Two types of three-level quantum counters, based on the Bloembergen and the inverted scheme, are discussed with respect to their application for up-conversion of far-infrared (FIR) radiation to visible light. It is reported on a tunable quantum counter of the inverted type: in optically pumped ruby, photons at 29 cm-1are absorbed by the transitionbar{E}(2E) →2bar{A}(2E) and lead to an increased R2- fluorescence radiation. By Zeeman splitting of the2Elevels, tuning of the detected frequency from 29 cm-1to higher and lower frequencies is possible (with 1 cm-1/Tesla). Using the 891-GHz radiation of an HCN laser we determine for the quantum counter a power conversion efficiency of the order of 10-6and a frequency bandwidth of 360 MHz. We find the interesting result that phonon bottleneck due to resonant trapping of 29 cm-1phonons can lead to an increase of the quantum efficiency by orders of magnitude.