A Perceptually Uniform Air Quality Index MeterI started this project during the wildfires of 2020. The air in Seattle was pretty awful, so many of us spent a lot of time refreshing the air quality report to determine if it was safe to go outside.
The index is helpful, and the visual isn't entirely unreasonable. However, I did notice that it had a few odd characteristics. Firstly, it seemed strange that each zone of the index was the same size, even though the ranges were different. For example, the lime green zone ranges from 0 to 50, but the purple zone ranges from 200 to 300.
A more proportionate meter would be spaced according to the values in each range.
There were also some concerns about accessibility in the colors. For example, if the meter were printed in black and white, there would be no apparent order in the shades. This is a violation of a principle called "perceptual uniformity", meaning that the changes in the values represented by the colors are reflected in the perceived differences between the colors themselves.
Take a look at this palette I created that is perceptually uniform. As the air gets worse, the color gets darker and more intense. In black and white, the model still works just fine. And, with the appropriate spacing accounted for, the fracility and imperative of staying in the lower rungs is even more apparent.