Let's see how it works sight and light. We explain, within the visual system, the visual perception, how we perceive the light that reaches our eyes and how it is interpreted by our brain to create images.

sight and light

how sight works

Since ancient times, the first function of the visual system was to provide information to carry out day-to-day tasks, information about the environment that surrounds us and that helps us make decisions from the point of view of acting.

If we are thirsty and want to drink, the first thing we should do is look around us and locate a glass and a pitcher of water. We must look where these objects are in order to carry out the motor action of drinking. If we do not know where these objects are, we will make search movements with our heads and eyes, which means using our peripheral vision to locate the objects and, immediately, focusing on them with our eyes. taint.

The information that reaches the brain serves to identify the objects and to start the motor process, in this case, to go and pick them up, that is, to calculate the distance, weight, etc., to extend the arm so that the hand can reach them and Execute the movement of pouring water and bringing the glass to your mouth.

This process involves three mechanisms:

  1. The look: to locate objects
  2. The motor mechanism: to grab it
  3. The visual system: to adjust the movement.

The set of these three systems is what we call Supervisory Attention System (“Supervisory Attention System” by Normann and Shallice, 1986) or Central Executive (“Central Executive” by Baddeley, 2007) which is summarized in: Where to look, what to do and what to look for.

Eye and brain connection

These systems are made in brain in different areas, visual tasks occur in the occipital region and partially in the parietal and temporal lobes; motor tasks in the parietal and frontal region and "schema control" in the dorsolateral region of the prefrontal cortex. These areas are reciprocally connected.

One of the most important points is where we look. Initially, it is the brain that directs the gaze towards what we have decided to look at, either directly on the object we want, because we know where it is, or looking around us because we cannot locate it and we search for it with movements of the eyes and head.

In this search process, the information we analyze is not "in detail" of all the objects that enter our campor visual, we identify these objects through a small group of data that, without “seeing them perfectly”, we know what they are and allow us to decide whether or not it is the object we are looking for, it is a scanner mechanism. The details that the brain uses of each object are the “salient” characteristics of those objects: color, shapes, contrasts, etc. This search and selection mechanism corresponds to a mechanism that we call “Top-down”.

The eye movement

One of the most important points in the study of the visual perception is that of the scanner mechanism. What leads us to move our eyes in certain directions, what are the details, the information, what does the brain assess during this scanning process to decide our subsequent actions.

We have a clear example in the table tennis player, the ball goes at a speed of 80 ms and a "follow-up" movement takes 200 ms, which implies that the player cannot follow it, he must have a visual strategy of trying predict where the ball will go in order to have time to place the racket in the right place, you should pay attention to the movement of the hand, position of the racket, direction of the opponent's gaze, etc., a ampa group of "signs" that you must know how to interpret to advance your movement.

Improve eye movement

Currently, through the “eye tracker” systems, we can analyze the eye movements, the points on which the gaze is fixed and thus try to discern how this scan is carried out, the relevant points, etc, to look for common mechanisms in each type of action, sport, etc, in order to get to know it better and, through specific exercises, being able to work on it and increase performance (driving vehicles, sports...).

El visual process It begins with the light that reaches the eye from objects, so it is essential that before starting the study of the mechanisms of perception, we review the most relevant points of light and the psychophysics of light.

How light reaches our eyes and forms images

Let's see the steps that light follows and the types of light that our eyes capture so that, through the visual system, it reaches the brain forming images.

Electromagnetic spectrum

The human eye is capable of detecting only part of the electromagnetic spectrum, the radiation band between 380 nm and 720 nm. Light reaches the eyes as radiation containing packets of energy called "quanta" or "photons". We can calculate this energy using the formula that establishes a relationship directly proportional to the speed of light and Planck's constant and inversely proportional to the wavelength of light or, using the formula that multiplies Planck's constant by the frequency of the light.

This mathematical relationship shows that quanta of short wavelength radiation, such as the blue band, have more energy than quanta of longer bands. From the clinical point of view, it is important since exposure to short-wave ultraviolet radiation produces more tissue damage than long-wave radiation such as infrared. Remember that UV type C damages the cornea, because they are absorbed to a greater degree at this level, while UV types B and A, especially damage the crystalline, that's why the former produce keratitis (when we sunbathe or ski without protection) and the others accelerate the process of Cataracts o macular degeneration after cataract surgery by allowing radiation to pass through retina.

Radiometry and photometry

radiometry refers to the power of a source of electromagnetic radiation, regardless of its effect on vision.

photometry It refers to the power of radiation with respect to the visual system and its clinical measurement is carried out by studying the photopic luminosity curve, which shows us how certain wavelengths of the visible spectrum of light are more effective with respect to vision.

The units of both are different, radiometry uses watts (radiant energy) and photometry uses lumen.

If we have 2 radiations of light with different wavelengths, one of 400 nm and the other of 600 nm, and both with a radiation power of 10 W, we will see that the 400 nm, with that power, does not produce visual stimulation, while than the 600 does, with an efficiency of 0.62. With this example we see how two stimuli of equal radiometric power, 10 W, produce different results in the vision, photometrically speaking.

The basic unit of photometry is the lumen, a measure of the power of luminosity, and by convention we say that 1 Watt has 680 lumens in a radiation of wavelength equal to 555 nm. In this way we can express the efficiency of all wavelengths, so a 650 nm radiation has an energy of, 0.1 x 680, equal to 68 lumens/W, where 0.1 corresponds to the efficiency of the 650 nm radiation obtained in the photometric curves of spectral luminosity.

Abney's Law

Most objects emit reflected light from a distant source. This reflected light usually has a mixture of different wavelengths so that we can calculate its luminosity by adding each of the energies that correspond to each of those wavelengths, the lumens are added and we obtain the total, it is the Additive Law of Abney.

How we see different types of light

light power: is the general, total, non-directional measure of the light emitted by a source, such as that produced by a light bulb, 1000 lumens in all directions. Light sources can be incandescent, when the light is generated by heat, or luminescent, when the light is produced by the excitation of individual atoms.

Luminous intensity: is the power of light contained in a given direction, in a cone with a given angle and is measured in candelas. One candela is one lumen per steradian, and one steradian is a cone of a sphere which is calculated using the formula:


where “r” is the radius of the sphere and A is the area of ​​the surface subtended by the sphere.

Luminance: quantifies the amount of light arriving from a surface, such as a sheet of paper, in a certain direction. Luminance is a perceptual term and we can make it equivalent to a physical attribute, and it would be brightness. Luminance is expressed in candelas per projected surface area: candelas/square meter (cd/m2).

It is interesting to know that the luminance or brightness of a surface, we perceive invariant regardless of the distance we are, remains constant even if we get closer or further away and this is due to the fact that the increase or decrease of candles is proportional to the change in size of that surface at the retinal level. If we move away, the size of that object becomes smaller, in proportion to the loss of reflected light, so its luminosity (luminance) does not vary.

Illuminance: Illuminance is the amount of light that falls on a surface and is expressed in Lux or lumens per square meter. It is important to remember that illuminance does not depend on the surface on which it is projected, this will be important when we talk about luminance, the light reflected by the surface.

It is of interest when making recommendations about the lighting conditions of a room or a larger area, such as a shopping mall, etc. A very strong illuminance can cause reflections that bother and cause visual fatigue that is not recommended.

Retinal Illuminance: Vision begins with the arrival of light on the retina and based on what has been said so far, we could apply the concept of illuminance and calculate the amount of light that reaches the retina based on the dioptric means and, especially, on the size of the pupil, diaphragm that regulates the passage of light to the retina. Retinal illuminance is expressed in one troland (td) as a unit and is obtained from the product of the luminance of the surface being viewed times the pupillary area.

Inverse Square Law

It refers to illuminance and tells us that the amount of light, in lumens, that falls on a surface is inverse to the square of the distance it is from the light source:

E = I / d2

Where E is the illuminance, I the intensity of the light source and d its distance from the surface where the light is projected. This formula assumes that the surface is perpendicular to the light source, if it were tilted, the amount of light that reaches it will be different and the formula changes by introducing the cosine of the angle formed by the light, the plane perpendicular to its line of diffusion and the surface on which it is projected:

E = (I / d2 ) cos θ

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The view and the light
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The view and the light
We explain the relationship between sight and light. What mechanism does the visual system use to capture the light that reaches our eyes and sends it to the brain.
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Área Oftalmológica Avanzada
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