Determining the invisibility of an optical cloak intended to hide something has become a complex problem in recent years. There are many invisibility mechanisms, the performance is quite different from technique to technique, and it is desirable to have a precise metric for their comparison. Here, we propose a simple metric that assesses the perceived invisibility. This invisibility index is based on the fact that the human visual system (HVS) is highly sensitive to spatial frequencies, and then uses the Fourier transform and the contrast sensitivity function of the HVS to assess invisibility.