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The minimum physical intensity, size, or duration of a visual stimulus (light, pattern, object) that can be reliably detected by the observer a specified percentage of the time (usually $50\%$ or more).
read more about visual threshold ...Absolute Threshold (minimum light detectable in the dark). Difference Threshold (minimum difference in brightness/contrast detectable).
In automated visual field testing (perimetry), the machine measures the local retinal threshold: the dimmest light stimulus that can be detected at specific points across the retina.
Thresholds are measured using psychophysical methods like the Method of Limits or the Method of Adjustment, often employing staircase algorithms in modern perimeters.
A stimulus presented at an intensity significantly above the absolute threshold, making it easy to detect, used in screening tests.
Yes. Due to retinal aging and changes in the optical media (e.g., cataracts), the light threshold generally increases (becomes dimmer) with age.
The ratio between the difference threshold and the background stimulus intensity, which remains relatively constant over a large range of light levels (Weber's Law).