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<   No. 4507   2021-05-04   >

Comic #4507

1 Ponsonby: I am just pondering something philosophical.
1 Mate: Do tell, Captain.
2 Ponsonby: How often must ships pass one another on a stormy night like this, unable to see each other, each crew sure that it is alone in the vast sea?
3 Mate: You know it’s the middle of the day?
4 Ponsonby: Wow, this is some storm.
4 Mate: Aye.

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The light has gotten more blue since a couple of strips ago, which is probably why Captain Ponsonby thought that night had fallen.

The cone cells in the retinas of our eyes provide us with colour vision, but they work best in bright light.[1] The more densely packed rod cells give us the high resolution of our vision, and they also operate well in dimmer light - however they don't provide the finer wavelength discrimination of the cones that leads to our perception of colour.

Of the three types of cone cells, the ones sensitive to long wavelength light (that we perceive as "red") drop off in sensitivity the most severely at low light levels. This means that as light grows dimmer, our ability to perceive the colour red fades away faster than our ability to perceive green or blue. Also, the rod cells are more sensitive to shorter wavelengths than to longer ones. Red objects become very dark and start to look black, while green and blue objects can still be perceived to have colour. Next the green sensitivity falls away, until at very dim light levels we effectively can't see red or green, and the only colour we're left with perceiving is blue. This is known as the Purkinje effect, and is important in the regime of mesopic vision - where the cone cells are working at reduced effectiveness compared to bright light, but are not yet totally dominated by signals from the rod cells.

This is the reason why we tend to think of night time scenes as bluish in tone, and why movie makers have long produced movies with night scenes having a definite blue tone to them - even if they're filmed during the day and made darker, the colour grader will usually make the scene more blue, by darkening any red objects more heavily than green and blue objects.

[1] For a refresher on colour vision in humans, see #3258

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