An application Note

The optimal contrast setting for LCD displays varies with ambient temperature. For most
applications this variation in contrast is tolerable over the “normal” temperature range of 0°C to +50°C. Most LCD modules are available with an extended temperature range option
which allows the display to operate from-20°C to +70°C. The changes in contrast are NOT
usually tolerable over this wide a range of temperatures, which means a way of adjusting the contrast voltage as the ambient temperature changes must be provided

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PDF =>
http://www.newhavendisplay.com/app_notes/Temperature_Compensation.pdf