Overview of human vision - Python Tutorial

Human vision, of course, is a fundamental factor that helps us process and see, visualize, and analyze any kind of visualization. So it's instructive to take a quick look at how our human vision actually works. So here we have a cross-section of a human eye. Light comes in, in this case, from the left-hand side of the slide, goes through the lens, gets projected into the retina at the back of the eye. In the retina, you have different kinds of cells, essentially that are responsible for processing different kinds of light. In particular, you have rods and cones. Rods process luminosity, so how bright or how dark a specific image is. You can think of them as being a black and white vision. Then you have cones that essentially process actual colors or all the colors that we're able to perceive. In particular, you have three different kinds of cones. One that processes blue, one that processes green, one that processes red. If you ever wondered why the RGB standard exists in video, this is the reason because that's exactly the colors that our brains and our eyes evolved to process appropriately. This is how essentially the visible spectrum is defined. And what this means in particular is that a lot of the colors that we think we see don't actually exist as colors themselves. What they actually correspond to are essentially different combinations of these three primary colors. So you can see here, for example, that the blue cones tend to respond to light between, say, 300 and maybe 550 nanometers while the red cones, for example, span the range from 400 to almost 700. So you see there is a huge amount of overlap for these two different kinds of cells, and actually a lot of the colors that we see are actually made-up of different combinations of these primary colors. So on the bottom part of the figure, we have the entire spectrum from violet on the left to red on the right. on the right that essentially shows you how to convert specific actual color so specific frequency and wavelength of light and how that gets perceived by the human eye so for instance if you look at the 600 nanometer range and just draw a vertical line straight up you will see that this line will intersect the green cones at around, let's say, 40 and the red cones at maybe 90. So what that means is that that specific wavelength of light, which is somewhere in between yellow and orange, is actually made-up, as far as our brain is concerned, as about maybe 80% of red and maybe 20% green. So this kind of helps you understand first how the RGB kind of standard works and why we represent colors with these three primary components and combinations of these components. But also, so it helps you understand that we have some colors that are much more easily perceived than others. And of course, the reason for that is that they tend to appear in nature in different contexts. So you have blue, of course, for the sky and the ocean, you have green for trees and grass, you have red for fire and light and sunsets, and so on. And another interesting aspect to notice here is that, for instance, when you look at the range of wavelengths that's covered by the rods that essentially control how bright you perceive a specific image to be, you notice that they're actually limited to a relatively short range of wavelengths. Particularly it goes roughly from 400 to 600 nanometers, and what this means is that you can have essentially how much brightness you want outside of this range that you will still perceive the image or the environment as being dark. So this is actually the reason why you might you might have come across essentially a night vision mode on your device or red light in specific contexts at night because essentially you're able to see in the red spectrum, kind of highlighting here between 600 and 700 nanometers, because there the red cones are still fairly sensitive, so you're still able to see fairly well, while at the same time it doesn't register as being a bright light so it doesn't destroy your night vision so you don't lose the ability to actually still see in the dark and if effectively get the best of both worlds we're able to see with limited light what because your brain is registering the the environment as being dark and opening up your pupils to let in as much light as possible and on the other hand you're able to to have a significant source of light that you're able to see well. Now, colors, as we perceive them, are essentially made-up of combinations of these three components. But, and of course, there's a huge range of possible colors we're able to perceive, even though these are not necessarily, or don't necessarily correspond to a specific wavelength of light. One obvious example is the color pink. there is no pink wavelength. Another example would be the color brown. There is no brown wavelength, but these are still colors we're able to use and perceive because they're made-up of combinations of other colors. It also means that we don't have a very clear idea or a very clear picture of where one color begins and another color ends. So here we have a quick visualization of a large online survey where essentially people were presented with colors within the the RGB space and asked essentially to name them and you can see how some colors or some color names tend to apply to a much wider range of colors while others tend to be very much more specific and this corresponds also to the range of wavelengths that are covered by each of the cones in your eye and also how sensitive they are at different wavelengths.