# Why does the human visual system produce a bright patch after staring at a bright light source and looking away?

If a person looks at a bright light source, such as a light bulb or perhaps the sun for a period of time, and then looks away and closes their eyes, they typically see a bright patch in the shape of the light source for a period of time. The amount of time varies, but can last for many seconds. What explanation can be provided for this effect?

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## migrated from dsp.stackexchange.comMar 3 '12 at 17:42

This question came from our site for practitioners of the art and science of signal, image and video processing.

isn't it a dark patch, not a bright patch? – rob Mar 6 '12 at 19:41

The human eye is an interesting device. One of the most amazing things it does is to adjust for the brightness, and it can do so over 10 orders of magnitude.

From a signal processing perspective it can be explained as follows:

There are a few different ways that this is done, but the most basic way is a high pass filter, with a cutoff of about .25-.3 Hz. So, if you look at a bright source of light for a period of time, then you will see the signal as too bright, until the filter can reset back to a zero mean signal. After that, the image will go away.

There are of course many subtleties, but that is the primary effect.

To illustrate this, I've included 2 plots, the first of a signal changing over a long period of time between 2 extreme values, the second of one changing quickly over time. These plots came from a class which I took and learned how to model the neuron of a fly's eye, which behaves very similar to a human eye. The signal at 0.5 corresponds to a gray type signal value, it's an artifact of some of the other processing steps. This is a complex model, but the crux of it is a high pass filter with a time step of 3 seconds. Time prior to 40 is to allow the filter to normalize prior to injecting a signal, it prevents having to know the initial conditions very well of the system.

The second figure shows what happens with a more typical transient time, on the order of 10 Hz. Note that the signal is passed through almost without any delay.

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If that is the case (time step of 3 sec) how do we watch movies on a screen or on a TV ? – Rajesh D Aug 31 '11 at 13:48
The behavior you describe is more like a low-pass filter, not a high-pass. – Jason R Aug 31 '11 at 14:25
It passes everything with a frequency higher than 1/3rd of a Hz. How is that a low pass filter? The DC is filtered out, which is also indicative of a high pass filter... I've tried to clear a point or two up, hopefully it's a bit clearer. – Pearsonartphoto Aug 31 '11 at 14:31
I think your "filter" analogy is a bit simplistic. The phenomenon that you describe (i.e. the average value of a burst of bright light persisting over a period of time after the light goes away) is a lowpass effect. Pupil control is really more analogous to a control system, where the brain reacts to the light saturation by adding loss to the light path. This isn't the phenomenon that the question asker was looking for, though; that is due to temporary memory in the retina itself. – Jason R Aug 31 '11 at 15:23
@Jason, this has nothing to do with pupil dilation and contraction. Flash blindness affects photoreceptors before the pupil can contract, hence the blindness – Phonon Aug 31 '11 at 17:11

In your eye there are light receptors of two types: rods and cones. Rods are sensitive to the level of brightness, but cannot discern color, while cones are not very light sensitive, but they allow for color perception. Both of these are types of photoreceptors which transduct physical light.

The physical response of rods and cones to light isn't immediate, and therefore there's a certain transient response of each photoreceptor to light. When the light you're looking at is too bright, some of your photoreceptor cells go into a kind of "saturated" state, and it takes a long time, because of that transient behavior, for them to stabilize back to "seeing" darkness again.

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The persistence that you observe when you look at a bright light for a short period of time is called flash blindness. The phenomenon is related to your retina's response to the burst of light. Quoting from the Wikipedia article:

Flash blindness is caused by bleaching (oversaturation) of the retinal pigment. As the pigment returns to normal, so too does sight. In daylight the eye's pupil constricts, thus reducing the amount of light entering after a flash. At night, the dark-adapted pupil is wide open so flash blindness has a greater effect and lasts longer.

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I removed references to signal processing. They are no longer relevant after the merge to CogSci. You can still read them in the original post. – Steven Jeuris Mar 3 '12 at 17:58