Mystery sensors on the Epson 4870 scanner - what are they?

I would like to know what are the two tiny devices (~1mm diameter and look a bit blue: silicon photo-diodes?) located adjoining the transparency brightness calibration lamp window?
I can see an aperture between them in the recessed bay, that is presumably used to measure the brightness of the scanner lamp and ensure it is warmed up and of a controlled brightness/colour prior to scanning a transparent target.
Are these two tiny devices just something comparable but for reflective scanning?
Grateful for any information,

Thanks, Bill