## 3 Results  
### Calibration of the UV Illuminance Uniformity  
Calibration of the UV illuminance uniformity was undertaken by generating a greyscale mask. Typically, the brightness can be set in an 8-bit greyscale from 0 to 255. However, this does not mean the LCD matrix can display all shades of grey. In addition, it is also not clear that the intensity of the transmitted UV radiation is proportional to the greyscale value. Therefore, the following measurements were taken initially:  
* UV intensity distribution on the LCD matrix, Figure 8;
* UV intensity as a function of greyscale at selected measurement points, Figure 9.  
In the research for this article, measurements were taken with a resolution of approximately 12 mm (measurement grid with 18 columns and 10 rows). It is not difficult to see that, in the case of the Anycubic M3 Premium 3D printer used in the study, the radiance intensity does not change with a resolution of every one greyscale unit. To evaluate whether the 12 mm measurement grid spacing significantly impacted the accuracy of the final correction mask, a high-resolution scan was conducted over a 31 mm segment, which includes more than two full standard grid areas. The scan was performed at coordinates x = 60 to 91 mm and y = 50 mm, in a region previously identified on the irradiance map as exhibiting high local variability. In our printer, the UV exposure  
Figure 8: Mean value of measured UV intensity on LCD surface. system consists of a single collimated lens with a custom-shaped reflective mirror. This design is intended to provide uniform light distribution without the overlap of artifacts commonly observed in systems using multiple parallel lens arrays (as found in some MSLA printers with tiled LED modules). In those systems, narrow bands of elevated irradiance often occur at the interfaces between adjacent light sources, leading to sharp local peaks in UV exposure. In contrast, the optical system used here is expected to produce smoother irradiance gradients. Based on the high-resolution scan and visual inspection of the irradiance map, we estimate that the spatial interpolation error does not exceed 0.05-0.1 mW/cm\({}^{2}\) because of the use of a grid of 12 mm instead of 1 mm, which remains below the minimum effective correction step of the LCD panel and therefore has no practical impact on the correction mask generation. As the change is stepwise and not linear, a tree regression model was determined for further calculations (Figure 10). Figure 10: Relative UV intensity. Figure 9: UV intensity as a function of greyscale at selected measurement points. However, to use this model for further calculations at any point in the matrix, the values of the measurements were normalized against the maximum value (3), as follows:  
\[I_{norm}=\frac{I}{I_{max}} \tag{3}\]  
As the waveforms are very similar, it was assumed that the model would be created using the data from row 2 and column 17. For convenience, the model operates on values relating to the reduction in brightness of the LCD matrix. That is, the value of adjustable brightness is 255--\(\Delta\)b\({}_{\text{n}}\). It was also necessary to develop a model to determine the value by which the brightness intensity of the LCD matrix should be reduced to achieve the expected relative UV (Figure 11). Based on the application of the models described above, maps were determined as follows:  
* The LCD matrix's expected relative brightness correction value (Figure 12a);
* Brightness values consider the constraints on the brightness adjustment of the LCD matrix (Figure 12b). Figure 11: A regression tree model to calculate the relative value of UV intensity on the LCD surface depending on the dimming LCD brightness level. These maps generated a greyscale image with a resolution of 7680 \(\times\) 4320 pixels as a correction mask. Due to the very light shades of grey, it is not included in the article. However, this is just a simple transformation from a color map to greyscale.