How do you quantifiably determine the point where something has slowly faded from view? Edit: an idea — instead of “can you still see it?”, ask “can you prove

How do you quantifiably determine the point where something has slowly faded from view? Edit: an idea — instead of “can you still see it?”, ask “can you prove you can still see it?”. If the disk had an arrow then statistically you could measure how many times you fail to correctly see the direction in which the arrow is pointing. As the disk gets deeper you’ll see the failure rate increase and maybe there is either a clear spike in failures or, more likely, a point where the error rate is above 50% / guessing for that particular field scientist’s eyesight. Maybe you could even calibrate eyesight so that results from different teams match up?