Low Cost Colour Measurements with Improved Accuracy

Daniel Wiese, Karlheinz Blankenbach Pforzheim University, Engineering Department Electronics and Information Technology Tiefenbronner Str. 65, D-75175 Pforzheim, Germany Contact: Fon: +49 7231 28 – 6658 ; Fax: - 6060 ; Email: kb@displaylabor.de Abstract Colour measurements are usually performed by measurement devices in the range of 1,000 to 100,000 €. We have evaluated the low cost approach with the MAZET MCTS 2 colour system regarding absolute accuracy and usability for display applications. The USB-powered sensor is available with standard software which is capable to measure absolute colour co-ordinates with an universal calibration. Additionally, this software allows calibrating the sensor to a special target (display, light source) with reference images or high end colour measurement devices. Because the original software is more dedicated to perform various measurement tasks rather than display relevant applications, two programs were developed by us. One is for automatic calibration with a YOKOGAWA colorimeter, the other one captures the most relevant optical PC-monitor parameters (max. luminance, dark room contrast ratio, gamma values, colour co-ordinates and gamut) by just one click. The accuracy of the MAZET MTCS 2 was evaluated in two ways: First, measurements and calibration with the original software and second by using of well known calibration algorithms in our own software. All programs show nearly the same results for measurements of PC-monitors: the absolute accuracy (with common calibration) we achieved is about ∆ E* ≈ 10 - 15 and, if the sensor is calibrated to the display, ∆ E* ≈ 1 - 3 ( ∆ E* is the colour difference acc. CIE 1976 LUV). Therefore the MAZET MCTS 2 shows an excellent performance when calibrated to the display under test. For absolute measurements its accuracy is however limited. 1 Introduction Colour measurements are necessary in many applications like evaluating of specifications as well as calibration of displays and control of colour light sources (e.g. LED backlights of LCDs). In multimedia systems usually device-dependant colour co-ordinates like RGB are used (e.g. stored in a file) and this leads often to colour shifts when displaying this on another device-dependant system like a monitor. So a better way is paved by device-in dependent colour systems like CIE. As a consequence of this, RGB-based devices have to be measured for grey scale, colours and gamut with standard colours (e.g. GRETAGMACBETH COLOURCHECKER) in order to generate corrections files (ICM). In display metrology it is always a task to find the right balance between accuracy and costs. There are several colour measuring instruments available, but high accuracy means also high price. On the other side, many applications require a colour measurement capability that is cost-effective and has a sufficient accuracy. The latter can only be achieved with low cost sensors that are calibrated to the specific display (target). In this article we focus on the cost-saving task by evaluating the accuracy of MAZETs [1] low-cost colour sensor MTCSICS on a new (2006) evaluation board (MTCS 2).