Links to blank web pages for using
a computer screen or tablet computer
as a photographic flat field

(White and with very light color tints
to overcome color bias in the screen)

WHITE

REDDISH

YELLOWISH

GREENISH

CYAN

BLUISH

MAGENTA

For those who need longer exposures:

GRAY

DARK GRAY

DSLR shutters are often uneven at exposure times shorter than 1/200 second.
Use exposures that long or longer when possible.