We present a detailed study of a statistically complete sample of highly inclined disk galaxies in the near-infrared K ^ { \prime } band .
Since the K ^ { \prime } -band light is relatively insensitive to contamination by galactic dust , we have been able to follow the vertical light distributions all the way down to the galaxy planes . The mean levels for the sharpness of the K ^ { \prime } -band luminosity peaks indicate that the vertical luminosity distributions are more peaked than expected for the intermediate sech ( z ) distribution , but rounder than exponential .
After fitting a generalized family of fitting functions characterised by an exponent 2/n ( n = \infty for exponential , n = 2 for sech and n = 1 for sech ^ { 2 } ; van der Kruit 1988 ) we find that the mean value for 2/ n in the K ^ { \prime } band equals < 2/ n > _ { K ^ { \prime } } = 0.538 , \sigma _ { K ^ { \prime } } = 0.198 .
Since projection of not completely edge-on galaxies onto the plane of the sky causes vertical luminosity profiles to become rounder , we have performed simulations that show that it is possible that all our galaxies can have intrinsically exponential vertical surface brightness distributions . We find that the profile shape is independent of galaxy type , and varies little with position along the major axis .
The fact that we observe this in all our sample galaxies indicates that the formation process of the galaxy disks perpendicular to the galaxy planes is a process intrinsic to the disks themselves .