Is this LED cover a commercial product?
Assuming it is pointed at a detector, it is most likely an optical collimating (focusing) lens. 

When when the photons are emitted from the LED, the photons spread out. 

If a spot lens is used, the radiant intensity increases by collimating the photons into a narrow beam.   

Example:

A 1200 mcd LED with a view angle of 60°
When collimated to a 1° beam, the intensity increases to 4,000,000 mcd.
Source:  Lumen Candela Calculator  

When comparing LEDs, if it is a visible color rated in mcd, you must convert the photometric mcd to radiometric Watts. 

The conversion factors for RGB are:
450 nm deep blue: 0.039
470 nm blue: 0.016
530 nm green: 0.002
625 nm  red:  0.005
660 nm deep red: 0.024
720 nm far red: 1.398

Deep blue is the most efficient LED radiometric and quantum (µmol) visible color.
Infrared is the preferred for photo detectors.   





Goolge Images of "led beam collimating lens"










  no, the lens is flat.


Notice how flat the exposed lens is.