Lux Measurement

Hi all i'm trying to create a feedback circuit using a ambient light sensor to record the Lux of my light source and adjust the current to the lamp as the lamp itself ages. The purpose of this is to have a system in place so that as the lamp ages the same amount of light is always coming out for its whole life. The problem i'm having is that my lumen range is between 1200 lumens to 2800 lumens and in this application when I convert that to lux it ends up being between 4million lux and 11 million lux. The light sensors I am using can not measure a value above 500,000Lx. I need some ideas as to how to measure the lux on this high lux lamp as well as keep the circuitry as small as possible. Does anyone have a solution to my problem. Any help would be great.

Thanks,
Dave