Such a wide opening single period is known to possess a diffraction envelope highly concentrated along the forward direction, limiting the chances of light to stray into the first order. This explains why the dashed curve in Fig. 4, while it enjoys a much increased center maximum, has a weaker first-order peak (around sing=0.25) compared to the solid curve. In summary, the first-order diffraction attainable by this pure amplitude grating is very limited.