Final answer:
The retinal image is ambiguous because it lacks sufficient information for definitive color or lightness perception. Equivalent stimuli are different inputs that result in the same perceptual experience, exemplified by color constancy and lightness perception parallels.
Step-by-step explanation:
When saying that the retinal image is ambiguous, it refers to the idea that the sensory information provided by the image alone isn't sufficient to reliably determine the true attributes of the visual scene, such as color or lightness. Our perception uses additional context and other cues to interpret these attributes accurately.
Equivalent stimuli are different physical stimuli that produce the same perceptual experience. An example of this in color perception would be experiencing color constancy, where an object appears to maintain its color despite changes in lighting conditions. The eye-brain system utilizes mechanisms, such as comparison of the light reflected from the object with the light from surrounding objects, to establish a stable perception of color.
The trichromatic and opponent-process theories offer explanations for how our visual system processes color. Experiments by Edwin Land with the retinex theory support the idea that our perception can discern full color through comparison of images. Similarly, when analyzing lightness perception, our visual system is sensitive to edges and contrasts rather than just to uniform light intensity.