Why in the experiment of double slits, in case of radiation of wavelength of red light, we can observe an envelop of the minimum?

Question

I have made an experiment in which i have to measure the intensity of the light generated by an $\text{He-Ne}$ laser, in the case of two slits, I pretend to observe an interference lattice. I have to compute the coefficient of visibility for the experimental data. In the lattice I have observed an envelop of the minimum, how can I explain this?