Visual acuity depends on the lighting conditions and especially the adaptive brightness, i.e. background brightness. With increasing background brightness, visual acuity initially grows rapidly and then only slightly increases from certain brightness values. Children are tested by age. Children under the age of 3 are tested by adapting to cover one eye, viewing the device, memory game and describing the image. Older children are tested using e.g. Snellen optotypes and Pflüger optotypes.

Papers:

C. Perera, R. Chakrabarti, F. M. A. Islam, and J. Crowston, “The Eye Phone Study: reliability and accuracy of assessing Snellen visual acuity using smartphone technology,” Eye, vol. 29(7), 2015, p. 888.

N. S. Anstice, and B. Thompson, “The measurement of visual acuity in children: an evidence‐based update,” Clinical and Experimental Optometry, vol. 97(1), 2014, pp. 3-11.

J. Habel, “Fundamentals of Light Technique: Visual and Vision,” Light,.(6), 2008, pp. 52-55.