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From the light source to the retina

The source of light to the retina

The vision is precious sense, with a highly complex mechanism. The retina is a neural tissue sensitive to light, which contains a layer of specialized cells called "photoreceptors" - literally "light receptors. The vision sensation occurs in the brain, but the visual process starts in the eye, specifically within the outer layers of the retina. An electric signal leaves the retina after traveling along the visual pathways, will become a visual sensation in a region of the brain called the occipital cortex. In physiological optics, the initial stage of the vision is of particular interest. It deals with the formation of a retinal image, born with the capture by the eye of the emitted light, and his 'projection' on the retina.

The light emitted by a source is often represented in a familiar way in the form of "rays": these rays are actually the direction of propagation of energy particles of light called photons. In the case of a star, the photons are emitted in all directions of space: the rays are emitted from the star in all directions of the surrounding space. An optical system like the eye can capture only a portion of the emitted light. However, this capture is not as 'passive' than a solar collector, which directly receives the rays of the Sun and simply convert light energy into electricity (it is in fact of the energy carried by the photons). The capture of light is intended to training from one image to the level of the retina. It resembles rather a photographic device that shares with the eye being provided with an "optical" (objective, generally consists of a group of lenses vs the couple formed by the cornea and the lens) and a "sensor" (film for film cameras, electronic sensor for digital cameras, and for the eye retina).

The role of the camera lens is to focus on the sensor from captured rays of the image framed in the viewfinder. This requires changing the path of the rays, as these are collected after they've diverged from their source. In addition, the image received by the sensor must be clear, thanks to the 'focusing' which is moving manually or automatically (auto focus) one or more of the objective lenses: we will see that the image sharpness is maximum in the plan where a maximum of light rays "intersect", so as to recreate, in the case of a star , a one-time image and so faithful to the source.

sour this light
Schematic representation of a light source; it emits light in all directions: the rays represent the direction of propagation of light, called photons, particles moving away straight from the source. Some of these photons can be captured by an optical system; to form a geometric image of the source, the rays should converge to a narrow region of the space that's there that an image sensor, a screen, or a retina would collect a "cleaner" image Captured photons flow is indeed concentrated in a small area, and the peak of light intensity (irradiance) can be understood as the image of the light source. If one collects a little light outside the maximum depth of field, the peak of illumination will be lower and more spread out: the image will not be a bright spot, but a less brilliant drive.

Similarly, if the vision in the eye is seen as sharp, then that light captured by the eye forms an image as close as possible to the source on the retinal screen. This means that the path of the light within the eye circles allows to focus the rays picked up by the eye in the plan of the retina. It is located at about 2 cm behind the cornea, the first optical interface encountered by the light located on the front of the eye. The quality of this focus on the retina is the quality of the received image: focusing light on the retina is the"optical step."  vision.

The eye, like the camera, is a light sensor whose purpose is to train a retinal image, that there will be translated into electrical signals that are then forwarded to the Visual brain areas by a chain of neurons, the emergence of the eyeball form the optic nerve. This transfer of information to the brain is neuro-cognitive step of the vision. The image perceived as Visual sensation is certainly, in some ways, quite different from the retinal image. The latter however dictates the content, and our goal will simply focus on the analysis of the quality of the retinal image as a reflection of the optical quality of the eye.

image formed on the retina
A portion of the light emitted by the sources within the observed landscape (Hong Kong Bay) is captured by the eye: this light must be focused in the retinal plane to form a sharp image. The sharpness of the image depends on many factors, of which the most important is that the rays emitted by each of the elementary light sources in the scene and collected by the eye are focused and intersects in the retinal plane.

The exploration tools used in refractive surgery are mainly optical vision stage. For example, corneal topography is the study of the cornea, which is one-half of the natural lens of the eye (the other half being made up by the lens). THEOPD allows a more direct study of the retinal image, thanks to the collection of parameters in relation to the light path in the eye.

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