Theory Thursday #3 - All cats are grey at night

by Massive Voodoo

Welcome everyone to the third Theory Thursday!

If you are not interested in understanding how the world works, especially light and shadow, color and harmony, you should better skip reading this post and future Theory Thursday editions.
Opposed to the very direct and practical tutorials you will usually find here, these series of posts will go in depth to answer questions that many painters didn't even ask themselfes.

Be reminded that these posts are written by Raffa alone and reflect his understanding of the topics.
He is not always right and if you found an error or you want to discuss, use the comment section!

This week the topic will be:
All cats are grey at night

Have you heard this saying? I am pretty sure most people have.
But how much truth is in this sentence?

Let's start with a photo taken at daytime.
It's a photo taken at the Miniatur Wunderland, such a crazy place :D

We cann all agree that night- and daytime look different by a huge degree. But why?
Let's first analyse the one thing we know for sure: it's darker!
At night there is less light than at daytime (we're not talking about places with artificial light like lightbulbs, fluorescent light or similar, just the moon)

So if we take the photo from above and make it a bit darker, we should have a night scene.

Hmmm, this doesn't look like a night scene at all, more like the look through sunglasses.

Let's try to understand how the eye works at night to get a better idea why "all cats are grey at night".

The eye is a really complex piece of evolutionary excellence. We can see at night and day.
Compare this to a camera and you will quickly notice how awesome an eye is and how much range we are able to see.

We talked about color cones in the first edition of Theory Thursday.
Color cones enable us to see colors and brightness levels.
But they have one limitation, they need a lot of light to work, so they only work at daytime.

In low-light situations, their brothers, the famous rods are used to see.
Rods, like cones, are located in the back-inside of your eye.
They are much more sensitive to light, so they are useless at day (too much light).
Also, they need much longer to adapt to new light situations.
While cones can adapt to a maximum degree in several minutes, rods need more around 30-45 minutes to fully adapt to a low light situation.

And, the most important difference to the cones for our question:
Rods are all the same - to see color we have three different kind of cones (Red, Blue and Green).
Rods are all the same, they do not detect wavelength of photons but the amount of photons incoming.
So they produce a monochromatic image. Basically a Black and White image.

So, we already answered the question!
That's the reason all cats are grey at night.

But... isn't this a bit boring? Yes!
Let's dive a bit deeper into night vision!

Rods are not detecting different wavelengths of light, but they have a sensitivity towards a certain wavelength - blue (~500 nm) and they are almost blind to others - reds (~620 nm).

If we look at the visible light spectrum,...

image from Wikipedia by Deborah S Krolls 

We will notice that the sensitive area of the rods is pretty much on one side of the visible spectrum.
While rods are sensitive to blues and greens, reds and yellows are much less strongly perceived by them.

You can easily see the effect on a red rose. 
Look at it at daytime and the red will look much brighter than the dark green stem.
If you look at it at night (without artificial light), the stem will look brigther than the red rose.

So, not only will everything be grey at night, colors will shift in the appearing brightness compared to daytime. 
This is very important when painting night scenes or trying to understand the theory behind it.

Did you ever notice that the military uses red light at night? 
(sometimes, other colors [blue light to see blood] are used for different situations, just to state this before we get a shitstorm in the comments :D )

It's a trick to keep the rods adapted to the dark light while beeing able to see better at night.
Rods are basically blind to red light, so the cones are used to see it. Because the cones are used, the rods stay adapted to night vision.

Let's have a look at the first photo again
At night, it would look more like this:
Pay special attention to the reds and yellows, blues and greens!

To finish this article my conclusion is:
All cats are grey at night, but orange cats are darker.

Hopefully you enjoyed this third edition of Theory Thursday and read you next week :)

Always remember, painting is not algebra or physics.
Theory Thursday is about fun facts and interesting things around us.

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