This article describes an accurate method to obtain the tone reproduction curve of display devices without using a measurement device. It is an improvement of an existing technique based on human observation, solving its problem of numerical instability and resulting in functions in log-log scale which correspond better to the nature of display devices. We demonstrate the efficiency of our technique on different monitor technologies, comparing it with direct measurements using a spectrophotometer. (c) 2007 Wiley Periodicals, Inc. Col Res Appl, 32, 310-319, 2007.