Stereoscopic vision: what is it, how it works, how it is measured?

Stereoscopic vision is an invaluable gift that nature has awarded to a person.Thanks to this mechanism, we perceive the surrounding world in all its depth and versatility. A 3D image forms the brain when a person looks at the visible objects with both eyes.

Stereoscopic vision enabled the modern person to create imitations of a stereo effect: 3D movies, stereotypes and stereophotos.All this makes the world around us even more delightful and mysterious.

What is stereoscopic vision and how does it work?

Definition of stereoscopic vision

Stereoscopic vision is a unique property of the organs of vision, which allows you to see not only the dimensions of an object in one plane, but also its shape, as well as the dimensions of the object in different planes.

Such a three-dimensional vision is inherent in every healthy person: for example, if we see a house in the distance, we can approximately determine what size it is and how far away it is from us.

Stereoscopic vision is an important function performed by the human eye.

Mechanism

On the retina of our eyes, a two-dimensional image is formed, however, a person perceives the depth of space, that is, has three-dimensional stereoscopic vision.

We are able to assess the depth due to different mechanisms. Possessing data on the magnitude of the object, a person is able to calculate the distance to it or to understand which of the objects is closer, by comparing the angular size of the object. If one object is in front of the other and partly obscures it, the front object is perceived at a closer distance.

The remoteness of an object can also be determined by such a sign as "parallax" movement.This is an apparent shift of more distant and intimate objects when the head moves in different directions. An example is the "railway effect when we look from the window of a moving train, it seems to us that the speed of closely located objects is greater than the speed of distant objects.

One of the important functions of stereoscopic view is orientation in space. Thanks to the ability to see objects extensively, we better orient ourselves in space.

If a person loses the perception of the depth of space, his life will become dangerous.

Stereoscopic vision helps us in many ways, for example, in sports activities.Without evaluating yourself and the surrounding objects in space, the performances of gymnasts on bars and logs will become impossible, pole vaulter will not be able to properly assess the distance to the bar, and biathletes will not be able to hit the target.

Without stereoscopic vision, a person can not work in professions that require an immediate evaluation distance, or associated with rapidly moving objects (pilot, train driver, hunter, dentist).

Deviations

A person has several mechanisms for estimating the depth. If any of the mechanisms does not work, then this is a deviation from the norm, leading to various limitations in assessing the remoteness of objects and orientation in space. The most important mechanism of depth perception is stereopsis.

Stereopsis

Stereopsis depends on the sharing of both eyes.When considering any three-dimensional scene, both eyes form various images on the retina. This can be seen if you look straight ahead and quickly move your head from side to side or quickly close one by one, then another eye. If you have a flat object in front of you, then you will not notice much difference. However, if the objects are at a different distance from you, then you will notice significant changes in the picture.In the process of stereopsis, the brain compares images of the same scene on two retinas and estimates their depth with relative accuracy.

Manifestation of a stereopsis

Disparity

This is the name of the deviation from the position of the corresponding points on the retina of the right and left eyes, in which the same image is fixed.If the deviation does not exceed 2 ° in the horizontal direction, and not more than a few angular minutes in the vertical direction, then a person will visually perceive a single point in space as located closer than the point itself fixation.If the distance between the projections of the point is less than between the corresponding points, then the person will feel that it is located beyond the fixation point.

The third option involves a deviation of more than 2°. If the vertical direction exceeds several angular minutes, we can see 2 separate points, which will appear located closer or further from the fixation point.This experiment underlies the creation of a series of stereoscopic instruments (Wheatstone stereoscope, stereo television, stereo range meters, etc.).

Manifestation of disparity

Allocate convergent disparity (at points near the fixation point) and divergent (at points located farther than the fixation point). The distribution of disparities in the image is called a disparity map.

Stereposise check

Some people can not perceive the depth of objects using a stereoscope. You can check your stereopsis with such a picture.

If there is a stereoscope, you can make copies of stereopairs, which are depicted on it, and insert them into the device. The second option is to perpendicularly arrange a thin sheet of cardboard between two images of one stereopair. Having installed them in parallel, you can try to look at your image with each eye.

Using a stereoscope

In 1960, a scientist from the US Bela Yulesh suggested using a unique way to demonstrate a stereo effect, excluding monocular observation of the object.This principle can be used to train stereopsis. Look at the pictures-autostereograms.

If you look into the distance, through the drawing, you will see a stereoscopic picture.

On the basis of this method, a device has been created that makes it possible to investigate the threshold of stereoscopic vision, an autostereogram.There is also a modified device that allows you to very accurately determine the threshold of stereoscopic vision.

Each eye is offered test objects that have the same area of ​​points and represent a shape of an arbitrary shape.In the case when the values ​​of the parallactic angles are zero, the observer can see in the generalized image points located in an arbitrary order. He will not be able to allocate a certain figure on a randomized background. Thus, the monocular vision of the figure is excluded.

Conducting the test

Moving one of the test objects perpendicular to the optical axis of the system, we see how the parallactic angle between the figures changes. When he reaches a certain value, the observer will be able to see the figure, as if breaking away from the background; the figure can also be removed or approximated to it.

The parallactic angle is measured by means of an optical compensator, which is inserted into one of the branches of the device. When a figure appears in the field of view, the observer fixes it, and an appropriate threshold of stereoscopic vision threshold appears on the indicator.

Neurophysiology of stereoscopic vision

Research in the field of neurophysiology of stereoscopic vision made it possible to identify specific cells in the primary visual cortex of the brain, which are tuned for disparity. They can be of 2 types:

1. Cells of the first type react only when the stimuli are accurately located on the corresponding portions of both retinas.
2. The second type of cell reacts only when the object is located beyond the fixation point.

   Scheme of volumetric vision

In addition, there are cells that react when the stimulus is closer to the fixation point.

All types of cells have the property of orientational selectivity.They have a good reaction to the moving stimuli and the ends of the lines.

There is also a struggle of visual fields.In the case when images on the retina of both eyes are very different, then often one of them is no longer perceived.This phenomenon means the following: if the visual system can not combine images on both retinas, it partially or completely rejects one of the images.

Conditions for stereoscopic vision

For a normal stereoscopic view, the following conditions are necessary:

• Normal functioning of the motor system of eyeballs;
• Good visual acuity of both eyes;
• The relationship between accommodation, fusion and convergence;
• A slight difference in the scales of images of both eyes.

If on the retina of both eyes, when viewing the same subject, the image has different sizes or a different scale, this is called aniseikoniey.

This deviation is the most frequent reason that stereoscopic vision becomes unstable or is lost.

Aniseikonija, as a rule, develops at presence of a different refraction of eyes (anisometropia). If it does not exceed 2 %, then it is possible to correct the usual stigmatic lenses. At a higher degree of aniseukonium, aniseikonic goggles have to be used.

Norms and indicators

The main reason for the appearance of strabismus is the violation of the connection between convergence and accommodation.With visible strabismus there is not only a cosmetic defect, but the visual acuity of the mowing eye is reduced. He can even turn off periodically from the process of perception of images (amblyopia).In the case of heterophoria, that is, latent strabismus, there is no cosmetic defect, but pathology will still prevent stereopsis. People with a heterophory that exceeds 3 ° can not work with binocular instruments.

The threshold of stereoscopic vision is determined by the influence of various factors, such as:

• Background brightness;
• Contrast of objects;
• Duration of observation.

Under good observation conditions, the depth perception threshold varies from 10 to 12 to 5 ".

Stereoscopic vision can be studied, assessed, evaluated in several ways:

• By means of a stereoscope according to the Pulfrich charts (the minimum threshold is 15 ") 4
• Using stereoscopes with a set of more accurate tables (measuring range - from 10 to 90 ");
• Using a device that includes the use of a randomized background (the permissible measurement error is 1 - 2 ").

Video

conclusions

So, stereoscopic vision is a complex mechanism, without which it would be difficult for a person to live in this world.It helps us to see objects not only on the plane, but also in space, allows us to calculate their size, volume, distance. You can check stereoscopic vision in several ways, as described in the article.

Also read about how the fields of view are explored.