## Materials and Methods
By considering an estimative \(n=1.5\) refractive index for the inspected material, Ducaram Kiss ceramics, for the central wavelength of the SS used, the axial range along the vertical in Figure 3 becomes \(z/n=2.5\) mm measured inside the material. Figure 2: Schematic diagram of the Master-Slave (MS)/Swept Source (SS)-Optical Coherence Tomography (OCT) system. SS: swept source; DC\({}_{1}\), 20/80 single mode directional coupler; DC\({}_{2}\): 50/50 single mode directional coupler; XYSH: two-dimensional XY lateral scanning head; L\({}_{1}\) to L\({}_{4}\): achromatic lenses; O: object to be imaged; PD\({}_{1}\), PD\({}_{2}\): photo-detectors; DA: differential amplifier; TS: translation stage. The outer surface of all the samples was positioned at a similar depth (\(z_{\text{surface}}\)) measured from the plane corresponding to a zero _OPD_; then, all _en-face_ images were acquired from a similar depth position (\(z_{\text{en-face}}\)). This depth of \(\sim\)0.375 mm was selected in order to provide as good as possible a reflectivity signal to characterize the granulation in the _en-face_ OCT image acquired from inside the material. In this way, as the reference and sample optical power levels were kept constant throughout all measurements, we ensured that changes in the brightness of the images are only due to changes in the optical properties of the samples. Moreover, by placing the samples as explained before, at a similar distance from L\({}_{2}\), we ensure similar lateral resolutions in all of the _en-face_ images. An immediate difficulty when the sample presents a curved input surface, for collecting an _en-face_ image from a fixed depth, as determined by the _OPD_ of the coherence gate applied, is that the _en-face_ slice does not cut a plane surface, but a curved surface from inside the sample. Therefore, the interpretation of what is being seen has to consider the curvature of the sample top surface. Therefore, maintaining the exact depth in all samples of the _en-face_ slices collected is somewhat difficult. Figure 3 depicts the depth selection where the _en-face_ images are acquired from. They are not rendered from the volumetric dataset, as performed by conventional OCT technology based on A-scans. They are generated in real time, directly, using a comparison method of optical spectra at the OCT interferometer output. We thus demonstrate that a single OCT _en-face_ image, at a suitably selected depth, can provide all of the necessary information for the evaluation of correct values chosen for the temperature settings of the oven. In order to better emphasize the effect of the temperature change, once the _en-face_ images are produced, the 500 horizontal lines on the \(y\)-axis in each image were averaged and the result was displayed as a function of the reflectivity with regard to the position on the \(x\)-axis. To quantify the effect of the change in temperature on the granulation in the _en-face_ images, a simple method was used: (i) each _en-face_ image was converted into a binary image and processed (the binary image replaces all pixels in the original gray-scaled image with the value 1 (white) if the gray-scale value is above a threshold (127 has been used on all our images) and replaces all other pixels with the value 0 (black); (ii) To quantify the granulation, the features identified in the binary image are counted and their size is  
Figure 3: Illustration of the guiding method to choose the depth from where the _en-face_ images are selected. The depth is changed by changing the spectrum stored (mask), as described in our previous reports using the MS technology. For all samples, their outer surface at \(z_{\text{surface}}\) was adjusted to correspond to \(OPD=0\), to insure that the _en-face_ images are acquired from a similar depth of \(z_{\text{en-face}}\) in all cases. estimated. This fulfills the aim of this study, different from that of imaging, oriented towards providing a simple, fast, and practical assessment of deviations of ovens from their best settings.