In order to achieve uniform dielectric barrier discharge at atmospheric pressure under large air gap

the evolution of air uniform discharge at atmospheric pressure was studied by means of ICCD high speed camera. The α coefficient of atmospheric air was measured by optical method. It is shown that the uniform discharge in atmospheric air is Townsend discharge throughout the discharge process. The discharge increases with the development of the applied voltage. The discharge intensity reaches the strongest when the current reaches the peak time.Moreover

the distribution of the light intensity in the discharge gap was recorded by taking side-view photograph using an ICCD camera with an extremely short exposure time and compared with the theoretical distributions

with different presumed α. The value of α was determined by the best fitting of the theoretical distribution to the experimental one

and was measured in atmospheric pressure air in the range of 29

3

2 V/（m·Pa） by using the light emission images. The results show that α/p0 and E/p0 in a certain range has a function relationship. The results of different studies in the same reduced field E/p0 are similar.The subtle differences come from two aspects:（1） their methods of measurement are different

（2）the air electronic attachment coefficient at low E/p0 value is very dispersed

thus the alpha coefficients of valid values are more scattered.