## 3 Illumination Systems' Performance Comparisons
### Output Radiation and Energetic Transmission  
As described above, the output radiation characteristics are important to ensure that the maximum amount of energy is being used effectively to illuminate the scene, as seen from the imaging side. Although this is unlikely a problem for the laser system, with a slightly diverging beam, the ray tracing simulations already indicated significant losses with the LED illumination system. We therefore measured the angular intensity distribution of the LED system with the optics described above, see Figure 2. The full width at half maximum dispersion angle of the LED emitter alone is specified at 120\({}^{\circ}\), which was confirmed by our measurements, with a full width at half maximum slightly below 117\({}^{\circ}\). With the optics proposed above, the dispersion angle of the radiation distribution has decreased to 25.9\({}^{\circ}\), suggesting a substantial improvement in energy transmission. On the other hand, and as expected based on the results of the ray tracing simulations of Figure 3, the VCSEL illumination system presented a radiation dispersion of less than 0.5\({}^{\circ}\). However, it should be noted that the divergence of the diffuse illumination distribution was a little over 2\({}^{\circ}\). The diffuse aspect of the illumination radiation is important to limit the shadowing effects of out-of-focus features [22], as described earlier in this manuscript. The illuminated area at the probe volume was circular for the laser system, whereas it was slightly oval for the LED illumination, due to the rectangular layout of the emitting area. The equivalent diameter of the illuminated area at the probe volume, taken at full width at half maximum was 15 mm for the VCSEL-based system, and 51.2 mm for the LED illumination. The global quantities reported above do not necessarily correlate with the light intensity at the measurement volume. We therefore measured the energy density at the probe volume for both systems. The energy density integrated over the field of view at 80 mm from the LED window cover is 0.004 \(\upmu\)J/cm\({}^{2}\) per pulse, under the pulse specifications given above. Inserting the 16 mm focal length aspheric condenser lens improves this figure to 0.058 \(\upmu\)J/cm\({}^{2}\). These results confirm both the magnitude of the improvement brought by the aspheric condenser lens, as well as the significant energy loss at the probe volume, over 99%, as predicted by the ray tracing simulations. Despite comparable total pulse energy at the source (0.38 \(\upmu\)J, compared to 0.34 \(\upmu\)J for the LED), the VCSEL-based illumination system delivers 0.18 \(\upmu\)J/cm\({}^{2}\) per pulse at the probe volume, or more than three times the energy density of the LED system. As anticipated from Figure 3, the low divergence of the laser illumination should result in only minor energy loss over distances relevant to long-distance microscopy. In contrast, the high divergence of the LED illumination system, despite the 16 mm focal length aspheric condenser lens, results in high energy losses at distances away from the emitter. Figure 5 reports the energy density per pulse measured as function of distance for the LED and VCSEL illumination system, according to the configurations given above (see Figures 2 and 3). The respective object planes for both illumination configurations are represented by vertical dash-lines, in blue color for the LED illumination system and red for the VCSEL. The results of Figure 5 confirm the fairly constant energy density for the VCSEL illumination system, and they highlight the decrease in energy density with distance for the LED system. Interestingly, the energy density as function of distance for the VCSEL illumination is not monotonic, but first increases and then decreases. This trend is the result of the optical arrangement with the plano-concave lens merging the beams from the six laser units, producing an energy density maximum at the converging location, as suggested by the ray tracing simulations.