APCA in a Nutshell

Accessible Perceptual Contrast Algorithm is a new method for calculating and predicting readability contrast. APCA is a part of the larger S-Luv Accessible Color Appearance Model known as SACAM (formerly SAPC). These models are specifically optimized for accessible color appearance on self-illuminated RGB computer displays & devices, and also for modeling accessible user needs, with a focus on visual readability.

APCA is the candidate contrast method for the future WCAG 3, and is also developing as the APCA Readability Criterion, an independent standard hosted by Inclusive Reading Technologies.

Lightness contrast Lc

The APCA generates a lightness/darkness contrast value based on a minimum font size and color pair, and this value is perceptually based: that is, regardless of how light or dark the colors are, a contrast value of Lc 60 represents the same perceived readability contrast. This is absolutely not the case with WCAG 2.x, which overstates contrast for dark colors to the point that 4.5:1 can be functionally unreadable when one of the colors in a pair is near black. As a result, WCAG 2.x contrast cannot be used for guidance designing “dark mode”.

The APCA contrast value is perceptually uniform, and pivots near the point where the CS curve flattens due to contrast constancy. Halving or doubling the APCA value relates to halving or doubling the perceived contrast. There is a subtle weighting for higher contrasts to smaller, thinner fonts, and a small boost for very dark colors.

Different Uses, Different Contrasts

The APCA-RC Bronze Simple Mode has a set of threshold levels related to use cases — for instance, Lc 90 is preferred and Lc 75 is the minimum for body text. This makes for an easy way to use ACPA, very similar to the old WCAG 1.4.3 in terms of ease of use.

The APCA also has optional advanced levels Silver and Gold which add in lookup tables, to associate font weight/size to the lightness contrast (Lc value). The lookup tables allow for even greater accuracy and therefore greater flexibility in design.

Failing Pass/Fail

A key takeaway is that a strict pass/fail with a blanket contrast ratio is not instructive as a guideline, and does not necessarily solve a given user need. In fact, user needs when it comes to contrast are conflicting—what is good for one can be harmful to another. This is even true of font size.

This points to the importance of real user personalization, an area where the technology is literally missing (and a work in progress). For the guidelines though, we can set ranges for targets and expectations toward making the web readable for all, with the recommendation that the user is able to choose at least one light mode or one dark mode color scheme.

Use-Case & Size Ranges

These general levels are appropriate for use by themselves, without the need to reference a lookup table. APCA reports contrast as an Lc value (lightness contrast) from Lc 0 to Lc 105+. For accessibility, consider Lc 15 the point of invisibility for many users, and Lc 90 as preferred for body text.

These define the basic minimum levels, what you might think of as AA in the old WCAG 2. For the equivelent to AAA, simply increase the contrast values by Lc 15.


Expanded Range and use cases

Instead of arbitrary pass/fail binary scoring, the APCA Readability Criterion has a sliding scale across the visual range, and is divided among use cases, as different uses have different contrast needs.

The overall approach improves design flexibility and readability at the same time. Readability is improved by increasing contrast in blocks of body text where it is most needed, and design flexibility is achieved by relaxing contrast for large non-text elements which do not need brute-force contrast levels, due to their larger size (i.e. lower spatial frequencies use lower contrasts).

For demonstration purposes, the example tool provides real-time updates of minimum font size and weight vs contrast: https://apcacontrast.com click on the color patches to bring up a color-picker.

More Info

See this Linktree for a short-list of resources for further reading: linktr.ee/myndex


Definitions of Terms Used In This Document

Study Volunteers Needed

Would you like to help create a more readable world for all? Please let us know! We have several studies planned for 2023, and it requires minimal time on your part to participate. Please send an email to perceptex@myndex.com with “volunteer” in the subject line.

APCA The Revolution Will Be Readable

APPENDIX: Additional Reading

APCA Readability Criterion

Maintained by Inclusive Reading Technologies Inc., a California Non-Profit.

Discussion Forum

Peer Review & Third Party Discussion of APCA

A listing of third party and peer reviews of APCA and related technologies. This directory includes journal-published peer-reviews, trade-published evaluations, and less formal comparative analysis, covering the usage, math, efficacy, implementation/integration, workflows, and more.

Published Articles on Color & Contrast by A.Somers

Uncle Sam saying I want you to use high contrast text