YSI 63 - Manual (Page 21)

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4.9 Display Backlight
At times it may be necessary to take measurements with the Model 63 in dark or poorly lit areas.
To help in this situation, the Model 63 comes equipped with a backlight which will illuminate
the display so that it can be easily read. To activate the backlight, press and hold the LIGHT
key. The display will remain lit as long as the key is pressed. When you release it, the light goes
out to preserve battery life.