## 3 Results
### Optimization of the Acriflavine Based Photopolymer Layer
#### 3.2.2 Optimization for 150 Micron Thick Layers  
Real-time monitoring of the diffraction efficiency during grating recording is considerably more challenging at 150 microns, so a stepwise recording with multiple samples is sometimes needed to properly examine the grating growth, especially at higher spatial frequencies (due to the high angular selectivity). In this section, a combination of real time recording and multiple Bragg curves is used to study blue-sensitive layers of 150 micron thickness. Firstly, real-time recording was done for 200 L/mm spatial frequency with higher (5 mW/cm\({}^{2}\)) and lower (1.5 mW/cm\({}^{2}\)) intensities. The growth curves of first-order efficiencies are shown in Figure 8a.  
Figure 8: Diffraction efficiency growth for 150 micron thick blue-sensitive layers. (**a**) On-Bragg efficiency growth curve of for gratings recorded with spatial frequency 200 L/mm, (**b**) a series of zero-order Bragg selectivity curves for grating recorded with 500 L/mm, for a range of exposure times, measured at 633 nm. (**c**) On-Bragg diffraction efficiency values obtained from (**b**). The equivalent diffraction efficiency that would be expected at 488 nm is also shown so that the optimal recording time can be identified.  
For 500 L/mm, individual gratings were recorded at a constant intensity (5 mW/cm\({}^{2}\)) but with different exposure times. The recording time for each grating g was increased from 10 to 40 s with an interval of 5 s. Figure 8b shows the results obtained in the form of zero-order Bragg selectivity curves, when the individual gratings were probed after recording by a He-Ne laser. Each curve graphs a set of data obtained by rotating the probe beam though a range of angles while monitoring the diffraction efficiency. The curves are arranged in Figure 8b according to the exposure time used to create the grating. They show gratings of increasing strength up to the optimum exposure and then the characteristic indications of overmodulation (growing sidelobes). The minimum value from each zero-order Bragg selectivity was taken to obtain the growth curve, which is shown in Figure 8c, along with theoretical values for the efficiency, which is for the same strength grating at 488 nm (using Equation (3)). Considering the growth of diffraction efficiency for both 200 L/mm and 500 L/mm spatial frequencies, the optimized time to record gratings with 5 mW/cm\({}^{2}\) for \(\lambda\) = 488 nm is estimated \(\approx\)16-20 s. As seen in Figure 6, the holographic lens to be recorded covers the range of spatial frequencies from 198 L/mm to 790 L/mm.  
Optimization of the Holographic Recording Conditions for the Fabrication of Individual Lenses at Blue and Green Wavelengths  
The next step was to record and test the individual lenses. As lenses consist of many spatial frequencies and slant angles in a single element, it is necessary to determine the optimal recording conditions for each lens, as well as to analyze the overall diffraction behavior to ensure that the expected microstructure is recorded. This makes it possible to (i) confirm experimentally which exposure conditions (from the range identified above) give the best performance across the lens aperture and (ii) to provide performance data of individual lenses as a comparator for the stacked lenses.