Legible in the room

Claim: text is readable from the back of the room: dark on light, high contrast, large type.

• Grade: measured for polarity; the minimum size is a convention.
• Evidence: review, strand 4 and strand 8; position, Known and Where practice runs ahead.
• Check: slide-type-size (heuristic), sufficient-contrast (exact, from reader-ergonomics).
• Reader styles: none changes it.

What it asks

• Use dark text on a light ground by default.
• Reach a contrast ratio of at least 4.5:1, or 3:1 for text of 18 point or larger. The rule is sufficient contrast.
• Set type large: 18 point or larger (Microsoft 2026d, convention), and count lines, 10 to 20 per slide (Beamer 2026, convention).
• In rehearsal, read the slide from the farthest seat (TED 2026, example only).

Why

Across three experiments, dark text on a light ground read best, both on a laptop and projected under several room lights. Pleasantness followed legibility (Greco 2008, secondary).

WCAG 2.2 asks for 4.5:1 on text and 3:1 on large text, meaning 18 point, or 14 point bold (W3C 2023b, convention).

Limits

• No study of projected size and comprehension in a room was found. Size research is for reading on a page or screen (Legge 2011; Rello 2016).
• Every numeric size is a convention.
  • 30 point (Kawasaki 2005), 17 point (Peyton Jones 1993), 18 point (Microsoft 2026d).
  • Type of at least 1/40 of slide height, and a title of 6 words (SSP 2026).
• Beamer 2026 itself says points mean little for projection.
• Practice splits on the ground. TED 2026 says bold text on dark. Beamer 2026 warns that inverse video fails in bright rooms. Schwabish 2014 reports dark ground in a dark room as usual advice and finds no reason given.
• Whether a dark ground beats a light one at room distance in a dark room is not known: advice says yes, the one projection experiment says no.
• AVIXA 2016 sizes an image from the farthest viewer; the formula is in the paid standard and was not checked (secondary).