Understanding Color in XR
Extended reality (XR)
Before diving into colors in XR space, we should understand three elements necessary for us to perceive color light, vision, and object.
💡 Light
The visible light is the small part within the electromagnetic spectrum that human eyes are sensitive to and can detect.
Visible light waves consist of different wavelengths, and the color of the visible light depends on it. These wavelengths range from 700 nm (red end) of the spectrum to 400 nm (violet end).
👁️ Vision
Though eyes detect the visible light, it involves a complex process after through which it is interpreted into different colors. Our eyes’ retina contains photoreceptors — specialized cells that are sensitive to light and relay messages to your brain. There are two types of photoreceptors: cones that are sensitive to color and rods, which are more susceptible to intensity sees things in black, white, and grey. You can “see” an object when light from the object enters your eyes and strikes these photoreceptors. Cones only work when the light is bright enough, but not when light is very dim. This is why things look grey, and we cannot see colors at night when the light is dim.
There are three types of cones in the human eye that are sensitive to short (S), medium (M), and long (L) wavelengths of light in the visible spectrum traditionally known as blue-sensitive, green-sensitive, and red-sensitive. These three types of color receptors allow the brain to perceive signals from the retina in different colors. It is estimated that humans can distinguish about 10 million colors.
🧱 Object
Objects appear in different colors because they absorb some colors (wavelengths) and reflect or transmit other colors. The colors we see are the wavelengths that are reflected or transmitted. White objects appear white because they reflect all colors. Black objects absorb all colors, so no light is reflected.
Creating colors with light
The primary colors of light are red, green, and blue. Mixing these colors in different proportions can make all the colors of the light that we will be able to see. The same method is used to create colors on digital devices like TVs, Projectors, and also XR devices. When colored lights are mixed together, it is called additive color mixing. It is done by adding one set of wavelengths to another. So if we combine all the colors of light, we will see white.
In contrast, with paints, subtractive color mixing is used to create new colors. Wavelengths are deleted by using the absorption property of paints. When the paints of different colors are mixed, both paints still absorb all of the wavelengths they did previously, so what we are left with is only the wavelengths that both paints reflect. So unless like lights, when we mix all of these colors together, the mixture will absorb all the light and only see black because no light will be reflected back to your eyes.
Each pixel in the digital displays creates color this way using additive color mixing, by the combination of red, green, and blue LEDs.
For example, in RGB, when R is set to 0, the Red LED is turned off. When the R is set to 255, the LED is turned fully on. Any value between them sets the LED to partial light emission.
So when RGB is set to (0,0,0), all LED is turned off and it becomes black, when it is set to 255,255,255 we see white, and at (127,127,127) we see grey.
As we saw earlier, Red, Green, and Blue are the digital primaries that cannot be created by mixing other colors but mixed to all other colors. Secondary colors are produced by mixing two primaries,
Red+Blue= Yellow
Blue+Green=Cyan
Red+Green= Magenta
Tertiary colors are produced by combining the primary and secondary colors. Tertiary colors are formed in sets as there are different transitional steps between both.
Similarly, by increasing or decreasing the intensity of the LED, different shades and tints of colors are produced.
How AR device works
Now we know how human vision works and digital displays produce colors. In XR, VR headsets are pretty simple when the LED is turned off, it turns black. But on AR and MR devices, when a LED is turned off, it creates a transparent screen, or if the intensity of the light is reduced the opacity increases. We need to better understand colors as it will also get affected by the environment.
Brightness of the environment,
The outdoor light level is approximately 10,000 lux on a clear day. In the building, in the area closest to windows, the light level may be reduced to approximately 1,000 lux. In the middle area its may be as low as 25 -50 lux. With additional lighting equipment depending on activity the light level is in the range 500 -1000 lux-. For precision and detailed works, the light level may even approach 1500 -2000 lux.
The color of a wall in the real world or the color of objects might also affect the color of the augmented objects. Lets see how different colors react to different environments,
Mobile AR
HMD
Hue
Shades and Tint
As you can see how the same color is visible to human eye differently based on the environment light, background color and type of the optical system on the device.
As designers when designing for AR we should be careful about the type of environment. Devices like lux meter or mobile apps to measure brightness comes in handy to understand the environment.
