Curl retention is dependent on humidity and heat set curls tend to fall out faster than water set curls. The ability of test products to prolong the curl is tested using tress image analysis.
 
By way of example, a wet set procedure involves applying a wet tresses  to a given confirmation using a pegboard. After drying and equilibration at low humidity, the relative humidity is raised to the required test level.   High quality digital photographs are taken periodically and the length of the tress is determined  using image analysis of the digital images.  Percent set retention at time t can be calculated from the tress lengths.