The most common applied passive technique is applying thermal insulation to reduce the rate of conductive heat transfer. In order to protect a structure from thermal radiation, it is possible to apply a coating with a low emissivity ε, reducing both absorption and emission. In reverse, this can be increased by applying a high emissivity.
In [Ruijl, 2001] a metrology frame is enclosed in an aluminium box, which acts due to its thermal capacity as a low-pass filter for dynamic thermal disturbances such as operators and environmental fluctuations. The high conductivity of aluminium creates a uniform temperature distribution over the shield, thereby also creating a uniform heat load on the metrology frame inside and thus reducing thermal gradients in the frame (see Figure 8).
Other techniques are for example creating a mini-environment, using IR-shields and applying large thermal masses, as illustrated in the GAIA satellite test set-up.
Figure 8: Thermal shielding: right beam enclosed by thermal shielding: gradient reduction ~Factor 40
