[BioC] backgroundCorrect offset value

[Prasad Siddavatam]

Hello Users,

I have a question regarding the usage of backgroundCorrect function in LIMMA.

when I do the following with offset 50, I am getting 2900 differentially
expressed genes
RG.b <- backgroundCorrect(RG, method = "normexp", offset = 50);

where as, when I do the following with offset 1, 
I am getting 1300 differentially expressed genes
RG.b <- backgroundCorrect(RG, method = "normexp", offset = 1);

Please advise which offset value to be used? Why is offset value making
so much difference?

I am using this for TWO channel data, which is read by "genepix".

Greatly appreciate your help.

Prasad