If not stated otherwise, a distance of 1 meter is assumed between eye and screen.
Screen resolution is taken from SystemInformation. Multiple monitors can have different screen resolutions, in which case the appropriate screen resolution has to be selected. The default is 72 dpi.
The reference for human eyes is a Snellen ratio of 20/20 (feet in non-metric units) or 6/6 (meters in metric units).
Some supported values for animal are "Dog", "Cat", "Mouse", "GuineaPig" and "Horse". Additionally, humans and color blindness types can be specified as:
The actual, physical size on the screen is very important for the accuracy of the simulation. The resolution of the screen is used to translate those into pixel on the screen. See "ScreenResolution".
ScreenResolution
"ScreenResolution" is taken from the SystemInformation. However, some system claim to have a different screen resolution than they actually have. The default for many systems is 72 dpi. Retina devices tend to have 144 dpi.
SnellenRatio
A common use of the Snellen ratio is in the Snellen eye chart (below the version in used in the US):
The minimum eyesight standard for driving is 0.5 (20/40 or 6/12).
Legally blind individuals have a visual acuity of 20/200 (6/60).