Scanning Near-field Optical Microscopies suffer from the low signal to noise ratio, due to the smallness of the diffracting probe used to get images. Therefore a lock-in amplifier is commonly used to perform homodyne detection. From the lock-in data, we reconstruct the near-field intensity diffracted by the probe-end in the case of approach curves. We show that the reconstructed and the detected signals can strongly differ. The reconstruction of the signal is necessary to give physical interpretation.