The term 'Just Noticeable Difference' refers to:

The term 'Just Noticeable Difference' (JND) relates specifically to the smallest difference in intensity between two stimuli that a person can reliably detect. In the context of visual perception, this often pertains to how much a light spot must differ in intensity from its background for a viewer to perceive that difference. This concept is closely associated with Weber's Law, which states that the perception of change in a stimulus is proportional to the initial stimulus's intensity.

When considering light and visual perception, the JND reflects the sensitivity of the visual system to changes in luminance. Therefore, the correct choice reflects the relationship between a light spot and its background, which is crucial for visual discrimination tasks in various settings, including clinical assessments of visual function.

The other options, while related to visual perception, do not directly capture the essence of what the JND specifically measures. The intensity of the light source and the thresholds or maximal responses of photoreceptors pertain to the limits of detection and response, rather than the specific differential sensitivity that JND addresses. Thus, identifying the difference between the light spot and the background represents the fundamental concept of detectable change characteristic of JND.