# Resolution Enhancement of Spherical Wave-Based Holographic Stereogram with Large Depth Range +
## 3 Reducing CDP Depth \(L\)
The differences among Figures 3a and 4a,b can also be understood through different weights of the first and second terms in Equation (5). When \(\Delta x_{1}\) = 5 \(\upmu\)m and \(L\) = 80 mm, the two terms have similar weights at large \(\Delta z\). By reducing \(L\), the reduction of the first term is comparable to the increment of the second term. However, when \(\Delta x_{1}\) = 3 \(\upmu\)m and \(L\) = 80 mm, the second term has greater weight. By reducing \(L\), the reduction of the first term is always less than the increment of the second term at large \(\Delta z\). That is why reducing \(L\) will always decrease the depth range in this case. When \(\Delta x_{1}\) = 8 \(\upmu\)m and \(L\) = 80 mm, the first term has greater weight. By reducing \(L\), the reduction of the first term is always more than the increment of the second term within the total depth range. That is why reducing \(L\) is always effective in this case. Thus, in different cases, the strategy of reducing \(L\) is different. Next, numerical simulation is performed. Figure 5a shows a bee model and a school badge built in the 3 ds Max modeling software. The camera array consists of 60 \(\times\) 60 virtual pinhole cameras, and the pitch and focal length are set as \(p\) = 0.3 mm and \(f\) = 3 mm. The captured EI array contains 3600 \(\times\) 3600 pixels with 5 \(\upmu\)m pixel pitch. The CDP is set at the center of the bee model, and the distance between the school badge and CDP is set as \(\Delta z\) = 20 mm. Figure 5b shows the numerical reconstruction with capturing depth \(L\) = 80 mm. By reducing \(L\) to 45 mm, Figure 5c shows better reconstruction quality at the CDP (see the foreleg of the bee), with comparable reconstruction quality at the marginal depth plane (the school badge). Further reducing \(L\) to 30 mm, Figure 5d shows the best reconstruction quality at the CDP. However, the reconstruction quality of the school badge is degraded, which means the depth range is decreased. This simulation verifies that a proper \(L\) needs to be chosen to enhance the resolution without decreasing the depth range. Next, an optical experiment is performed on a binary amplitude-only hologram which is printed on a glass substrate coated with chromium film. The amplitude-only hologram \(I(u,v)\) is encoded as:  
\[I(u,v)=2\text{Re}[H(u,v)r^{*}(u,v)]+C, \tag{6}\]  
where \(r\) (\(u,\,v\)) is the reference plane wave with incident angle of \(3^{\circ}\) and \(r^{*}\) (\(x,\,y\)) is the conjugation of \(r\) (\(u,\,v\)). \(C\) is a constant real value to make \(I(u,\,v)\) non-negative. Then the hologram is binarized by setting the mid-value as the threshold. The hologram contains \(3600\,\times\,3600\) pixels with \(5\,\upmu\)m pixel pitch. A solid laser with wavelength of \(510\) nm was used to illuminate the hologram after being collimated by a lens. Figure 6a-c show the reconstruction results with capturing depth \(L=80\) mm, \(45\) mm and \(30\) mm, respectively. It is easily confirmed that reducing \(L\) will always improve the reconstruction quality near the CDP (see the bee model). But to avoid the quality degradation at the marginal depth (see the school badge), a proper \(L\) should be chosen.