THE SENSE OF VISION

The appearance of a food item is the first attribute humans tend to perceive, hence it is no surprise that vision in sensory evaluation is very important. The physiology of vision is very complicated with many structures and abilities. 70% of all sensory receptors in the human body are located in the eyes. Light entering the eye passes through the lens and is focused on the retina. The retina is the sensory tunic of the eye and consists of two layers, the pigmented layer and the nervous layer. It is within the nervous layer that photoreceptors process light stimuli to form our vision. These photoreceptors,rods and cones, sense and encode the patterns made by light in our surroundings. Rods are responsible for dim-light and peripheral vision and are more sensitive to light than cones. However they do not provide either sharp images or color vision. Conversely, cones serve to operate in bright light and provide high-acuity color vision. As light excites these photoreceptor cells, electrical signals are sent through sensory neurons of the bipolar cells and ganglion cell axons to the bundle of nerve fibers called the optic nerve. This nerve from each eyeball is responsible for sending the signals to the brain(Marieb, E.N.)

Visible Light Spectrum

Visible light travels in the form of wavelengths and is composed of small packets of energy called photons or quanta. Thus, light could be viewed as photons traveling in wave-like fashion at very high speeds. As visible light passes through a prism, the waves are bent to produce a band of colors called the visible spectrum. This spectrum progresses from red to violet. Objects thus have color because they absorb some wavelengths and reflect others. White objects reflect all wavelength of light and in contrast, no pun intended, black objects absorb all wavelengths. The perceptual colors of the surfaces of objects can be characterized by three phenomenal qualities:

1. Hue - the actual color name (red, blue, and green)
2. Saturation (chroma) - clarity/purity of the color, the šgreyĆ scale
3. Intensity - lightness, light to dark(Wyburn et al.).

illustrate the way in which our vision can play tricks on us. Our perception is not always reality.

Color perception in sensory evaluation is important and some concerns are:

  • Color vision differs among subjects. Degrees of color blindness exist, i.e. inability to distinguish blue from green. Some people also have highly sensitive color perception, allowing visual differences to be detected that the sensory tester may not see.
  • Consistent responses about an object color are often given by subjects even when filters are used to mask differences.
  • Perception of color is influenced by background and relative sizes of contrasting color.
  • The texture and gloss of a surface can also affect perception of color in sensory testing.
  • The eveness of color as opposed to an uneven or blotchy appearance can affect sensory perception(Meilgaard et al).

    • Sensory Physiology Nancy L. DeVore

    Updated 8/29/97. Send mail to Food Resource, Oregon State University, Corvallis, OR.