A meteor stream consists of many meteors of different sizes. If we assume that in a certain size range falls the number of meteors of a certain size exponentially with the size, so you can quantify it in a formula. The larger a meteor, the greater the brightness when entering the Earth's atmosphere. The population index quantifies this exponential relationship between the number of meteors and their brightness.
The population index r describes the brightness distribution of the meteors of a meteor stream. He characterizes the factor by which to increase the number to the nearest meteor weaker class size down.
Can one observe a certain number of meteors so at a certain ambient brightness, so you can watch as many meteors at about one magnitude lower ambient brightness r- time.
The ZHR of a meteor stream is by definition specified for a limiting magnitude of 6.5 mag. Since the actual limiting magnitude, however, often varies therefrom, can be determined using the population index of the number of observed meteors the ZHR. The other way round you can from the ZHR and the border size to determine how many meteors you, if the radiant at the zenith and the sky is cloudless, should observe. This number should be possible to calculate by.