Thermographic Measurements of Hot Materials Using a Low- to High-speed RGB-camera : Prospect of RGB-cameras Within the Field of Thermographic Measurements

Abstract: Monitoring the thermal behavior of material while heated or cooled is of great importance in order to understand the structural behavior of materials. This thesis aims to investigate the prospects for imaging hot materials using an RGB-camera. The main motivation of using an RGB-camera is the very simple set-up and, in comparison to thermal IR-cameras, low price. A method and code enabling thermographic measurements in the temperature range of 800°C up to 1500°C has been produced. Calibration of the RGB-camera was made, the accuracy was predicted to be poor within the temperature range of 1000°C up to about 1200°C. The poor accuracy of the calibration within this range has its source in the non-linear (and irregular) response of the CMOS sensor prohibiting a valid exposure time function to be accurately determined. The calibration is thus performed with different settings (i.e different exposure times and aperture settings) but without any correction for the setting change. The validation experiments were performed in (or very close to) the temperatures where the temperature error was predicted to be the largest. An under-estimation of approximately 30-50°C in the temperature range between 950°C and 1015°C could be seen corresponding to an absolute error of about 3-5% in this temperature range. The accuracy is however thought to increase with the temperature above a temperature of approximately 1250°C . It is possible to perform a temperature transform of target images with temperatures above 1500°C since the Look-up-table produced for the temperature transform extends to about 3000°C . However, the accuracy is unknown since no calibration or validation experiments within these ranges were performed. The result of the calibration and experiments along with the theoretical assessment within the thesis gave background to the discussion of optimal imaging system for thermographic measurements. In order to receive more accurate temperature measurements, a CCD-sensor is to prefer since producing more uniform images with a more linear and predictable response. This would most likely enable the implementation of the camera setting influence. To receive better color accuracy but mainly to prevent channel overlap a system using three sensors instead of one (as for the current imaging system) is to prefer. This would result in larger freedom of channel choice and thus, the temperature transform can somewhat be customized for the specific measured temperature ranges. A brief discussion concerning the overall choice of camera was also brought up. Since the temperature range is large and the red, green and blue channels are sensitive to temperature changes the demands on the sensor dynamic range will be high in order to receive a linear response, or even a fully predictable response, across the whole temperature range. A suggested option to the RGB-camera, still offering the very simple set up, is a dual-wavelength camera in the near (or medium wavelength) infrared range.