The diploma work ˝Measurement of hardness with the Right-Light method˝ was elaborated to determine the measuring error when the hardness of softer materials is measured by Brinell method. Namely, softer materials indicate a tendency to deform plastically in the immediate vicinity of the indentation, which increases its apparent diameter and consequently the lower hardness is measured. By using Ring-Light illumination, however, this additional deformed area does not affect the size of the measured indentation diameter, allowing more accurate measurements. In order to determine the minimal material hardness, when hardness measured by those two illumination modes becomes significantly different, we measured a considerable number of samples and compared the results. From the obtained results we determined the limit minimum value of the material hardness (> 250 HBW), to which the Brinell hardness measurement with classical illumination is still acceptable. For a more detailed analysis of the apparent increase in the indentation diameter due to plastic deformation, the sample with the largest difference in the measured hardness values was cross-sectioned through the indents’ height. With optical microscopy, we analyzed the deformation area just below the indented surface area and in the specimen matrix. At the edge of the indent, the deformation area is clearly visible, which apparently increases the indents' diameter. Microhardness measurements confirmed that the sample just below the surface of the indention is work hardened, while the analysis of the microstructure did not show any changes between the matrix and the edge of the sample. We also checked the influence of the surface roughness and find out that if the scatter of the results has to be as small as possible, measurements of hardness must be done on ground surfaces.