# Holographic Lens Array for Solar Collector with Large Angle and Expanded Spectral Width
## 3 Experimental Results  
### Acceptance Angle of HLASC  
Based on the above principle, we fabricate an HLASC using a lens array with a single lens aperture of 1 cm and a focal length of 1 cm. The holographic recording material used for fabricating the HLASC is a photopolymer, and its post-processing only requires a simple step of UV curing. Additionally, the holographic material is placed parallel to the lens array with a distance of 2 cm between them, and the off-axis reference light is tilted at an angle  
Figure 4: Flowchart for flexible design of HLASC with different parameters.  
of 70\({}^{\circ}\). The wavelength of the laser used for fabrication is 532 nm, the total laser power is 300 mw, and the total exposure time is 10 s. The size of the HLASC is 5 cm \(\times\) 5 cm, and the HLASC is composed of twenty-five OHLs. To compare the collection angles and spectra, we also remove the lens array and fabricate a holographic grating using two parallel lights as a reference. To avoid the influence of ambient light on the diffraction spectrum to be measured, we used an approximately collimated white light flashlight with an LED light source to simulate broad-spectrum sunlight for the following measurements.  
Firstly, the acceptance angle of the HLASC is tested. We use the white light flashlight to illuminate the fabricated HLASC and the reference holographic grating. A white cardboard is placed on the side to receive the collected light, and a spectrometer is placed in a fixed position facing the white cardboard to measure the spectrum of the collected light. We test the collection spectra of the HLASC and the reference holographic grating at different incident angles by changing the direction of the flashlight. Then, we process the measurement results by integrating the spectral collection curve to obtain the overall collection intensity of the HLASC and reference holographic grating, and normalize the values at different angles according to their respective maximum values. The results are shown in Figure 5. The green part represents the results of the proposed HLASC, the blue part represents the results of reference holographic grating, the dots represent the measured values, and the solid line represents the fitted curve. We define 20% of the maximum collection intensity as the effective collection threshold. The results indicate that the effective collection angle of the HLASC is greater than 54\({}^{\circ}\), which is significantly higher than the 8\({}^{\circ}\) collection angle of the reference grating. According to Equation (1), under these fabrication parameters, the theoretical collection angle of the HLASC is 53.13\({}^{\circ}\). The actual measured value is higher than the theoretical value, which may be due to the fact that the collection wavelength is no longer limited to the recording wavelength, and the collection angle will also be further expanded slightly. It should be pointed out that the spectrum of the flashlight is only in the visible light band, and we only use it to verify the effectiveness of the angular and spectral expansion method. There is also the possibility of the selective collection of ultraviolet and infrared bands of sunlight, as long as the recording conditions are designed reasonably according to the method shown in Figure 4. The parameters of the lens array used for fabricating the HLASC are also not limited to the above parameters.  
Figure 5: Normalized collection intensity at different incident angles.