19:32 Nov 14, 2010 

Whether porosity refers to that of some analysed material or to an X-ray image for instance does not change the oirigin of this imaging vocabulary.
As you indeed prefer void to pore in this context, you may wish to consider "void ratio" for "porosité", although they slightly differ in the original equation.

18:30 Nov 14, 2010 

"Porosité" is not referring to the porosity of an analysed material, e.g. rock - that would then be translated as "porosity".
The "pores" are on the image itself, therefore I have translated as "voids"
Here is the 2 paras. of text preceding that are included in the question - it may help with context:
"Deux lignes parallèles aux parois sont tracées, l'une localisée à proximité du bord et l'autre sensiblement au centre du canal, tel que representé sur les figures jointes.
Pour chacune de ces lignes (bord ou centre du canal) un nombre de pixels égal à zéro (solide) ou égal à 1 (pore) est obtenue.