posted on 8 May 2013 by guy
last changed 8 Jun 2016
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ages: 8 to 99 yrs
budget: $0.00 to $0.00
prep time: 0 to 0 min
class time: 5 to 20 min
This lesson contains a short explanation of positive afterimages and some suggestions on how best to see them.
When light shines on the retina, chemical and electrical signals are sent to the brain, providing the basis for our sense of vision. When the light is turned off, the brain continues to receive and process visual signals for a short while. These signals lead to the perception of positive afterimages.
activity — seeing with your eyes closed
Stand in front of a window (or a bright computer screen) and cover your eyes with your hands until your eyes have become somewhat dark adapted (a half a minute should be enough). Uncover your eyes for about half a second to get a quick glimpse of the view in front of you and then cover them up again. When your eyes are covered again, you should continue to see an image of the view through the window, which will fade away after just a few seconds. This is a positive afterimage, caused by a persistent signal from your visual system, which continues for a little while after the retina has been exposed to light. The image might be seen a little more clearly if you repeatedly uncover and recover your eyes in quick succession.
This afterimage is distinct from the negative afterimage that can sometimes be seen against a bright background (see our lesson on Yang and Yin - negative afterimages for more on negative afterimages). The positive afterimage results from photochemical processes in the retina and the brain, which are initiated by shining light on the photoreceptors, but which do not stop immediately when the light is turned off.
In this effect, the different photoreceptors (and other color perception mechanisms in the brain) have slightly different persistence times. (See our lesson on Color Perception for more on photoreceptor structure and behavior.) Although all three types of red, green and blue sensitive cones might be stimulated by a white scene, when the lights are turned off some cones may persist longer in sending a signal, thereby producing a colored afterimage. After I look at a very bright pale sky, the positive afterimage I see behind my closed eyes quickly turns a bright magenta (indicating that signals from my red and blue cones are the most active), and then after less than a second changes to blue (indicating that my red cones are no longer active and my blue cones continue to transmit). This experience varies for different brightness, different exposure times, and from one individual to another, but it can be quite dramatic.
what's going on?
When light shines on the retina, chemical and electrical signals are sent to the brain, providing the basis for our sense of vision. When the light is turned off, the brain continues to receive signals from the retina while the processes that produce these signals shut down, and as the signals already produced complete their journey to the brain. This continued visual signal, even after our eyes are closed, is known as persistence.
We can sometimes see persistence effects for some colors for a relatively long time (a few seconds), as we do by looking out a window and then covering our eyes, but the afterimage dims very rapidly. Moreover, we perceive reliable full-color persistence, with all three types of cones transmitting more or less equally, for only a short period. In low light conditions, bright, full-color persistence usually lasts about 1/25th of a second. It can be even shorter in bright light conditions.
Any dark interval shorter than our persistence time is imperceptible; our brain continues to receive a visual signal for that time, and doesn't perceive that the lights are out. This feature of our vision is used to advantage in many situations to give the sense of continuous vision from a sequence of discrete images.
Movie projectors operate by showing a sequence of still frames on the screen, interspersed with brief periods (7 to 10 milliseconds) when the light is blocked by the shutter and the film is advanced. As long as the frame changes are short enough, we fill in the dark periods with the persistence of our visual systems.
Fluorescent lamps do not give out a steady light, and generally depend on persistence in the human visual system to keep us from noticing the flicker of illumination. The illumination in these lights ramps up and down with each cycle of alternating current that is applied. In the Unites States, household current reverses 120 times a second, causing a brief period of minimal current, and minimal light each time. The ballast in a fluoresent tube tries to prevent this drop in current, and some ballasts work better than others in this regard. For older magnetic power ballasts, the drop in illumination can be significant, and without the persistence of our visual systems, we would notice a flickering from these light sources.1
Visual persistence also plays a role in the glow trails from sparklers. Anyone who has ever lit one of these fireworks and waved it around knows what I'm talking about. As the sparkler passes through the air, it appears to leave a glowing trail behind it. Of course there is no real trail, it's just that the bright image of the sparkler persists in our visual system even after the sparkler has moved on to another location. The net result is that the sparkler leaves behind a trail of visual images in our brain.
In a similar vein, some popular toys use a moving rod or fan blade mounted with LEDs to spell out messages.
The thaumatrope is another vintage toy that uses visual persistence to blend two images together in the brain. See our lesson on The Thaumatrope for some instructions and materials to build your own.
question to ponder
- How do you tell the difference between the colored afterimages from persistence (positive afterimages) and desensitization (negative afterimages)?
The positive colored afterimages you see behind your closed eyes do indeed look a lot like the negative colored afterimages you see in our lesson on Yang and Yin — negative afterimages. However, negative afterimages occur when the retina is being stimulated by looking at a white background. The colors appear because some cones in the retina do not respond to the stimulation because they have previously been desensitized. In that case, the colors produced are complementary to the colors that caused the desensitization. In contrast, positive afterimages occur when there is no stimulation (you have your eyes closed and covered by your hands), and some cones continue to transmit color signals for a little while. These colors are the same as the colors that previously stimulated the retina.
David P. Sterna and Mauricio Peredo at Nasa have posted a more advanced explanation of fluorescent tube plasmas and ballasts at http://www-istp.gsfc.nasa.gov/Education/wfluor.html.
- 1. At the end of their lives, some fluorescent tubes will develop flickering at lower frequencies than the power frequency, which we actually can see as flickering. Moreover, even the normal current fluctuations sometimes produce an audible noise, which leads to a hum that many people associate with fluorescent lighting.