A figure looks sharp on the laptop and faint on the projector. What should change before the talk?
Leave margin for a hall that steals contrast and for eyes that miss color. Use dark ink, thick lines, colors that differ in lightness, and names on the lines.
Expect the room to wash the screen out.
Becausea projector cannot make black. Its black is the bare screen under the room’s lights, so every lamp raises the black and lowers the contrast. Suppose room light adds a tenth of the screen’s white everywhere. Black on white then falls from 21:1 to under 8:1.
Give text and lines more contrast than the minimum.
Becausethe web’s accessibility guidelines, WCAG, figure their minimums from the file’s colors alone. They ask for 4.5:1 for ordinary text, and 3:1 for large text and chart lines. In the example room, a gray that just passes at 4.5:1 drops to about 3.5:1.
Draw lines at 2 points or more.
Becausethe 0.25 to 1 point that the journal Nature asks for suits a page in the hand, not a hall. From six screen-heights away, for example, a 1-point line on a slide 540 points tall spans about one minute of arc. That is the stroke width of the 20/20 line on an eye chart, and acuity falls with contrast.
Choose colors that differ in lightness as well as hue.
Becauseabout one man in twelve has a color vision deficiency, most often trouble telling red from green. Room light also thins every color toward white. A difference in lightness survives both. On a white slide, pair a near-black with a middle tone, such as navy and vermillion. Each clears 3:1 against white.
Write each name on its line.
Becausea word needs no color vision, and WCAG asks that color never be the only signal. A reader who sees two lines as one hue can still tell which is which.
Another way
Vary the line itself, one solid and one dashed.
Becauseshape needs no color vision and survives a grayscale printer. Past two or three lines the patterns crowd, and labels do the work more quietly.
Check
Turn the slide to grayscale and stand back six times the height of the laptop screen. Every line should still show in its own gray, and every label should read easily. Lines that turn the same gray may look alike to a reader with red-green deficiency.
KeepDraw for the back row, lights on.