Dear Mr. B:

You wrote:

 When I read articles, I see different uses of a noun or -ing  after a preposition, and I get confused about what the author really means.

For example: 

For measuring brightness, methods are mainly based on the evaluation of the intensity reflected in the specular direction, when the surface to be analyzed is illuminated by a beam of light with a fixed angle of incidence.

For measurement of GSH uptake, cells were first pretreated with 0.25 mM acivicin for 15 min to inhibit GGT activity (Visarius et al., 1996; Lash and Putt, 1999).

After measurement of basal radial artery FMD the NOS inhibitor, NGmonomethyl-l-arginine (l-NMMA), was infused via the upstream brachial artery at a dose (4 mol/min) known from our previous studies to induce maximal reduction of radial FMD [13].

After measuring the produced ANG I, the endogenous ANG I obtained prior to incubation was subtracted.

 In assessment of the pupillary light pathway, both the direct response and the consensual response are tested.

In assessing prognosis, the speed at which tetanus develops is important.

In assessing alcohol intake, the history should also focus on whether alcohol abuse or dependence is present.

 Would you please help me with this matter by giving me more examples? (If you feel necessary)

What is the difference between the noun and -ing as a verbal in meaning? And when do we prefer the use of one to the other one?

You pick very dense material! The gerund “-ing” as in “for measuring” or “after measuring” refers the action. For example in the sentence that begins “for measuring” , you could say “in order to measure” or “when one measures.” It is no different with the sentences using “assessing.”

A measurement or an assessment is a specific act.

Having said that, they often can replace the other form but the wording would have to change. You would not say “for measurement of brightness” in the first one unless you were referring to a specific instance in which the brightness was measured. Even then, you would change the wording to “for a (or the) measurement of brightness.”

The first and last “assessing examples” could be exchanged as long as you used the articles and prepositions correctly. However, using “assessment” in the second sentence really does not fit because the example given is clearly hypothetical. (Also the expression “assessing prognosis” seems odd—after all, a prognosis IS an assessment. Unless one is examining various prognoses, it sounds stilted or redundant.)

The differences are subtle, and many times they can be exchanged with the other, as long as the articles and prepositions are used correctly. The advantage of using a gerund is that it can have direct objects or complements. The advantage of using a noun ending in “-ment” is that it takes modifiers like adjectives and prepositional phrases more naturally.