光子气体和低温下强简量子气体都不是经典理想气体，通常的热波长没有意义。但是，它们存在每个粒子的平均

动量，因此可以定义广义热波长.这个热波长是系统量子相干性的一个量度，在非简并条件下，实物粒子系统广义热波长回到

通常的热波长.简并条件下，热力学系统的线度必须大于广义热波长，因此导致了对热力学系统出现相关现象的最小线度，即

最小线度判据.本文利用这一判据给出了一些典型系统的最小线度.

Photon gas and highly degenerate quantum gases are not classical ideal gas at all，and each of them has its own mean momentum from which a generalized thermal wave length can be thus defined，and the usual thermal wave length is an irrelevant concept.For a non-degenerate system，this wave length converges to the usual thermal one form a matter particle.For a degenerate system，the dimension of thermodynamic system must be larger than this generalized thermal wavelength，which leads to a criterion on the minimum dimension of the thermodynamic system.For some typical systems，we use the criterion to give their minimum dimensions.