In modern web design, color contrast can be measured one of two days—[[Color contrast can be measured simply using the WCAG 2.x relative luminance contrast ratio|simply, by WCAG's relative luminance formula]], or more precisely, by APCA. APCA stands for the Accessible Perceptual Contrast Algorithm. ## The Problem with WCAG What I like about WCAG's relative luminance formula has been that it's pretty simple. It's pass/fail. It's also founded on visual acuity, i.e. AA conformance is meant to help people with 20/40 vision, AAA conformance helps people with 20/80 vision, etc. But that simplicity is exactly what undermines it. There's a much wider range of human *perception*, and perceptual impairments. These include visual acuity, contrast sensitivity, and color vision deficiency. And contrast, believe it or not, is not a simple measure of the difference between colors (or "relative luminance"). Contrast between colors is sensitive to context. ## How APCA Works ## Understanding APCA Results ### Bronze Simple Mode APCA seems a bit harder to understand, but my best understanding right now is this. It has a "Bronze Simple Mode" which offers a set of threshold levels related to use cases. * APCA has a range of Lc (lightness contrast) values that go from 0 to LC ±106. * Lc 15 is the point of invisibility for many users * Lc 90 is preferred for body text * [See here for more details on the ranges](https://git.apcacontrast.com/documentation/APCA_in_a_Nutshell#use-case--size-ranges) ### Silver and Gold Lookup Tables But there is more. Silver and Gold add lookup tables, to associate font weight and sizes to the lightness contrast (Lc value). See, for example, the Figma plugin Zebra and how it depicts these lookup tables: ![[APCA_SilverGoldRatings.png]] Notice the colors, which indicate spot readability and fluent readability. The goal is *fluent readability*. > - **The contrast sensitivity threshold** **(CS)** is the point of “just noticeable differences” (JND). That is, the point between visible and invisible. > - Fluent readability refers to critical contrast. This is the smallest amount needed for best reading speed and comprehension. Peer-reviewed science tells us¹ contrast should be at least ten times the JND. The preferred contrast reserve is twenty times threshold for best fluent reading. > - Spot readability means readable without significant effort. Spot reading is the lowest readable level, where the contrast needs to be three times the JND. This low level is useful for disabled controls, copyright bugs, and other non-content. ## Reference [Baldwin, Nate. "APCA (Accessible perceptual contrast algorithm)" (Color & Contrast, accessed July 2024).](https://colorandcontrast.com/#/apca) [Myndex. "Why APCA as a New Contrast Method?" (2022, Myndex Research).](https://git.apcacontrast.com/documentation/WhyAPCA)