So when we display data the data will then need to be perceived by a human observer. That perception starts with the human retina. And so, we need to understand how the retina perceives light. We're going to learn about the eye and how does the eye sense light. And based on that knowledge of how the eye physically senses the light, we can figure out how small the details of a visualization can be and what color should you use in a visualization. So we're going to focus on the eye, and the mechanics of the eye. And this is a image of the eye and illustration from Grey's Anatomy. And the eye consist of a lens that focuses the light that receives from the aperture that is control by this iris. And it focuses that light onto this retina. And this retina contains the sensors that basically determine what light is hitting, what part of this image plane. So we don't have an image plane, it's more of a hemisphere. And the important part here is this fovea centralis. This is your center of vision. When you look at something, it's because that something you're looking at is getting focused onto the fovea centralis. This is where you have a lot of sensors that can tell a lot of detail about what you're looking at, then less sensors as you get farther away from that. You also have an optic nerve that connects to your retina, and you don't have any sensors here to be able to see the light that's coming in, so there's a blind spot that we have, and we don't even realize we have the blind spot, because our perceptual system is covering it up with, in a way that we don't even realize is happening. So this leads to visual acuity, the notion of visual acuity. How sharp is your vision? What can you see, how small of a detail can you present to a viewer? And we measure our visual acuity when we go to an eye doctor, when we look at one of these eye charts and it tells us our Snellen ratio. 20/X, which means you can distinguishes at 20 feet what the average person can distinguish at X feet. So if you can only see this letter E, then you have 200 vision, meaning you can see this E at 20 feet when the average person can see this E even as far away as 200 feet. And if you can see below this red line, you have better than 20/20, better than average vision. And the reason you can distinguish these letters [COUGH] is that their features project to a certain distance apart on the retina, and if your lens is doing a good job of focusing that information, then the sensors on your retina can resolve those details. If your lens is not doing such a good job, then the image that's getting presented isn't going to be resolved as well by your sensors. And so, your retina basically is going to consist of rods and cones. Rods perceive the brightness and cones perceive color. And there's three varieties of cones. Cones that perceive mostly red colors, [COUGH] cones that perceive mostly red colors, cones that perceive mostly green colors, and cones that perceive mostly blue colors. And then, there's a variety of additional processing that happens by the ganglion's, these other nerve cells after the retina. But this rods and cones are useful to look at for the moment. You have 80 million rods and only 5 million cones. So your intensity vision is much better by your color vision. Your rods are denser away from the fovea. Your fovea, your center of vision, is mostly cones, as it turns out. So astronomers have learned, when they're looking at very dim stars, to look slightly away from the star, look slightly off to the side of what they're studying. So that their rods can process the information and not their cones, because their rods are more sensitive to the changes in light. And they're quite sensitive, they don't even turn on when it's daylight. And then, you have cones, and they're, again, sensitive to various colors or various wavelengths of light. They're very dense in the fovea, anywhere from a 100,000 to 325,000 cones per millimeter squared in the fovea and they can distinguish about 150 hues and combined between the different hues, a hue is a colour in the rainbow. And the different shades and intensities and saturations, you end up with about 7 million different shades that you can distinguish with your eye. Depending on how on your lens, all lenses exhibits some form of chromatic aberration. And that means that refraction depends on the wavelength of light. Light at the red wavelengths is going to focus at a different location than light in the green wavelengths to the blue wavelengths, and this creates a dispersion that can create problems of focus, and this is why blue blocker sunglasses or amber sunglasses which are nothing more than just amber shaded filters on your sunglasses. They're going to age our vision because they're filtering out blue light and blue tense to focus off the retina whereas red and green tend to focus closer to the retina. So the blue wavelengths are actually blurring the image and the red and green wavelengths are generally sharper on the retina. So it'd be good to avoid pure blue text or at least desaturate it with a little bit of red or green colors to aid in focusing on edges of text or important figures. There's some interesting properties of color, this isn't necessarily chromatic aberration, but we tend to perceive the red is being closer than the blue, and this is our part of our perceptual system. For color perception, there's a difference in brightness, as well. Blue is generally a darker color than red or green. If you look at the pure luminance of a color, the luminance of that color will be 30% of its red component + 59% of its green component + 10% of its blue component. That means that green is six times as bright as blue, and green is twice as bright as red and red three times as bright as blue. And if you have yellow, you can create yellow in the additive color system by red plus green. Yellow is very bright. It's 31% plus 59% gives you 90% brightness. And you can figure out the luminance of these other colors based on that. Also, it's important to acknowledge that a good number of people, in particular, 10% of males, are color blind. And if you're displaying information with color, you may want to add additional ques to that information. So that people that are color blind can also understand it, and also to pay attention to contrast. Because if you're displaying something only with color, then people who are color blind aren't going to be able to tell the difference. Just color contrast and so you may want to change the brightness of the color as well and use this formula to make sure enabling colors have different brightnesses and addition to different use. And these colors, these red, green and blue colors yet change into a different color space. Space brightness, brightness shade. And then, a brightness minus blue, and a red minus green channel. So you get a color space that goes from black to white, yellow to blue, and red to green. This is the black white channel, this is the yellow blue channel. And this is the red green channel. That's what actually gets into the brain after some processing of the retinal image. And so, you're cones and rods will start to get exhausted [COUGH] if you stare at the plus sign in the middle of the screen here, you'll start to see that the purple dot that's missing instead of missing will start to turn green because it's turning into the complement of the purple color. And once it starts, if you keep staring at the plus eventually that green dot will eat up the rest of the dots and you're left with a solid gray screen. And this is all due to, basically, some exhaustion of the rods and cones in your perceptual system. So the human retina perceives light using rods and cones. Cones perceive color, rods perceive brightness. We have more cones near our fovea, our center of vision, and more rods in our peripheral vision. Also, we tend to expect warmer colors to be in the forefront, and cooler colors to be in the background. [MUSIC] [SOUND
