2.2.- What background brightness do I have to select?

In the iPad you can select the percentage of brightness of the screen, however you need to know what is the background luminance for that brightness. The relationship between the percentage of brightness and the luminance can vary between models and has been reported by Livingstone et al.¹ For example, tablets at 75% brightness varied from 165–202 cd/m2, and charts ranged from 270–418 cd/m2 which means a less variation between different iPad models than different clinical ETDRS charts.¹ You can also found relationships between brightness and luminance in other works.^2–5

The international standard normative recommends a background luminance from 80 cd/m² to 320 cd/m² and close to 200 cd/m² for measuring visual acuity.⁶ However, this luminance differs from the value of 85 cd/m² reported in some consensus for clinical trials.⁷ According to this, our recommendation is that you use around 42% of brightness for measuring at around 85 cd/m² or 82% of brightness for measuring around 200 cd/m². If you have the PICO device you can select exactly the brightness you need for a particular value of luminance.

Very important !!, do not forget to Switch Off the Auto-brightness from SETTINGS -> GENERAL -> ACCESSIBILITY -> DISPLAY ACCOMMODATIONS -> AUTO-BRIGHTNESS -> OFF

1. Livingstone IA, Tarbert CM, Giardini ME, Bastawrous A, Middleton D, Hamilton R. Photometric compliance of tablet screens and retro-Illuminated acuity charts as visual acuity measurement devices. PLoS One. 2016;11(3):e0150676.
2. Rodríguez-Vallejo M, Monsoriu JA, Furlan WD. Inter-display reproducibility of contrast sensitivity measurement with iPad. Optom Vis Sci. 2016;93(12):1532-1536.
3. Aslam TM, Murray IJ, Lai MYT, et al. An assessment of a modern touch-screen tablet computer with reference to core physical characteristics necessary for clinical vision testing. J R Soc Interface. 2013;10(84):20130239.
4. Tahir HJ, Murray IJ, Parry NR a, Aslam TM. Optimisation and assessment of three modern touch screen tablet computers for clinical vision testing. PLoS One. 2014;9(4):e95074. doi:10.1371/journal.pone.0095074
5. Bodduluri L, Boon MY, Dain SJ. Evaluation of tablet computers for visual function assessment. Behav Res Methods. 2017;49(2):548-558. doi:10.3758/s13428-016-0725-1
6. ISO 10938:2016. Ophthalmic optics. Chart displays for visual acuity measurement. Printed, projected and electronic.
7. MacRae S, Holladay JT, Glasser A, et al. Special Report: American Academy of Ophthalmology Task Force Consensus Statement for Extended Depth of Focus Intraocular Lenses. Ophthalmology. 2017;124(1):139-141. doi:10.1016/j.ophtha.2016.09.039