I am doing some quality control on a bunch of affymetrix chip and I noticed a consecutively bright spot in every single one of these chips. I assume this has a technical use since I observed this in different chips. Can anyone tell me what it is? The image below is an example with exceptionally low overall intensity but that spot in the middle is shiny as always. Thus particular chip is Affymetrix U133 Plus 2.0. The image is generated by `image()` function of R on the affy object.

<img alt="" src="http://i.imgur.com/IcggNRi.png" style="height:611px; width:670px" />
That's the control probe set. You **should **see that on every chip, so it's useful as a QC indicator.