Dell Latitude monitor light fading in and out

I have a new Dell Latitude. When I first started working with it, a box asked if I wanted the screen to adjust to ambient light. Being new at this, I said yes. Now, I have a monitor which is constantly getting brighter and dimmer even within a few seconds. Does someone know how to stop this? Thanks.

 

If you hold down the "FN" key on the keyboard and tap the left arrow key, it toggles the Ambient Light Sensor on and off.

 

Thanks very much. What a relief!