In order to determine the absolute threshold, you would go through a number of trials. During each trial, you would signal when you are first able to detect the presence of light. The smallest level that you are able to detect half of the time is your absolute threshold for light detection.
In hearing, absolute threshold is the smallest level of tone you can detect, considering you have normal hearing and there are no interfering sounds. For example, you might not be able to hear someone whispering from 20 feet away, but if that person speaks to a normal voice, it may reach your absolute threshold.
