Absorb:
Shine a light with known intensity into the object.
Measure the intensity of the light that comes out.
Any light that goes in but doesn't come out the other side
was absorbed by the object.
(Note: You also need to be aware of any light that bounces back from
the first side without entering the object. That light is reflected from it,
and can't be expected to show up at the other side.)
Transmit:
Any light that goes in and DOES come out the other side
was transmitted by the object.
