Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

Читать онлайн книгу.

Essentials of Veterinary Ophthalmology - Kirk N. Gelatt


Скачать книгу
of Vision

      Introduction

      Physical Optics

      Light

      Light has alternately been described as a wave or as photon particles. However, these descriptions are not mutually exclusive. Both models also are applicable in the eye: the wave theory explains the physical changes light undergoes during its passage through the eye, and the particle theory explains the energy transformation that occurs when light strikes the outer segments of the photoreceptors. Hence, the first part of this chapter discusses light as a wave, while the second part discusses it as a particle.

Schematic illustration of light as a wave, which is characterized by two parameters.

      As light strikes the photoreceptor outer segments, it is absorbed by a visual photopigment. The function of this two‐part molecule reflects the principles of quantum physics, as it utilizes both the wave properties and the particle properties of light. The first part of the molecule, the opsin, determines the wavelength of the light that the photopigment will absorb, thus determining color vision. The second part of the molecule, the visual chromophore or retinal, uses the energy of the photon to undergo isomerization (from 11‐cis‐retinal into all‐trans retinal in the case of rhodopsin), thereby initiating conversion of a light stimulus into an electric signal. This process, the phototransduction process, which is discussed in detail later in this chapter, is the first step in the propagation of a visual signal.

      Photometry

Source Luminance (cd/m2)
Sun 109
Car light 107
Incandescent tungsten lamp 106–107
Fluorescent lamp 104–105
Clear sky at noon 104
Full moon 103
Street lamp 0.1–1.0
Moonless night sky 10−3 to 10−6

      a In general, only the photopic system is active at a luminance >3 cd/m2; at a luminance <0.03 cd/m2, the scotopic system functions alone. Both systems are active at intermediate luminance values, which are defined as mesopic vision.

      Luminance is measured using photometers, which are divided into two major classes. Visual photometers provide a subjective reading, because the observer compares the illumination of the measured light with that of a standard light. Photoelectric photometers convert the measured light into an electric current, which is displayed by the instrument. Photometry measurements are extremely important in electroretinographic (ERG) recordings because they are used to describe such variables as threshold, ambient light, and stimulus parameters.

      Transmission and Reflection

      Human vision is limited to a wavelength range of 380–780 nm. This limitation is a result of two factors: the first is the absorption spectrum of the opsin component of the visual photopigment, and the second limiting


Скачать книгу