This paper is going to focus on the problematic of intra ocular pressure (IOP) measurements, performed by non-invasive methods. More specifically the devices that will be subject to the investigation are non-contact tonometers that use a concentrated air stream and optical sensors to determine the IOP within a human's eye. The paper deals with issues connected with this measurement method, by presenting a standard that is capable to calibrate these devices with traceability to the basic SI unit and measurement uncertainty. By introducing an independent and traceable calibration method of these device into the clinical practice the reliability of the measured IOP that is the primary indicator of eye glaucoma can be improved.