Nikon D5100 Reference Manual page 19

When the camera is ready to measure white
balance, a flashing PRE(_-,-,=)will appear in
the viewfinder and information display.
5
6
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.
Check the results.
If the camera was able to measure a value for
white balance, the message shown at right
will be displayed and Gd 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 no ud
will appear in the viewfinder for about eight
seconds. Return to Step 4 and measure
white balance again.
/1_, ",,
85