## 3 Description of Pseudo-Coloring Method  
The specific nature of the compromising emanation signals (spurious emission) causes the images reproduced from them to differ significantly from the images obtained from sources such as cameras, X-ray scanners, or tomographs. Due to the low SNR values, coherent summation and filtering are typical methods of improving their quality. The resulting image is also subjected to contrast-enhancement operations. However, they are still presented using a palette of shades of gray. The perception of such an image (detail distinguishability) may be difficult, especially in the case of small changes in the signal value (Weber-Fechner Law). Moreover, for the human eye, image blur strongly reduces the sensitivity to luminance contrast, but the sensitivity to chromatic contrast is not reduced. For this reason, a more effective method of improving the readability of the image created from compromising emanation signals is to manipulate its color rather than its contrast. The approach proposed here consists of the conversion of images saved in the bitmap format, i.e., conversion from a one-dimensional color space (8-bit channel, grayscale color scale) to a three-dimensional space (24-bit channel), using the RGB color space (Figure 7). The 8-bit channel is associated with the 8-bit A/D converter used in the process of recording emission signals. Therefore, the software used for data visualization developed by the authors originally enabled the recording of the reproduced data in the form of grayscale images with an 8-bit depth. The method of multi-level thresholding of image pixel amplitudes for each RGB channel presented later in the article allows us to obtain a color image. The exponential function (Figure 8) was used to build the LUTs, which is described by Equation (1); the mathematical notation is presented for the R channel (other channels G and B are described analogously). The variables of the function are the width of the \(A_{E}\) waveform characterized by the zeros \(x_{1E}\) and \(x_{2E}\), the \(B_{E}\) shift of the function waveform along the pixel amplitudes of the colored image, the maximum \(C_{E}\) function (maximum value of the function waveform amplitude), which may be in the range \([0;255]\), and the minimum \(F_{E}\) function, which may be in the range \([0;255]\). Changing these parameters (Figure 8) enables the construction of LUTs with different values for each RGB channel. \[E_{R}(x)=\left\{ \begin{array}{l}F_{E_{R}}+\left(C_{E_{R}}-F_{E_{R}}\right) \cdot e^{C_{E_{R}}(x)}\text{ for }x\in\left[x_{1E_{R}},x_{2E_{R}}\right]\\ F_{E_{R}}\text{ for other }x\in\left[0,x_{1E_{R}}\right)\cup\left(x_{2E_{R}},255 \right]\end{array} \right. \tag{1}\]  
where:  
\[G_{E_{R}}(x)=\frac{-22.5}{A_{E_{R}}^{2}}\cdot\left(x-B_{E_{R}}\right)^{2} \tag{2}\]  
\[x_{1E_{R}}=B_{E_{R}}-\frac{A_{E_{R}}}{2} \tag{3}\]  
\[x_{2E_{R}}=B_{E_{R}}+\frac{A_{E_{R}}}{2} \tag{4}\]  
\(A_{E_{R}}\)--the width of the shape of the exponential function for the channel \(R\) defined as:  
\[A_{E_{R}}=x_{2E_{R}}-x_{1E_{R}}, \tag{5}\]  
\(B_{E_{R}}\)--value of \(x\) for which the exponential function \(E_{R}(x)\) has a maximum in the range \(\left[x_{1E_{R}},x_{2E_{R}}\right]\) (it is the middle of the range \(\left[x_{1E_{R}},x_{2E_{R}}\right]\); \(x_{1E_{R}}\) and \(x_{2E_{R}}\)--corresponding minimum and maximum value of range of \(x\) in which the values of exponential function \(E_{R}(x)\) are higher or lower than \(F_{E_{R}}\); \(C_{E_{R}}\)--maximum or minimum value of the \(E_{R}\) function set in the range \(\left[x_{1E_{R}},x_{2E_{R}}\right]\); \(F_{E_{R}}\)--value of the \(E_{R}\) function set in the range \(\left[0,x_{1E_{R}}\right)\)\(\cup\left(x_{2E_{R}},255\right]\). The shape of the analyzed function is not accidental. It is related to the form of characters contained in the reproduced images from the compromising emanations. The information diffusion channel, through which the radiated emission signal penetrates from the source to the receiver, has the features of a high-pass filter.