Screen brightness varying in lenovo thinkpad t420 while using on battery

I have owned Lenovo ThinkPad t420 laptop in which I have windows 7 operating system and I am facing problem with my laptop from last some days. In my laptop whenever I use my laptop by connecting the charger at that time it works fine but when I remove the charger and after that when I use my laptop at that time brightness in my laptop changes unknowingly. now please tell me how to stop this issue.

First of all you have to check certain things in your laptop. Check whether you are using latest graphic drivers for your laptop or not. You have to also make sure that your BIOS version is updated. If all these things are latest and then after also you are facing this problem then I think you are facing this problem because you have checked "Display Power Saving Technology" option in your graphics option. if you want to solve this issue then you should try these steps to solve this problem. First you have to right click on your desktop and after that you have to select the graphic properties. Now after opening the graphic properties you have to select the power option and after selecting the power option you have to select the power feature and after that you have to select the "On Battery" from drop down menu option in Graphics Power Plan Settings. now after doing that you have to uncheck Display Power Saving Technology" option and after that you have to click on the ok. Now after clicking on the ok you have to check whether you are having the same problem or not. i think after trying this solution your problem will get solved. if you need any more information regarding this then please let me know.

first I thought that this problem occurs because of the some LCD related issue and after reading your reply I have update my graphic driver and BIOS also but after that also I was also facing the same problem and then I have uncheck the Display Power Saving Technology option from the graphic properties in my laptop and after doing that my problem get solved. Thank you for helping me.