Hardwood Flooring and IR - What causes the color change?

[zero-degrees]

I've always been curious by this but haven't been able to find anything online in the past... Curious if anyone here knows the actual root cause of this.

When IR hits true hardwood flooring - not laminate - it shows some pieces as an almost natural bare hardwood while other pieces still hold there stained color... Below are two example pics of what I'm talking about. The daytime picture you'll see the normal cherry stained flooring and the nighttime IR you'll notice how some of the landing actually looks like specific pieces are bare hardwood...

 

[bp2008]

I bet your wood was stained with more than one brand of stain. No doubt the stain-makers don't pay any attention to IR reflectivity properties.

 

[alphawave7]

Or different wood densities (esp. hardwood vs. soft), which have different IR absorbance/reflectivity properties.