## 4 Discussion  
In the equipment TEMPEST evaluation, mainly upper case letters are used, because it facilitates the video recovery process, which represents the \"worst case scenario\" mentioned repeatedly in the TEMPEST procedures. Thus, the video recovery process for the font Arial was repeated, but this time with the use of the upper case letters and numbers, as illustrated in Figure 12. We performed the tests for secure asymmetrical font again and the results are shown in Figure 13.  
Figure 11: Image recovered with Times New Roman font—R&S FSWT receiver. Figure 12: Image recovered when Arial font with upper case letters was displayed. Figure 10: Image recovered with Font 28 with blank spaces: secure asymmetrical and secure symmetrical—R&S FSWT receiver. Color coding in RGB format (red, green and blue) is done by combining in some proportions these 3 primary colors, with values between 0 and 255. Thus, the blue color is coded for example [0,0,255] or \"0000 FF\" in hexadecimal format (HEX) while black is represented by [0,0,0] or \"000000\" (HEX) and the white color of [255,255,255] or \"FFFF\" (HEX). The inverted video option practically means the reversal of the white color with the black, meaning the consideration of the values [255,255,255] for black and [0,0,0], for white respectively. So it does not represent an additional processing of the video CE signal, but it is only the option for a TEMPEST engineer regarding the visualization of the restored image. In the opinion of our specialists, the images restored with the \"reverse video\" option enabled are more intelligible and less tiring for the eyes. Due to the fact that the TEMPEST specialist spends several hours each working day with such images displayed, this technical aspect related to the working mode is very important to them. The restored images illustrated in Figures 6-13 were performed with the \"reverse video\" option enabled. Without activating this setting, the image shown in Figure 14 is obtained. We can consider that the text is intelligible for the font size of 36, 28 and 24 and not for all the letters, this statement being valid only for size 36 in case of Arabic numerals, and even at this size, the number 2 can be easily confused with the number 8. However, it can be observed that the date of the measurements cannot be understood for the secure asymmetrical font with a size less than 28 and for the secure symmetrical font with a size less than 24. We consider that for the font size 12, no alphanumeric character is intelligible, regardless of the font considered, secure asymmetrical or secure symmetrical. It is also true that the tested fonts are difficult to track even in the Word Office editing environment and that the human operator needs an adaptation period to use these fonts. In conclusion, we consider that the fonts are efficient in terms of resistance to the video recovery process and that the secure asymmetrical font is more efficient than the secure symmetrical one, although the characters of the secure asymmetrical font have been restored with a better contrast. The documents edited with size 12 are impossible to recover if any of these fonts would be used. Figure 14: Image recovered when secure asymmetrical font with upper case letters—without reverse video option enable. Figure 13: Image recovered when secure asymmetrical font with upper case letters was displayed. Next, we considered that dimension 24 represents the intelligibility limit for the two TEMPEST fonts analyzed. Thus, we continued to use as a test video signal a page with size 24, corresponding to all four previously used fonts. The image recovery results corresponding to this video signal can be seen in Figure 15.  
Here, we can identify all the numbers for the secure symmetrical font and partially for the secure asymmetrical. The most readable characters are 'b', 'g', 'k', 'p', 'q','s' and 't', while the numbers 2, 3 and 4 can be identified, even for the secure asymmetrical font.