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The EYE: Scanning

Scanning

When a point source of light is focused on the retina:

  • some nerve fibres in this area which were previously 'silent' begin to 'fire'; these are called 'on-fibres';
  • other fibres previously discharging become silent; these are termed 'off-fibres';
  • other fibres rapidly adapt to the existing light and discharge briefly at the beginning and end of the light stimulus only; these are called 'on-off-fibres' and are more numerous than the other two types.

Thus, if an image is fixed on the retina, after the initial stimulus a large number of the nerve fibres become inactive, resulting in a rapid fading of the image. To prevent this, the eye executes a continuous scanning movement consisting of small oscillations at about 30—80 Hz which keeps the on-off-fibres firing at the border of the image.

Persistence of vision

If the eye observes a series of static images changing at a frequency of greater than 60 Hz the eye will not perceive them as still images but rather a steady smooth movement. This is used in film and TV transmission and explains why a series of still images drawn for a cartoon are perceievd as animation.

In the early days of film production, it was scientifically determined that a frame rate of less than 16 frames per second caused the mind to see the film as flashing images. Audiences still interpret motion at rates as low as ten frames per second or slower (as in a flipbook), but the flicker caused by the shutter of a film projector is distracting below the 16-frame threshold. Modern theatrical film runs at 24 frames a second. This is the case for both physical film and digital cinema systems.

See a sample question - Q7 on this page of past paper questions.