Abstract: A real-time calibration instrument on a processless writing device reads a pattern of marks produced by the device and adjusts writing parameters in real time through feed back mechanisms to modify the characteristics of the marks and their pattern. The calibration instrument includes a photoelectronic camera defining an optical axis perpendicular to the recording medium, and the marks are illuminated coaxially along the same axis to capture an image of the marks without distortion. According to other features, the recording medium is rotated on a drum relative to the recording head and the illumination is provided by a stroboscopic source synchronized with rotation of the drum.