LEDs emit red, green and blue light to the surface to investigate, and photodiode detects the scattered emission. We have two tasks: LEDs must don't irradiate photodiode, and as much as possible scattered by surface light must be collected by photodiode. 

We made some test holders to make a selection. We tested 5 types of holders and select the bes.

Fig. 1. Scetches of differend LED- Photodiode holders.

Fig. 2. 3d models of  LED-Photodilde holders.

Fig. 3 Slicer picture of LED-Photodilde holders.