Accommodation and acuity under night-driving illumination levels.

Laboratory experiments are described in which the monocular changes in the refractive error and acuity of six young, normal, adult subjects were measured as the field luminance was reduced from approximately 100 to 10(-3) cd/m2. It was found that, at luminance levels equal to those recommended for road lighting (about 1 cd/m2), acuity fell from its photopic value of > or = 6/6 to about 6/9, with little change in the measured refraction. Marked changes in refraction, i.e. night myopia, only started to become manifest when the luminance was further reduced to below about 0.03 cd/m2, much less than that applying under normal night-driving conditions. Direct experiments under street-lighting conditions confirmed the absence of any significant night myopia. It is concluded, therefore, that neural changes, rather than night myopia, normally are responsible for the acuity loss suffered by drivers at night.