The diffraction of light is given by the Fraunhofer diffraction integral, provided that the distance of propagation is much larger than the transverse size of the initial optical field. (Over shorted distances one would use the Fresnel diffraction integral.) The Fraunhofer diffraction integral is essentially a Fourier integral. Therefore, one can refer to a diffraction pattern that is seen over such a large distance as a Fourier transform pattern.
The transmittivity of a transparent object should be seen in association with its reflectivity. When light falls on the object, some light is transmitted and some is reflected. The sum of the power that is reflected and that which is transmitted must add up to the power of the incident field. Therefore, the transmittivity and the refelctivity should add up to 1. In accordance with the definition for reflectivity, the transmittivity is the ratio of the transmitted optical power to the incident optical power.
In the context of the diffraction pattern produced by the transmitted light, the transmittivity is a function of the transverse coordinates, giving the transmitted light a specific profile which is then propagated to produce the diffraction pattern. One can think of a thin transparent film having a varying transmittivity over its surface.