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From: Rafael C. Caruso

Date: 2022 Mar 18, 05:45 -0700

During the transition from night through twilight to day, the sensitivity of the rod pathway decreases as background (sky) brightness increases. In these conditions, the rod pathway is said to "saturate", and loses its ability to mediate the detection of dim stimuli.  If you wish, the "common name" for this property of human vision in bright light, is "rod saturation".  In daylight (so called "photopic" conditions, with bright background brightness), maximum light sensitivity is mediated by cone photoreceptors and their pathway to the brain. This reaches its peak sensitivity in the fovea at the very center of the retina. Therefore, averted vison does not help when attempting to see a small and dim target (e.g., Venus) in daylight, and foveal vision is ideal for this task.

There is also the 'blue field entoptic phenomenon' , especially noticeable when looking into bright blue light, such as a cloudless sky. Whereas red blood cells absorb blue light, white blood cells do not, so they let the light through to the retina. Light-sensitive cells in the retina read this as a signal of increased brightness, causing us to see the white blood cells as spots.

-K