Readability of displays in bright outdoor surroundings is a problem for many types of visual displays. The wide range of lighting conditions that these displays should be able to operate in, makes the design of the test set-up very challenging. Mobile displays of today usually have LCDs with LED backlight or OLEDs, which gives the possibility to control the light levels from very low luminance or even zero luminance to very high luminance levels with good contrast and color rendering. There are many displays that can show high luminance (>1000 cd/m2) and with that are considered or claimed to have daylight or sunlight readability. However, these statements are rarely supported by scientific tests with test subjects. This paper will discuss and analyze the conditions for performing such experiments. Experimental results that the visual acuity is negatively affected by strong illuminance are presented.