Accurate, absolute, and reproducible measurements are the expected qualities of any instrument. A differential image motion monitor (DIMM) is presented which assesses astronomical seeing with those requirements. It has been designed (with collaboration of the staff of the Department d'Astrophysique de l'Universite de Nice and the Instituto de Astrofisica de Canarias: DA/IAC) also to be reliable, simple, transportable, and easy to set up, operate, and maintain. Its accuracy has been studied, both from theoretical and experimental points of view, mainly with respect to CCD sampling, photon noise, and threshold influence on centroid determination. The DA/IAC DIMM has been cross calibrated with the existing ESO DIMM. It monitors the seeing with a temporal resolution better than half a minute. This type of instrument is essential for testing new and already existing sites and to ascertain that the surroundings of telescopes do not generate additional turbulence. It is also indispensable to optimize astronomical instrument selection in 'flexible scheduling.'