Direct and in-direct measurments I have seen and heard this many times that a certain (physical) measurement is "direct" or "indirect". What is the characteristic or definition that sharply separates these two notions?
 A: I'm guessing a direct measurement is one you would read directly from your measuring instrument, like a weight on a scale or the size of an object read from a tape measure.  An indirect measurement might require a calculation, like the mass of a planet from the orbital period of a moon.
A: There is no sharp distinction. The usage of these terms simply is one of context to indicate that one measurement requires less calculation to infer the desired quantity from the experimental data when compared to the other method.
The reason for that is a deeply philosophical one: Every scientific measurement is always an indirect one. For example, if you want to measure the mass of a stone, you put it on a balance, sure, but really that is not a direct measurement either, if you have a digital balance, your actual measurement is of the electronics creating an electrical signal on the display, which in turn you don't see directly either but you see photons from the display hitting your eye, which really is a measurement of the response of your retina to it's current illumination, which really is a measurement of your synapses propagating electrical signals ... Etc.
The one exception I know is in contemporary dark matter detection, where "indirect" became a standing term that refers to the search for astrophysical signals, and "direct" a term to refer to searches for signals generated in laboratory experiments. But that's just unfortunate and inaccurate jargon.
A: Consider measuring the absorbance, at 600 nm, of a solution of copper sulfate in water. The solution is blue, so it absorbs 600 nm light.
For an indirect measurement, a cuvette containing the copper sulfate solution could be placed into a spectrophotometer. The intensity of 600 nm light transmitted through the cuvette would be recorded in the usual way. Then another measurement would be performed, on a cuvette filled with water. This cuvette content is the reference or blank. Ignoring photodetector dark response, etc., then the transmittance is the ratio of the two transmitted light intensities, i.e., intensity transmitted through the copper sulfate solution divided by intensity transmitted through the blank. The absorbance is calculated as the negative logarithm of the transmittance.
For a direct measurement, consider a pulsed dye laser producing light pulses at 600 nm wavelength. Then, via one of the several photoacoustic spectroscopy methods, a photoacoustic response will be produced when the cuvette containing copper sulfate solution is illuminated with laser pulses at 600 nm. The response, whether from a piezoelectric transducer or a microphone, is directly proportional to the absorbance. For the blank, very little response is produced because water has low absorbance at 600 nm.
So the difference is that the direct measurement method produces a response that is proportional to the absorbance while the indirect method uses measurements from which the absorbance is calculated.
