If you re an iPhone user and you are facing the brightness dimming issue on your IOS device, then you need to make sure that your battery health should be good.
1) Unlock your iPhone & check the battery percentage
2) If it is low, then try to connect the charger
3) If the brightness is still low, then you have to replace the battery of your iPhone from the nearest Apple Service Centre.
If you want to know more solutions regarding this problem, you can read here: https://emailspedia.com/why-does-brightness-keep-going-down-on-my-iphone/