I am using a lens testing interferometer, where I record 4 to 5 interferograms with a 90$^{\circ}$ phase step between consecutive interferograms. In addition to the interferometer, I have also created a Python script, where I simulate the process using computer-generated interferograms and try to make everything as realistic as possible.
An important criterion in my analysis is the data modulation per pixel $\textbf{γ}$. According to the literature, the data modulation for the 5-step Hariharan Phase-Stepping Algorithm that I am using, can be calculated from the 5 recorded interferograms(Malacara, e) as per the formula shown below:
$$\gamma = \frac{3[4(I_4-I_2)^2+(I_1+I_5-2I_3)^2]^\frac{1}{2}}{2(I_1+I_2+2I_3+I_4+I_5)}$$
In my simulations the fringe contrast is a constant 1.0, as expected. However, when I calculate and plot the fringe contrast from real interferograms, I see that the average value is about 0.35 and that there is a visible fringe patters across the plot, meaning that something is modulating the fringe contrast.
My question is: Could you explain possible causes (error sources) that can cause this fringe pattern, the ripples and the overall drop in the fringe visibility?
