Glossary: Terminology Emerging From APCA Research

[Myndex]

TERMINOLOGY EMERGING FROM SAPC/APCA RESEARCH

This was moved from the use case thread to make it easier to access and update:

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.)

COLOR and LIGHT

• Field Adaptation or alternately screen adaptation: When viewing self-illuminated displays, field adaptation refers to the light adaptation as a result of the total screen luminance, which is a peripheral adaptation, larger than the local adaptation of text and adjacent background, and smaller than the global/ambient adaptation.

• Y_s
  Estimated Screen Luminance — Y_s means the adjusted relative luminance of the display screen in the SAPC standard observer environment. This is similar to Y of XYZ. Y_s however, it may have modifications for a particular colorspace or vision accommodation.

• L^s
  Screen Lightness — 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/SACAM model.

• L^c
  Lightness/Darkness Contrast — L^c or "Lightness Contrast" is the perceptual lightness contrast value generated by the APCA algorithm, based on achromatic luminance. Lightness Contrast is engineered to follow human supra-threshold perceived contrast of two elements of different luminances, and for specific spatial frequencies.

• H^c
  Perceived Color Contrast — this refers to the hue/chroma/saturation differences between colors, irrespective of lightness, and recognizing that spatial frequency affects hue markedly different than luminance (i.e. 1/3rd the resolution of luminance).

• C∩
  Color Set — beyond measuring a simple color pair, a color set is the color pair, and also the larger page background, larger page RMS contrast, and the ambient environment (and in some cases additional stimuli).

TEXT SIZE and CONTRAST

• 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) in conjuction with the font's critical weight, and at or above the critical font size (or element size) for a given use case.

  • Regarding Critical Size and Contrast
    • See Bailey/Lovie-Kitchin for the definitions of critical size and critical contrast—these terms mean the nexus point wherein increasing further does not result in an improvement in reading speed nor comprehension.

• Critical Weight — the font weight, or element thickness, needed to achieve a specific critical contrast level for a given color set.

• 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.
    • NOTE: in the world of typography, there is the term "font contrast" but this does not mean the contrast against a background, it means the contrast of the thicker strokes against the thinner strokes within a glyph.

PAGE and LAYOUT

• Whitespace Contrast Effect — 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 different page fonts on a page.

• WCAG 3 Reference Font or APCA Reference Font — a specific defined reference font face that is used for comparison to a desired page font, to determine offsets or ratios for specific font metrics, most importantly weight and x-height.

• Equivalent Readability Size (ERS) — A Ratio (or offset), of the difference in x-height of a specific Page Font relative to the x-height of a WCAG3 or APCA reference font. (For all uppercase fonts, then relative to the uppercase X height).

• 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 or APCA reference font.

USER PERSONALIZATION

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

  • Edit: this is now popularly being referred to as "dark mode"

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

  • Edit: this is now popularly being referred to as "light mode"

• HiCon — a high contrast mode, where the standard colors on a page have increased distance. IMPORTANTLY, APCA guidelines suggest that smaller thinner text elements are given greater contrast increased than large bold thick elements (which in some cases should not be increased).

• LoCon — a lowered contrast mode, where the standard colors on a page have decreased distance.IMPORTANTLY, APCA guidelines suggest that large bold thick elements are given greater contrast decreases than smaller thinner text elements, such as body text (which often shold not be reduced).

• FlexZoom or Proportional Zoom — zooming textup to specific sizes, and not by a uniform percentage. I.e. the smallest text zoomes the most, the lagest text zooms by a smaller percentage, and only to maintain "being larger" but not by the sam proportions. This recognizes that there is a limit to how large a large font shold be zoomed.

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