OK -

I visited the "Sparkfun" website and found information in TSL230R-LF-e3.pdf. A quick (actually a little more than that) viewing of the included "Photodiode Spectral Responsivity" chart indicates sensitity close to the "Extended Red" IR films we have today - starting at 300nm and ending ~ 1100nm, with peak at 700nm. That is close enough (from memory of film sensitivity) for use without filtration.
As a matter of fact, I can't think of a filter MILD enough to improve the "matching" - at all. Even a protective pice of glass MIGHT detract from th operation of this sensor.

With this in mind, I would trust information from this sensor - MUCH MORE than I would put faith in my own perception - remember, IR can NOT be seen.

Now ... "Sparkfun". What a web site!!! An Electrical/ Microprocessing Geek could overload here!!
I thought I was into some pretty "heavy" stuff in the Companies I have worked for - but these guys....

Great Googly Moogly ...!!!