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Emergent Techniques for Assessment of Visual Performance (1985) / Chapter Skim
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DARK-FOCUS: ANOMALOUS REFRACTIVE ERRORS AND ACCOMMODATION
Pages 14-21

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From page 14...
... It is not uncommon, for example, to find two pilots who both have excellent visual acuity in the examining room, yet, at night or in a bright empty sky, one can consistently detect and identify targets faster than the other. Recent research on visual accommodation reveals one of the mechanisms responsible for individual differences in target detection and recognition, and it has led to the development of a new approach to predicting and to optimizing performance under low visibility conditions.
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From page 15...
... All measures were taken with a laser optometer in total darkness with the subjects' normal refractive correction in place. SOURCE: Leibowitz and Owens, 1978.
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From page 16...
... With weaker stimuli, the eyes' focusing range gradually diminishes, producing response functions with progressively shallower slopes. With very weak stimulation, accommodation remains at the dark-focus regardless of stimulus distance (Johnson, 1976)
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From page 17...
... , and it may be related to problems of size and distance perception that occur with artificial display systems. This normal variation of the eyes' focusing behavior can greatly hinder the ability to detect weak stimuli and to resolve fine details, and it provides a basis for understanding and correcting a variety of anomalous refractive errors that have puzzled vision specialists for years.
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From page 18...
... study were both clinically emmetropic -- i.e., standard clinical tests of refractive error indicated that neither subject required corrective lenses. We want to emphasize that the same dark-focus prescription is not appropriate for all low visibility conditions.
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From page 19...
... It allows correction of anomalous refractive errors that can seriously limit detection and identification under low visibility conditions, and it may be useful for prescribing glasses to optimize performance on near visual tasks. These applications follow the principle of matching the individual's dark-focus to the visual task by optical means.
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From page 20...
... At present, it appears that the primary target populations among Navy and Air Force personnel are pilots, who should benefit from correction of empty-field and night myopia, and personnel such as operators of radar and video display terminals, who have critical near vision tasks and may benefit from optical corrections for near visual tasks based on the dark-focus. The working group therefore recommends that initial screen studies and field evaluations concentrate on pilots and radar operators and others engaged in demanding visual tasks.
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From page 21...
... . Although the mechanisms for intraindividual variations of the dark-focus are still obscure, they probably reflect changes in the tonus of the ciliary muscle due to prolonged focusing effort or to systemic variations in autonomic arousal.
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Key Terms

  • anomalous refractive
  • contrast sensitivity
  • resting focus
  • visual tasks
  • visibility conditions

  • stimulus conditions
  • low visibility
  • space perception
  • individual differences
  • near visual

  • binocular vergence
  • dark retinoscopy
  • resting state
  • standard clinical
  • accommodative responsiveness

  • sensitivity function
  • retinoscope beam
  • complex visual
  • optimizing visual
  • visual acuity

  • visual performance
  • initial resting
  • reduced stimulus
  • optical corrections
  • night myopia

  • reduced stimulation
  • spatial frequencies
  • clinical assessment
  • corrections based
  • night driving


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