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Huygens-Fresnel principle is well used in analyzing wavefront propagation in vacuum. However, we should clear if this
principle is available in the space including absorptive objection, such as a grating. In this paper we analyze if a grating
can diffract a beam when the grating set on the focus point and the grating pitch is larger than the Airy disc, especially
the phenomenon in a special interferometer [1]. The special interferometer shown in reference [1] has an extended
incoherent light source modulated by a binary grating. The light source is imaged onto a sinusoidal transmission grating
by the tested lens. The pitch of the binary grating is half of the period of the sinusoidal grating. Light coming from an
arbitrary point of the light source is focused onto the sinusoidal grating. Two kinds of analysis method are considered.
One is using Huygens-Fresnel principle as described in reference [1]: the concave beam can be resolved to a pair of
waves. Each wave is diffracted by the sinusoidal grating. By using the modulated extended light source, only one color
interferogram interfered by -1st and +1st order beams can be observed. By shifting the sinusoidal grating; the phase of the
interferogram can be modulated; therefore, the intensity after the grating will change. The intensity variation is the
contrast of the interferogram. The other analysis method is to calculate the intensity after the sinusoidal grating as in a
normal Ronchi test. If two points on the light source separated by distance of the binary grating's pitch is picked up, the
intensity of each point after the grating changes sinusoidally when the sinusoidal grating is shifted. Because the period of
the modulation grating is half of the sinusoidal grating's pitch, the intensities of the two points change with "pi" phase
difference. The total intensity after the sinusoidal grating does not change even if the sinusoidal grating is shifted. We
found that the confusion in reference of complex transmission and amplitude transmission is one reason of the
difference. We also reviewed the developing of Huygens-Fresnel principle from Maxwell equations. To get
Huygens-Fresnel principle, condition of non absorptive elements is needed. Because transmission grating is an
absorptive element, wavelet should be integrated before the grating. We also found that the substrate influences the
phase of complex transmission when NA of the test is large. We performed an experiment to show that the contrast is
almost zero, not 21% as calculated by the theory shown in the paper of refer.
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