[Dale(294engr]]

Gary,
A separate emitter & detector would eliminate the black IR absorption problem
at the cost of a bit more complexity,

i.e. two wired & optically aligned devices per location
(+ may need to shield ambient w/ tube or use modulation)

(1 loc pulse width of 7" ball or
2 loc's for distance traveled reducing error from dia & beam pos on ball)

Some teams may only have balls that are half black.

then I suggest the simpler method, spray paint test ball (black half or all)

Verified - works/repeatable:
simpler: one device position per loc, self aligning / shielding (sensivity adj)
I set up this test in 15 min w/old real time scope (DSO or storage better)
I was impressed with the results of two sensor time repeatability and

promising 1 sensor approach using 7" ball width to measure exit velocity (pro: sensor close to exit, con: error if beam not centered on ball) looks very promising, i.e. "simple as possible but not simpler" A.E.

??? re: retro-refl
Our IR & Vis Banner Sensors contain emitter & detector & range adjustment in one compact package and rely on retro reflected beam.

Broken beam approach requires physically separate Emitter & Detector & likely a hood (or modulated?) and good stable optical alignment

Dale(294engr]