In problems where is a two dimensional temperature field the integral simplifies to

(7)

Since a path length difference of will generate one fringe shift, the temperature difference required for this purpose is

In air and in water, it is . Since density decreases with increasing temperature at constant pressure, both derivatives are negative under normal conditions.

For laser . Hence per fringe shift for a laser is in air and in water. Note that the value itself decreases with increasing geometric path length . Hence the sensitivity of measurements can be adjusted by designing apparatus of varying dimensions in the direction of propagation of light. In high temperature application L is made small while in problems involving small temperature differences can be large. As discussed later, the largest value of is, limited by refraction errors.

In summary, each fringe of an interferometer represents a line of constant phase, constant refractive index, constant density and hence constant temperature, namely an isotherm. This aspect is useful in qualitative interpretation of interference patterns.