Hi, in Unreal 4.18, you can use this function to get the average pixel intensity as the light estimation: UGoogleARCoreFrameFunctionLibrary::GetLatestLightEstimation. You can use the value as a multiplier to your scene lighting like directional light and ambient light.
You can checkout our HelloAR sample for details: GitHub - google-ar/arcore-unreal-sdk: ARCore SDK for Unreal