USE CASES AND APCA TERMINOLOGY

[Myndex]

IMPORTANT TERMS TO KNOW

TERMS EMERGING FROM SAPC/APCA RESEARCH

In the course of research here, we have a few terms that are specific to the use case(s) and I'd like to clearly define them. These terms were created in the interest of clear and plain terminology that is descriptive and easily understood with little to no special explanation. I.e. the terms themselves are intended to be easy to grok, to help keep things simple, short, and digestible. (Some of the definitions need to be reworked, and moreover, visual aides created.)

Screen Lightness (L^s) — L^s means the perceived lightness/darkness of the display screen in the standard observer environment. This is similar to L^* (Lstar) of CIELAB and CIELUV. L^s is part of the SAPC model.

Lightness/Darkness Contrast (L^c) — L^c or "Lightness Contrast" is the perceptual lightness contrast value generated by the APCA algorithm. Lightness Contrast is engineered to follow human supra-threshold perceived contrast of two elements of different luminances.

Readability Size — Readability size is a combination of the fluent critical font size (relative to visual angle) at or above the critical contrast (in the standard observer environment) in the given use case, using L^c.

Readability Contrast — Readability contrast is a combination of the critical contrast (using L^c) at or above the critical font size.

Weight Contrast — this defines the contrast of high spatial frequency items, especially stimuli that have a stroke width less than 4px. It applies to fonts and any "stroke type" stimuli. The weight contrast of stimuli thicker than 4px, or a visual angle of 5 arc minutes, is fairly constant supra-threshold (contrast constancy). The weight contrast of thinner stimuli are relatively more affected by spatial frequency than by luminance.

Whitespace Contrast — refers to the readability contrast increase caused by increasing whitespace (line spacing, letter spacing, etc....)

Page Font — a specific font face as actually used on a page of content. There may be several differentpage fonts on a page.

WCAG3 Reference Font — a specific defined reference font face that is used for comparison to a page font, to determine offsets or ratios for specific font metrics.

Equivalent Readability Size (ERS) — Ratio (or offset), the difference in x-height of a specific Page Font relative to the x-height of a WCAG3 reference font.

Equivalent Readability Weight Ratio (ERW) — Ratio (or offset), the difference in perceived weight contrast, measured in L^c, of a specific page font relative to the weight contrast of a WCAG3 reference font.

WoB (White on Black text or "Reverse") — Light text on a darker background, results in a negative L^c value (APCA).

BoW (Black on White text or "Normal") — Dark text on a lighter background, results in a positive L^c value (APCA).

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USE CASES CLARIFIED & RELATED CONDITIONALS (APCA)

FLUENT

• Fluent Readability, Block/Body Text

  • Defined as:
    • a block or column of more than two continuous lines of text that:
    • uses a body font with an x-height of ~19px or LESS.
      • This means 34px Verdana or 42px Times (or smaller)
      • This is relative to the non-zoomed initial page load at 100%
      • MINIMUM font size: x-height of 9px (16px Verdana)
      • MINIMUM weight: 400 for fonts under 24px (x-height 12)
      • Minimum weight 300 for fonts above 24px.
      • MINIMUM contrast: Lc 75
    • is not a nav, menu, tool-tip, etc (those fall under fluent content)
  • slightly higher readability contrast/size is required for body text than other fluent content
  • is substantially impacted by line height and other whitespace contrast metrics
  • normally has the highest spatial frequency of any content on a page
  • ZOOM: Body text has zoom priority over other adjacent text items.
    • When zooming text independently of other page items, body text should be the last text type to break or go into a horizintal scroll.
    • UNSETTLED: Should body text be the keystone text for zoom operations?
    • MORE WORK NEEDED: in terms of achievable guidelines on zoom amounts and relative zoom.

• Fluent Readability, Primary content that is not body text

  • up to two lines of continuous text (per the normal non-zoomed layout)
    • No larger than an x-height of 80px (Verdana 144px or Times 178px)
    • No smaller than an x-height of 8px (Verdana 14.4px or Times 17.8px)
    • MINIMUM weight: 300 for fonts under 24px (x-height 12)
    • Minimum weight 200 for fonts above 24px.
    • MINIMUM contrast of Lc 60
  • includes headlines, captions, and images of text, if they contribute to content
  • primary navigation and primary menus
  • asides, tool-tips, spoilers, "continued"
  • user entered text in forms, fields, etc
    - does not include text the user controls or adjusts in size or color
  • ZOOM: Fluent text has zoom priority divisions (work needed)
    • RELATIVE: fluent content text that is normally larger than adjacent body text should remain larger than body text when zooming, but the percentage of increase can and should be relaxed such that larger text does not increasein the same percentage as the smaller body text.
    • MORE WORK NEEDED: in terms of achievable guidelines on zoom amounts and relative zoom.

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SPOT (NON-FLUENT)

• Spot Readability, secondary and ancillary "non-content"
  • the most obvious case is copyright, byline, footer matter
  • Must still be well above the legibility limit
  • Does not have to meet critical size nor critical contrast
    • No smaller than an x-height of 5px (Verdana 9px or Times 11px)
    • contrast no less than Lc 30
  • Category can include secondary/tertiary nav such as to TOS/EULA
  • This category includes placeholder text in forms, and disabled controls.

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NON LEXICAL

• Non-Text Discernible Elements

  • Discernible means that while non-lexical, they carry specific meaning
  • Examples:
  • Emoji/color pictogram: ✈️ ❤️ 🚫 ⏮⏹▶️⏭⏏️ 🚴‍♀️ ☠️
  • Unicode ones: ☏ ☹⃠ ⇦ ⇨ ♘ ☜ ✇ ⃣
  • Solid vs outline require different contrasts, much like fonts: ♜♛♕♖♠︎♣︎♥︎♦︎♤♧♡♢
  • Coga issues: which is the home button? 🏚 ⌂
    • which is the hamburger menu? 🍔 ☰ ䷀
    • what are dots supposed to do if I click them? ⫶⃨
    • if I click this will I be happy? ☺︎
    • Is this radiation or a roll of recording tape? ☢︎
    • do I click this to call? ☎︎ Or does it just open a phone book?
    • Hey this one is hollow ☏ what does that mean? Is it supposed to be so much harder to see?

• Non-Text Functional Elements

  • button shape or form (not the text)
  • input field, forms, text areas (but no the text)
  • element focus and focusing indicators
  • inline link indication (but not the text itself which is separate)
  • meta states, visual user feedback (i.e. a blink on enter, etc).
  • temporal/spatial state changes (including state changes to text, separate from the text's contrast).
  • EXAMPLE: Buttons and WCAG2 vs WCAG3 APCA:
  • 
  • And that one single pass WCAG 2 did correctly, the background was a fairly high #b8b8b8, with black #000 text. In every other case, both the text and button were darker than #a0a0a0. The reason I mention this: one of the proposed SCs for WCAG 2 was "at least one color must remain above the equivalent of #a0a0a0". While it is not a perfect solution, it helps WCAG 2 avoid dark color pairs, where it is known to fail by incorrectly passing color pairs that shouldn't.

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• Aesthetic and Divisional Elements
  • Simply anything that is purely decorative, non-readable, not needed for understanding content.
  • It can include contrasts that may be helpful for organizing content but are not strictly necessary.
  • These have no minimum contrast standard EXCEPT:
• Aesthetic and Divisional Elements SHALL NOT distract nor interfere with readable content.
  • GUIDANCE: avoid placing text on textured backgrounds.
  • GUIDANCE: take care to avoid placing decorative icons near functional (clickable) icons
    • EXAMPLE: is one of these the home button? 🏚 ⌂ 🏠🏡⌂⌂
    • EXAMPLE: when does a letter pictogram mean check email? ✉︎ 📬 📩 📨 📧 📥 📤 𖧕 𓊭 𓊭 ✉️ 💌
• If the aesthetic or divisional element helps a non-excluded element achieve a needed level of contrast, then the aesthetic element is not considered decorative, but is then grouped with the element type being helped.

EMPIRICAL BASIS FOR THE FOREGOING (IN PART):

Reading rate (wpm)	Minimum acuity reserve
Spot (∼40 wpm)	1.3:1 (0.1 log unit or 1 line*)
Fluent (≥100 wpm)	2:1 (0.3 log units or 3 lines*)
Maximum	3:1 to 8:1 (0.5 to 0.9 log units or 5–9 lines*)

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"Normal" Vision Defined

Normal Vision is a specific definition, and a somewhat clinical definition:

• Snellen acuity of 20/20 or lower (20/16 is "perfect" acuity, 20/200 is SSA disabled)
• Peli-Robson contrast sensitivity of 1.95 or higher (2.25 is "best")
• Farnsworth Munsell Hue Color TES of 60 or less (TES 0 is perfect)
• 80% Visual field with MD no lower than -2db (0 is best, -20db is SSA disabled)
• The age-related baseline normal is ages 20 thru 40.
  • Below 20, contrast sensitivity is still developing, so young normal includes a lower contrast sensitivity.
  • Above 40, presbyopia is a normal development, so mature normal includes a lack of near-distance acuity.1

Footnotes:

• _The WHO sets average vision at 20/40. For the US, EU, Japan.
• Depending on the study, "normal" typically includes "with refractive correction" if the correction can achieve the required score.
  • An example: "normal with correction needed for presbyopia", presbyopia being "normal" for over age 40 for instance.
• SEE ALSO the SAPC Standard Observer Model_

copyright © 2019-2021 by Andrew Somers and Myndex Research™. All Rights Reserved.

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Myndex Infographics

Critical Font Size

NOTE: review is ongoing in particular to align actual fonts to relevance with this table.

Physical Device Visual Angle per Distance

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CITED RESEARCH (selects)

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TERMS FROM RELATED RESEARCH OR STANDARDS

From Choeng/Lovie-Kitchin/Bailey/Bowers, also Legge, Arditi, Whittaker, and others. This includes some of the terms as commonly used in research.

• Acuity reserve The difference between the current print size and the smallest print size giving the maximum reading rate (critical print size) is the patient's acuity reserve. (AR as measured in lines is valid only when using a near-chart with a logarithmic progression of sizes in 0.1 log steps, such as Bailey–Lovie word or text charts or MNRead acuity charts.)
• Near visual acuity: the smallest print that can be resolved at the near test distance (example, 40 cm/16") indicates near visual acuity
• Text visual acuity (text VA): threshold print size
• Reading Rate (RR) in WPM: words per minute
• Maximum reading rate (MRR): the mean of the reading rates for print sizes at and above CPS
• Standard word: A standard word is six characters.
• WPM: words per minute
• Print size (N point, Lovie-Kitchin definition) Point measurement refers to the overall dimension of the type body from the top of the ascending letters to the bottom of the descending letters, with the lower case letters being half the point size; 1 point = 1/72 inch (0.35 mm). Point sizes are commonly preceded by N, for example, N8 (8 point print), which was first used by Law simply to indicate ‘near’.
• Critical print size (CPS): the smallest print size that gives maximum reading rate
• The M print size notation refers to the distance in meters, at which the overall size of the lower case letters subtends a visual angle of five minutes of arc.

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FOOTNOTES

• This first post may be edited periodically.

• When discussing the areas of vision and readability, there are a few terms that are important, which while they are used in research are not generally used in accessibility. However they are increasingly important to know as modern vision research makes its way into accessibility.

• On the Visual Contrast Wiki, the RESOURCES PAGE has a glossary and definitions, mostly they are the terms used in vision research or in colorimetry.

• Here I am going to try to limit to only those terms directly relating to readability.

• LOW VISION

  • LEGGE: low vision patients needed magnification that gave at least five times threshold size and a five-character field of view was sufficient.