When the monitor is calibrated with the built-in sensor, why does its contrast fall (black level rise)?

If the ambient light level changes when calibrating, the built-in sensor cannot measure correctly. This may result in a contrast ratio that is too low (and a black level that is too high). Please be careful to stabilize ambient light when calibrating with the built-in sensor. In order to minimize the influence of ambient light, we recommend you do the following: -Shut out external natural light in the room through the window; close curtain or pull down the blinds. -Use the monitor hood. -While measuring, do not move your face or another object close to the monitor and do not look into the sensor. -Avoid placing the monitor in an environment where ambient light can directly enter the light receiving section of the sensor.