[Validation of constancy testing of film processing according to Regulation 6868-2]

Constancy tests of film processing according to DIN 6868-2 require the measurement of the quantities "Empfindlichkeitsindex EI" (Speed Index) and "Kontrastindex KI" (Contrast Index), aimed at determining changes of film processing that can influence the resulting radiographic image. In this study it was investigated whether these quantities are actually suited to adequately describe processing-induced changes of the sensitometric properties of X-ray films. For this purpose, processing-induced changes of the parameters EI and KI were compared with corresponding changes of the parameters "Lichtempfindlichkeit LE" (Light Speed) and "Lichtkontrast LK" (Light Contrast), both used for acceptance tests of film processing according to DIN V 6868-55. The results of this study can be summarised as follows: 1. There was a very good correlation between the quantities EI and LE. 2. Changes of the quantity KI were generally not correlated with changes of LK, for example significant changes of LK were not always reflected by changes of the quantity KI. The following conclusions are drawn: 1. The quantity EI is well suited to indicate changes of the sensitivity of film processing. 2. The use of the quantity KI for constancy testing is questionable, and it is suggested to refrain from the measurement of KI.