KYORITSU 3131A - Manual (Page 15)

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7. Back Light Function
To facilitate working at night or dimly lit situations, a back light function is
provided which illuminates the display.
To operate this function, the back light button must be pressed and released
while pressing the test button. The back light continues illuminating for
approx. 40 seconds and then turn off automatically.
When the test button is released, the back light will turn off even within the
lighting time.
Fig.7 How to use the back light button