The entopic effect of the blue sky or the so-called Shearer effect consists in the fact that, looking with a defocused gaze, into the clear blue sky, you can see many small light points flying with a train along a small trajectory, and then quickly extinguish like sparks.
Illustration of the entopic effect of the blue sky.
To see this effect, you can just look at the blue screen, for this you need to relax your eyes and try not to move them and at the same time defocus your vision as if you are looking far through the screen, after 15-20 seconds you will be able to notice very small sparks, the most difficult don't move your eyes.
The thing is that photons of light, falling into the lens of the eye, pass through two layers of neurons before reaching the photoreceptor cells. Such a design can be compared with a camera in which a processor would also be located on top of a photosensitive matrix.
An enlarged fragment of the diagram of the retina with two layers of retinal neurons (ganglionic and bipolar cells) and the third layer of photoreceptors (rods and cones) depicted on it.
Of course, the retinal neurons themselves are practically transparent to light, otherwise we would not be able to see anything.
The vessels of the human retina.
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But like any cells, retinal neurons need nutrition and oxygen, for the delivery of which is the network of the thinnest vessels that cover the entire area of the retina.
And the erythrocytes moving through the vessels - red blood cells responsible for supplying cells with oxygen - are not transparent even by their name.
And here it is important to clarify that we see red blood precisely because of erythrocytes, and red because they are filled with hemoglobin protein molecules - a special protein for transporting oxygen and CO2. The maximum of the absorption spectrum of oxygenated hemoglobin (HbO) is in the blue part of the spectrum, so the light reflected from the hemoglobin contains very little blue, which is why we define it as red.
The absorption spectrum of hemoglobin (bold red line) superimposed on the absorption spectra of the four types of photoreceptoros.
But what does the flashing and dying lights have to do with it? - After all, if the vascular network, filled with red blood cells, absorbs the blue part of the spectrum, we should just see the red network of blood vessels. The adaptation mechanism plays an important role here, the visual system is good at ignoring static visual signals, this can be easily demonstrated using the example of the image below, it is enough to fix your gaze at the black point and try not to move it for 10 or more seconds and you can gradually notice how a gray background around the point becomes smaller and disappears, our visual system considered this signal unimportant, since it does not affect anything.
Adaptation to the vascular network takes place according to the same principle, we do not even need to make efforts to fix the gaze, because the vessels are simply part of the retina and move along with the movement of the gaze. As a result, our visual system “adds” additional blue color to the entire red mesh from the vessels, restoring the original picture.
The funny thing is that the effect of flickering lights on a blue background does not occur at all due to erythrocytes, but due to the fault of white blood cells - leukocytes, immune cells. and due to the fact that leukocytes are larger in size than erythrocytes, when they move through the thinnest vessels, they form small congestions and a space not filled with erythrocytes forms in front of them for a short time and the full spectrum falls into such gaps, as a result of which “re-adaptation” and we see a bright dot with a small trail in the direction of the leukocyte movement. And if all the erythrocytes left the vascular network at once, then before death from hypoxia, the retinal neurons could show us something like this:
But, fortunately, this does not happen normally, and we see only small gaps in the adaptation stencil, in the thinnest vessels where only one leukocyte can pass at a time, and this effect is not observed in the very center of the visual field, since there are no vessels there. this is necessary to ensure maximum resolution. This effect has found its application in ophthalmology as a test for assessing blood flow in the retinal vessels, the patient is shown a bright blue screen and then asked to compare the number of bright dots that he saw with several samples. Also in favor of such an explanation of the effect of passing sparks, the fact that the pulsation of bright points coincides with the heart rate speaks.
Author: Nikita Ivanov