areas showed that at frequencies where the object is visible for a duration as long as, or longer than the break continues. The playback rate must be increased so that movement is perceived.

With a few exceptions, all test participants clearly identified a break in movement, whereby blinking objects changed to one moving object. At the frequencies 1:11, 1:12, and 1:13 about 50% of the test participants perceived no clear break. Either a constant or no movement at all was perceived.

Noticeable here is the fact that the study area for these frequencies had to be settled at lower frame rates. However, as with other frame rates, an increase from frequency to frequency was noted.

In another part of the investigation, the pause was extended to decrease the object’s visibility. The areas to be examined were once again settled at lower frame rates, and it was observed that nearly half of the test participants consistently perceived no movement.

In summary

The following frequencies and frame rates work well:
0:1 up to and including 1:9 All test participants perceived movement.

In the second part, the remaining test participants perceived movement at the frequencies 5:1, 6:1, and 9:1.

The frame rate 30 at the frequencies 5:1, 6:1 worked well. For 9:1 the frame rate 36 worked best.

Peculiarities in the study

Some test participants reported a different phenomenon during the survey, which is why we extended the test with additional parameters in terms of obstacles we put in place, based on assumptions we made.

We made five general assumptions:

1.The movement comes and goes from one direction.
Some test participants reported that the movement would “run out of one side”. At first, we suspected there was a correlation to the right or left-handedness of the test participant. However, it turned out that this perception correlated to the side from where the object first appeared to be visible.

2. Within the same frame rate, movement can turn into a perceived blinking.
This phenomenon occurs right at the beginning of a test: The perceived change can be attributed to whether the person focuses on the mid-point between the objects, or the objects themselves.

When test participants focused on the obstacle, they perceived a blinking. When test participants focused back on the object, a perceived movement was triggered by their eye movement.

3. An object flashed faster than the other.
see point 1

4. The movement describes a form.
Often, test participants described that the movement would run in pendulum or a circular motion. For this reason, we added additional obstacles. We predicted they would either cancel each other out, or reinforce the perceived movement and influence the direction or shape described. (see more under “Obstacles”.) An explanation for why participants viewed pendulum motion in the first place was not found.

5. The object is scaled up, or the distance of the objects to each other is increased. The object appeared larger, or the distance between objects increased.
We were not able to determine what caused this phenomena, due to the small amount of data points available.


We placed additional obstacles in order to study the frequencies, where test participants perceived either no movement or complete movement, then examined a random sample afterwards.

The same frequency was presented to a test participant once more. In nearly all cases, the result was confirmed to be consistent with the initial observations. Afterwards we successively added obstacles, which led to the following results:

  1. The short horizontal bar caused a visible separation, which is why a movement was perceived even stronger. At frequencies where participants initially observed motion at all frame rates, their perception suddenly changed to a blinking movement.
  2. The long horizontal bar worked as a unifying element. Movement suddenly dissolved in all cases. However, the manner in which the movement disappeared differed amongst test subjects. For some, the movement was reported as leaving “through a pipe”. In others, the movement was perceived as “going behind” the obstacle.



The addition of an obstacle in a cross shape brought no meaningful results. In cases where participants noticed movement, they reported a jumping “behind” or “over” the cross. Either way, the movement was perceived as a blinking motion. In those cases, the cross shape simply reinforced the perception of blinking.


Vertical bars
Vertical bars used as obstacles worked as separating elements, which dissolved the perception of movement and triggered a perceived blinking.

Central vertical bar
The central vertical bar worked as a separating element that dissolved the movement and caused a blinking instead. Participants reported in a few cases that a circular motion was maintained though not intensified.

Vertical bar on the baseline.
Participants reported that a circular or pendulum movement was rather intensified and additionally supported.

Vertical bar positioned above the center point.
A rotating movement underneath the vertical bar was always perceived clearly. An initial pendulum movement turned into a rotating movement.


Vertical bar with offset center point, and positioned to the right.
Circular movements always appeared somewhat ovate, or directing towards the rear. There were also cases where this obstacle triggered separation and caused one movement to appear as two. The obstacle caused the impression that the two objects would bounce against a wall and bounce back.