Why does purple light appear blue in cameras?
By: Selman Ay
Tags: Branding, Color, Research, Design, Experience

I knew that the first thing to do was some research. I thought there would definitely be a detailed article about this but I could only find limited information that explained cause and effect. But there must be a satisfying answer to this question. After acquiring some foreknowledge, I determined 5 questions that would help me reach the result:

What is the difference between purple and violet?

How do our eyes work?

Why do people see the color violet?

Why can’t some cameras capture violet?

Why can some cameras capture violet?

Now we can start by answering the first question.

What is the difference between purple and violet?

The basic difference between violet and purple is that purple is a color that can only be detected by the brain. On the other hand, violet is an actual colour which can be seen in the visible spectrum. Scientists reached this information by doing observations on the electromagnetic spectrum.

The color range which can be seen on the electromagnetic spectrum.

Purple is a subjective color that can be seen with the cooperation of the human brain and eyes. Purple color doesn’t exist in the real world because purple light doesn’t have its own wavelength. It is a mix of the two main colors which are red and blue. Purple light seems more saturated and warmer than violet.

On the other hand, violet has its own wavelength and it is between the colors of blue and ultraviolet. When you break the sunlight using a prism, you can see violet. This special color can be objectively identified with a physical point of view and it’s less saturated and thus colder than purple.

Besides, the color in the rainbow which is supposedly known as purple is actually violet.

Purple color palette and violet color palette.

In many cameras and color films violet appears blue but purple still appears as purple.

You can try this effect on the color palette above by using the camera on your phone. If you’re already reading this article on your phone, I’m sharing a photo that I took below. But you should definitely try it.

You can more clearly see the difference between the purple colored World logo and violet colored Getir logo. The actual colors of the logos are shown below the photos.

A brief comparison between purple and violet.

Now that we’ve learned the difference between purple and violet, we can move on to the next question. How do our eyes work?

How do our eyes work?

There are two different photoreceptor cells in our eyes. These are rods and cons. Rods are sensitive to the luminous intensity. They allow us to identify objects when it’s dark. Because they are not as sensitive as cons when it comes to perceiving colors, we see objects as uncolored when it’s dark.

The cons are responsible for perceiving colors. And they operate best when the ambient is bright.The wavelength of light absorbed by the retina and each of the cone cells respond differently, allowing us to distinguish colors. Cons have different types, each of which responds to different wavelengths. These types are:

Blue cones (S cone cells (S means small), these type responds to shorter wavelengths) Green cones (M cone cells (M means medium), for medium wavelengths) Red cones (L cone cells, L means long; for longer wavelengths)

A simplified scheme of the eye.

Now that we have an idea of how our eyes work, we can reach a more accurate result of our first question by using the information we’ve learned in this article.

Why do people see the color violet?

As a result of a strange response of the cone cells to longer wavelengths, we see the color violet. Yes, the thing that allows us to see this color, stems from a cell response that is strangely named and acts unusually.

Approximate response range of different cone cells in the human eye to light.

To speak more technically, we can say that the cone cells in our eyes respond to short and long wavelength lights, namely blue and red, around 440 nanometers (nm). And this is one of the most important features which allow us to see violet.

Another feature that allows us to see violet is that the L Cones have a strange, bimodal (having two modes) response in long wavelengths(shown with red).

S cones have a broad but well-defined response at the blue end of the short wavelength spectrum. Likewise, M Cones respond to a wide range of propagation on medium wavelength green.

Also, some people can see violet as blue. Although this situation is very rare, its reason is based on some types of color blindness. So if you see a color violet but your friend or customer claims it’s blue, believe them. You can access my other article about color blindness from the link below.

Why can’t some cameras capture violet?

Because sensors are used that cannot imitate the strange response of the cone cells to red light.

Here you can see the light-response curves of a camera that captures violet as blue.

The light-response curves of our phones’ cameras are similar to those of the graph above.

You too can compare the reaction time of the cone cells in the human eye and the camera of our phones with the help of the last two graphics above.

Most cameras, including phone cameras, respond to red light in 600 nanometers. That’s why these cameras cannot capture violet.

Why can some cameras capture violet?

Because sensors are used that can imitate the strangeness of the cone cells in the human eye. This enables them to capture violet just like the human eyes can.

More expensive cameras try to imitate the violet effect in the human eyes.

The graph above shows the response range of a Canon 450D to different lights. This camera can capture violet because it responds to red light in around 440 nanometers which is almost the same as the human eye. Thus us humans can see violet in images that this camera captured.