Night myopia and driving.

Current knowledge of the refractive shifts (twilight myopia, night myopia, dark focus) that occur as the luminance is progressively lowered from photopic levels is reviewed. In complete darkness, myopic changes of the order of 1 dioptre are typical in young adult subjects. These gradually diminish as presbyopia is approached. The possible role of these refractive changes in causing visual difficulties for young drivers on the road at night is considered. It is concluded that high mesopic levels (about 1 cd/m2) of road luminance produced by street and vehicle lighting are normally too great to allow significant refractive shifts to occur, although acuity will be reduced due to light-dependent neural changes. Under these circumstances it appears unlikely that specific night-driving prescriptions will be beneficial: the accurate correction of normal refractive errors is, however, more important during night than day driving due to the greater blur associated with the larger night-time pupil. Suggestions are made for further experimental work in this area.