Imaging technique for inspecting the inside of coal-fired boilers in operation using a visible-light camera

We developed an imaging technique for inspecting the inside of coal-fired boilers during operation by using a visible-light camera, which has a higher spatial resolution and depth of field than mid-infrared image sensor camera systems commonly used for imaging the inside of boilers. Two main problems are associated with the use of a visible-light camera: the low light transmittance of combustion gases in the visible spectrum and the large non-stationary variations in intensity levels. To overcome these difficulties, a narrow-band imaging technique using a bandpass filter was employed to identify the wavelength range with the highest transmittance through gases. Contrast adjustment and noise reduction via conditional averaging of the temporal images were performed during post-image processing. Results obtained from the tests of several bandpass filters indicated that bandpass filtering centered at 650 ± 5 nm is optimal for obtaining clear images. Adaptive histogram equalization was found to be more effective for contrast enhancement than other standard methods. Additionally, a clearer image can be obtained by conditionally extracting and averaging temporal images with high Pearson product-moment correlation coefficient values for the average of all images. Finally, a clear image of the inner wall of a 1000-MW-class coal-fired boiler during operation is presented to demonstrate the feasibility of the proposed method. Graphical abstract