Nikon D3100 Reference Manual page 107

When the camera is ready to measure white
balance, a flashing D (L) will appear in
the viewfinder and information display.
5
Measure white balance.
Before the indicators stop flashing, frame the
reference object so that it fills the viewfinder
and press the shutter-release button all the
way down. No photograph will be recorded;
white balance can be measured accurately
even when the camera is not in focus.
6
Check the results.
If the camera was able to measure a value for
white balance, the message shown at right
will be displayed and a will flash in the
viewfinder for about eight seconds before
the camera returns to shooting mode. To
return to shooting mode immediately, press
the shutter-release button halfway.
If lighting is too dark or too bright, the
camera may be unable to measure white
balance. A message will appear in the
information display and a flashing b a
will appear in the viewfinder for about eight
seconds. Return to Step 4 and measure
white balance again.
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