The fundamental question regarding vision is the visual threshold: under optimal viewing conditions, what is the minimum amount of light required to elicit a light sensation in the observer? This condition requires the observer to remain in darkness for a sufficient period of time to fully adapt to the darkness. This also involves the probability of seeing and individual variations, but a numerical value can be used to illustrate this: for 50% of people, there is a 50% chance of visual perception. A large object presented to the eye for a long period of time can be seen when its luminance L is 10-6 candela/m2. This value is often cited in practical lighting design. The general rule is: the smaller the object, or the shorter its presentation time, the higher the luminance required to elicit a visual sensation.