Physics of Vision

5 Key excerpts on "Physics of Vision"

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  eBook - ePub

  Digital Video Quality

  Vision Models and Metrics

  • Stefan Winkler(Author)
  • 2013(Publication Date)
  • Wiley

    (Publisher)

  2

  Vision

  Seeing is believing. English proverb

  Vision is the most essential of our senses; 80–90% of all neurons in the human brain are estimated to be involved in visual perception (Young, 1991). This is already an indication of the enormous complexity of the human visual system. The discussions in this chapter are necessarily limited in scope and focus mostly on aspects relevant to image and video processing. For a more detailed overview of vision, the reader is referred to the abundant literature, e.g. the excellent book by Wandell (1995).

  The human visual system can be subdivided into two major components: the eyes, which capture light and convert it into signals that can be understood by the nervous system, and the visual pathways in the brain, along which these signals are transmitted and processed. This chapter discusses the anatomy and physiology of these components as well as a number of phenomena of visual perception that are of particular relevance to the models and metrics discussed in this book.

  2.1 EYE

  2.1.1 Physical Principles

  From an optical point of view, the eye is the equivalent of a photographic camera. It comprises a system of lenses and a variable aperture to focus images on the light-sensitive retina. This section summarizes the basics of the optical principles of image formation (Bass et al. , 1995; Hecht, 1997).

  The optics of the eye rely on the physical principles of refraction . Refraction is the bending of light rays at the angulated interface of two transparent media with different refractive indices. The refractive index n of a material is the ratio of the speed of light in vacuum c 0 to the speed of light in this material c : n = C 0 /c . The degree of refraction depends on the ratio of the refractive indices of the two media as well as the angle ϕ between the incident light ray and the interface normal: n 1 sin ϕ 1 = n 2 sin ϕ 2 . This is known as Snell’s law .

  Lenses exploit refraction to converge or diverge light, depending on their shape. Parallel rays of light are bent outwards when passing through a concave lens and inwards when passing through a convex lens. These focusing properties of a convex lens can be used for image formation. Due to the nature of the projection, the image produced by the lens is reversed, i.e. rotated 180° about the optical axis.

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  Information Photonics

  Fundamentals, Technologies, and Applications

  • Asit Kumar Datta, Soumika Munshi(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  Chapter 3 Vision, Visual Perception, and Computer Vision 3.1 Introduction Human visual perception is an active process of the brain starting with primal processing in the visual system. Visual perception (or vision) can be defined as the process or the result of processes of building an internal rep-resentation of an object or a scene in the mind of the viewer. This encom-passes entities or relations that are believed to exist in an external reality and that can be derived by processing reflected light rays (or an absence thereof). In other words, visual perception is the human ability to interpret a scene by processing information that is contained in visible light by the eye-brain mechanism. 3.1.1 Human visual system The human visual system can be regarded as consisting of two parts. In the first part the eyes act as image receptors which capture light and convert it into electrical or neuronal signals, which are then transmitted to image processing centres in the brain. These centres process the signals received from the eyes and build an internal replica of the scene being viewed. While the eyes convert visual signals to electrical impulses via a chemical process. In the second part the brain partly acts by simple image processing and partly by building and manipulating an internal model of the scene. 3.1.1.1 Structure of the human eye The anatomical structure of the eye can be divided into three structural regions: (a) protective structure of the eye consisting of the orbit, the eyelids, conjunctive, and the sclera; (b) anterior segment consisting of the cornea, the aqueous humour, iris, the crystalline lens, and the ciliary muscle; and (c) posterior segment consisting of the retina and the vitreous humour. 85

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  Physical Aspects of the Human Body

  • Hartmut Zabel(Author)
  • 2023(Publication Date)
  • De Gruyter

    (Publisher)

  11  Physical aspects of vision

  Physical properties of the visual system
  Refractive power of the cornea	43 dpt
  Refractive power of the lens	15 dpt
  Combined optical power of the eye (relaxed state for infinite object distance)	58 dpt
  Average refractive index	1.4
  Accommodation width	Young: 12 dpt; old: 2 dp
  Resolving power	3 μm
  Visual acuity	1’
  Intraocular pressure	10–25 hPa
  Visual field	Horizontally: 200°, vertically: 130°
  Total number of cones	7 × 106
  Total number of rods	120 × 106
  Density of cones and rods in the fovea	Cones: 120 × 103 /mm2 ; rods: 50 × 103 /mm2
  Density of cones and rods outside the fovea	Cones: 50 × 103 /mm2 ; rods: 120 × 103 /mm2
  Spectral range of the eye	400–700 nm
  Dynamic range of the eye	106 –10−6  cd/m2
  Number of muscles controlling the eye ball	9
  Number of pigments in a rod/cone	109
  Membrane potential of cone/rod in dark state	−30 mV
  Number of layers in the retina	5
  Cell types in the retina	6
  Thickness of the retina	100–300 μm

  11.1  Introduction

  It has been estimated that 80% of our sensory information from the environment is perceived through our eyes. The processing of visual perception takes up one-fourth of our brain. Infants’ visual intelligence develops earlier and completes before verbal skills improve. All these facts show how important the visual system is [1 ]. Loss through blindness is a serious handicap.

  This is the first of two chapters on our sensory system. The next chapter deals with the auditory and vestibular systems. Of course, there are many more senses in our bodies that we constantly draw information from such as pressure, taste, smell, and temperature. The visual and auditory sense are, however, excellent examples of how biology converts physical properties, such as light and sound, into action potentials (APs). The APs are stored and processed in our brain to enable decision-making using memory and experience.

  As we will notice, visual perception is an extremely complex and intriguing signal transduction. Roughly speaking, one can distinguish three main parts contributing to visual perception: (1) optics of the eye; (2) photon detection and initial image processing within the retina; and (3) signal transmission and further processing in the visual cortex of the brain. This chapter gives a brief overview of mainly the first two parts with some remarks on the third part.

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  The Resource Handbook of Electronics

  • Jerry C. Whitaker(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  Whitaker, Jerry C. “ Light, Vision, and Photometry ” The Resource Handbook of Electronics. Ed. Jerry C. Whitaker Boca Raton: CRC Press LLC, ©2001 © 2001 by CRC PRESS LLC Chapter 5 Light, Vision, and Photometry 5.1 Introduction Vision results from stimulation of the eye by light and consequent interaction through connecting nerves with the brain. 1 In physical terms, light constitutes a small section in the range of electromagnetic radiation, extending in wavelength from about 400 to 700 nanometers (nm) or billionths (10 -9 ) of a meter. (See Figure 5.1 .) Under ideal conditions, the human visual system can detect: • Wavelength differences of 1 millimicron (10 Ä, 1 Angstrom unit = 10 –8 cm) • Intensity differences as little as 1 percent • Forms subtending an angle at the eye of 1 arc-minute, and often smaller objects Although the range of human vision is small compared with the total energy spec-trum, human discrimination—the ability to detect differences in intensity or qual-ity—is excellent. 5.2 Sources of Illumination Light reaching an observer usually has been reflected from some object. The original source of such energy typically is radiation from molecules or atoms resulting from internal (atomic) changes. The exact type of emission is determined by: • The ways in which the atoms or molecules are supplied with energy to replace what they radiate 1 Portions of this chapter were adapted from: Jerry C. Whitaker and K. B. Benson (eds.), Standard Handbook of Video and Television Engineering , 3rd ed., McGraw-Hill, New York, NY, 1999. Used with permission. © 2001 by CRC PRESS LLC • The physical state of the substance, whether solid, liquid, or gaseous The most common source of radiant energy is the thermal excitation of atoms in the solid or gaseous state. 5.2.1 The Spectrum When a beam of light traveling in air falls upon a glass surface at an angle, it is re-fracted or bent.

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  Visual and Non-Visual Effects of Light

  Working Environment and Well-Being

  • Agnieszka Wolska, Dariusz Sawicki, Malgorzata Tafil-Klawe, Małgorzata Tafil-Klawe(Authors)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  2   The Biological Bases of Photoreception in the Process of Image Vision

  A vision is not just a picture of what could be; it is an appeal to our better selves, a call to become something more. Rosabeth Moss Kanter

  2.1 Optics of the Eye – From the Pupil to the Retina

  Numerous studies have been devoted to vision and visual perception in human beings. Though far from easy, it is important to understand the complex and still not entirely discovered workings of sight. The following chapter will try to elucidate the biology of photoreception in the process of image vision.

  Figure 2.1 provides a diagram showing a schematic cross-section of the human eye (A) and the morphology of the retina (B). Light enters the eye through the cornea (a transparent external surface) and passes through the pupil (the aperture that allows it in). The diameter of the pupil is controlled by the muscles of the iris so that an optimum amount of light can be let in under different conditions. Behind the iris lies the crystalline lens focusing the incoming light on the retina. Thus, the eye can be compared to a photographic camera: it has a variable aperture system (the pupil), a lens system, and a retina that corresponds to the film.

  FIGURE 2.1 Cross-section of the human eye (A) and morphology of the retina (B).

  The amount of light entering the eye through the pupil is proportional to its area. The pupil diameter in the human eye can vary from 1.5 mm to 8 mm and is regulated by the iris, whose major function is to admit more light in the dark and less in daylight. The variations in the pupil’s aperture make it capable of allowing 30-fold changes in how much light enters the eye.

  The lens has just the right curvature for parallel rays of light to pass through each of its parts and be bent exactly enough for all the rays to pass through a single focal point. The more a lens bends the light rays, the greater is its refractive power measured in terms of diopters. In children, this refractive power can be increased from 20 diopters to about 34 diopters, which is an accommodation of about 14 diopters. The elastic lens capsule can change shape (become more or less spherical) in response to the activity of the ciliary muscle, controlled by the autonomic nervous system. This refractive power influences visual acuity, or clarity of vision – in the human eye it is about 25 seconds of arc for discriminating between point sources of light. A person with normal visual acuity looking at two pinpoint light spots 10 m away can barely distinguish the separate spots when they are 1.5 to 2 mm apart.

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