The Mach-Zender interferometer can be operated in two modes, namely the (a) infinite fringe setting, and the (b) wedge fringe setting. In (a) the test and reference beams are set to have identical geometrical path lengths and fringes from due to density, and so temperature , changes alone (In this discussion, changes in density due to changes in pressure are taken to be of secondary imporatnce; this assumption is valid for many buoyancy-driven flow experiments). Since each fringe is a line of constant phase, it is also a line of constant refractive index, a line of constant density, and, hence temperature, and hence an isotherm. It can also be shown that the fringe thickness is an inverse measure of the local temperature gradient, being small where gradients are high. The infinite fringe setting is employed for high-accuracy temperature measurements in the fluid. In (b), the mirrors and beam splitters are deliberately misaligned to produce an initial fringe pattern of straight lines. When a thermal disturbance is introduced in the path of the test beam, these lines deform and represent the temperature profiles in the fluid. The wedge fringe setting is commonly employed for wall heat flux measurements.

The specifications of the components of a Mach-Zehnder interferometer employed by the author are presented below.

Table 1: Specifications of the He-Ne Laser

Make	Spectra-Physics
Model	Spectra-Physics 127
Output Power	60 mW(maximum), 35 mW average output
Wavelength	632.8 nm
Color	Orange Red
Coherence Length	20-30 cm
Power Consumption
Efficiency
Beam Diameter
Beam Divergence
Amplitude Noise, 10 Hz-2MHz
Amplitude Ripple, 45 Hz-1kHz
Life Time