Visual acuity may be defined as the power to see clearly, or to discriminate detail in objects. This power may be measured by the angle subtended at the nodal point of the eye by the detail discriminated or, more practically considered, by the angle subtended at the frontal surface of the eye. It may also be measured in terms of the size of the detail and its distance from the eye. It differs greatly with the intensity of light employed, and the amount of difference varies in individual subjects. So great is the effect of intensity of light and so poorly has it been subjected to control that it might be well to emphasize the importance of the factor by qualifying the definition to read: the power to see clearly, or to discriminate detail, at a given intensity of light. An entirely correct expression of acuity of course
FERREE CE, RAND G. TESTING FITNESS FOR NIGHT FLYING: VISUAL ACUITY. Arch Ophthalmol. 1938;20(1):58–79. doi:10.1001/archopht.1938.00850190070007
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