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Light, Sound & Audiovisual Perception

Light

Prior to the 19th century, using the term "visible light" would have been totally meaningless; but, in 1800-1801, William Herschel and Johann Wilhelm Ritter further expanded both ends of the light spectrum as defined by Isaac Newton in 1666 on having proven that light contains invisible forms of energy which extend beyond the range of red and violet. Advances made in the field of physics have resulted in light being defined as a minor part of a larger whole which includes electromagnetic radiations ranging from gamma-rays to radio waves in which what is referred to as Light is distinguished solely by being visible to the human eye.

Waves & Particles

All forms of energy, which are grouped together under the heading of electromagnetic energy, can be defined as either waves or particles. Based on their total energy, they must be defined as being particles; and in the specific case of light, the term "photons" or "quanta" of light is used. When defined in terms of how their vibrations evolve, the term "wavelengths" is employed.

THE ELECTROMAGNETIC SPECTRUM

This spectrum can be considered as being a continuum. The values shown are those approaching the mid-point dividing two bands of energy

Properties Of Visible Light

Sources of light give off energy spanning the full breadth of the spectrum, but may do so to different degrees of intensity on the different wavelengths. Within this scope, sunlight entails a certain balance of intensities among waves of different lengths, and it is this specific point of balance, which is referred to as white light. Other light sources give off their radiation with balances of intensities giving rise to what is known as colour dominants. The concept of light intensity denotes the total amount of radiation given off by a source per unit of surface area and is measured in lumens or in candles. Likewise, the luminosity that an object reflects (brightness) is measured in lumens or in candles per unit of surface area.

The concept of colour temperature, which is measured in Kelvin degrees (Kº), is linked directly to the combustion temperature of the light source in question. The higher this temperature, the higher its colour temperature, the greater the total amount energy of the radiation given off and the shorter the wavelength of the dominant radiation will be.

The physicochemical properties of the materials which do not give off any visible radiation cause them to reflect and/or absorb the light given off to a different degree for each wavelength, which, combined with the balance of intensities which characterizes each light source, gives rise to the different colours in which matter is perceived.

Sound

Just like light, sounds comprise part of a much broader family of waves ranging from the vibrations caused by earthquakes, entailing frequencies even much lower than one cycle per hour, up to ultrasound. In this family of the longitudinal waves, which are perceptible to human ear, are defined as being sound waves.

Vibrating Matter

Longitudinal waves are vibrations of matter which (involving stages of compression and rarefaction) are indicative of the absorption and transmission of the kinetic energy transferred by any type of impact. The total amount of energy in movement, the surface area covered and the types of vibrating movement involved (depending upon the material of which the object is comprised and upon its shape) determine the properties of these waves. When the waves strike and penetrate different mediums, their properties vary and change through the processes of reflection, refraction or diffraction. These vibrations (the waves) vanish into empty space. The rate of speed at which they travel also depends upon the medium through which they are moving. For sound waves, the speed in dry air at sea level is of 331.4 meters/s; in the atmosphere with a 50% RH, it speeds up to 340 m/s and, in glass, nears 5000 m/s.

Sound Waves

A longitudinal wave of any type is characterized by its frequency [T] or period, which measures the length of one cycle, by its amplitude [A] or height of the wave front, and by its intensity. Waves of the same frequency or amplitude can be of different heights or periods. Sounds may be comprised of several frequencies, which can be in harmony (musical notes) or out of harmony (described as noise) with one another. Their intensity depends upon the amount of energy, which is transmitted per cm² per unit of time; and, with regard to sound waves, as a result of their intensity being linked to their frequency and amplitude, determines their loudness.

The Sense of Hearing

Sound waves, which are picked up by the external ear (a great aperture microphone) are converted into vibrations and are amplified by the tympanic membrane (eardrum) and the incus, malleus and stapes all working together as one. When these vibrations reach the cochlea through the scala vestibuli and scala timpani, they come to the cochlear duct, which is located between the above-mentioned canals and contains some 5,000 individual Corti bodies, each one of which vibrates under the effect of a certain range of frequencies.

All of these organs function as a whole like an analogue-digital converter, which transmits a specific signal in response to each frequency detected. Statistically speaking, it has been established that the human ear is capable of hearing frequencies within the 20-20,000 cycles/second range, at intensities of 0 decibels [dB] (hearing threshold) up to 120 dB (pain threshold). The hearing sensation (loudness) is not the same throughout all of these ranges. A sound of 80 dB is audible throughout the entire range of frequencies, whilst a sound of 20 dB will be heard solely within the 200-15,000 cycle range.

Wave Velocity Perception

Wavelengths vary depending upon the direction in which the source transmitting the waves involved is moving, becoming shorter in the direction of movement and lengthening in the opposite direction. On shortening, light waves move toward violet, and sound waves toward high frequencies; on lengthening, they move toward red and low-pitched tones. The speed at which sound travels makes it possible to immediately distinguish whether the source of the sound heard is still or is moving.

The Sense of Sight

Parts of the Eye

The human eye, likened to an optical device, can be described as being a dark room equipped with lenses and devices for adjusting focus and brightness. In the eye, the dark room is the corpus vitreum or vitreous body. The lens assembly is comprised of the cornea and the crystalline lens. Focus is achieved by means of the combined effect of the optic nerve and the sclera. Brightness is adjusted by the contraction of the iris, which can expand or contract the diameter of the pupil within the 1.8 mm - 10 mm range. And the retina serves as the screen or film on which a latent image is formed to then be transmitted to the brain and processed in a region devoted especially to vision.

Light & Colour Perception

There are two types of light-sensitive organs scattered irregularly over the retina, that is, the cones, totalling some 7,000,000 in number and basically concentrated around the macula lutea; and the rods, totalling some 170,000,000 and scattered over the entire surface of the retina. Apparently, when light strikes the surface layer of the retina, it triggers a primary, definite response reaction of a pigment-related type. The cones and rods, which are located in the next layer, react in turn (to strong, coloured lights and to dim, colourless lights), and these reactions (acid reaction to light / alkaline reaction to darkness) give rise to electrochemical impulses, which, by means of the optic nerve, transmit the visual sensations in each eye to the organ in the brain specially devoted to vision.

The human eye’s is capable of distinguishing among a wide range of differing degrees of brightness, being able to discern brightness values within the a theoretical scale of 1-10,000,000.

Field of Vision, Line of Sight & Peripheral Vision

Although both eyes working together cover a 210º horizontal field and a 120º vertical field of vision, the ability to see is not the same everywhere on the retina, and despite this wide-angle field of vision, the object being viewed must remain aligned on a dual imaginary line of sight which runs through the centre of the pupils, connecting it with the macula lutea (the area most sensitive to light and to colour) in each eye. Training one’s sight steadily along this double, synchronized line requires employing the mechanism known as visual accommodation (focus) known as binocular fixation, which, on the vision in both eyes converging on the point/object being observed, achieves the maximum degree of visual acuity. This need for convergence sets a minimum limit on the distance at which it is possible to focus clearly on objects. In research experiments, it has been established the outer limit of the human focal range is located approx. 12 cm from the back of the retina. When focusing on objects located more than 6 meters away, the human eye focuses in long-range perspective.

Audiovisual Perception

Stereoscopic Vision & Persistence of vision

Seeing is not a continuous process. Each light-sensitive cell takes time to complete its electrochemical reaction, to discharge and to then ready itself once again. In research experiments, it has been proven that vision is discontinuous, and that the brain’s vision centre recreates 10-12 images/second.

In some way which has not as yet been fully elucidated (perhaps because the information furnished by each eye includes the location of the exact point from which each signal is coming), the transmitting organs or the brain itself organizes the signals given out by the cones and rods into "trains of signals" which are "routed" from the retina to the vision centre in the brain and are "sidetracked" in an orderly fashion until the time at which they are fully "scanned" and interpreted in the proper order. In any event, the sight mechanisms do not allow a new image to come up until they have completely "scanned" the previous one. This stop-start aspect of vision is provided for by means of a "memory" mechanism, which "holds" each image in the brain’s vision centre until it is replaced by the next one in line. This "memory", formerly referred to as "retina lag", is now known as persistence of vision. By combining (or making changes in) the information received from each eye (binocular vision), due to how the pupils are spaced apart, the minor differences in alignment and positioning perceived by each eye make it possible to see objects in perspective (in 3D).

The Binaural Effect

Steadily training one’s line of sight is one aspect of vision. The eyes see solely what they look at, in this regard, are similar to a film camera. To the contrary, the sense of hearing always covers the same field to either side of the listener, making it possible --in conjunction with the relatively slow rate of speed at which sound travels and with the differing reception of reverberations (echoes) caused as the result of sound waves ricocheting off the surroundings-- to sense the presence of invisible, "still" objects and shapes and the movements of the sound-emitting sources.

False Horizon

By means of the mechanisms of binocular vision and the persistence of vision, outdoor images are perceived in perspective, and each image received is superimposed on the following one for purposes of viewing continuity. But the eyes (and the entire body) are in constant movement, and these consecutive images rarely fully coincide. The light coming from one same point on an object strikes the retina at a different place each time. Under these conditions, the superimposing of different consecutive imaged should be perceived as "blurriness".

At the same time as the stop-start process of vision is taking place, the semicircular canals in each ear --organs, apparently, no way involved in the sense of hearing-- provide constant, exact information regarding the position of the head in relationship to the vertical plane of Earth’s gravity. In some way, this stereo information picked up by the ears in the semicircular canals to be processed by the hearing centre in the brain is "compiled" with the eye’s stereo information processed by the vision centre in the brain and, by means of an analytic integration element as yet to be definitely located, provides the information necessary to afford the possibility of what we refer to in general as "audiovisual perception" to construct and perceive a constant false horizon in the consecutive information from the retina fitting together and sharply aligning with one another.