Film speed
The system was later extended to cover larger ranges and some of its practical shortcomings were addressed by the Austrian scientist Josef Maria Eder (1855-1944) and Flemish-born botanist Walter Hecht (de) (1896-1960), (who, in 1919/1920, jointly developed their Eder-Hecht neutral wedge sensitometer measuring emulsion speeds in Eder-Hecht grades). Still, it remained difficult for manufactures to reliably determine film speeds, often only by comparing with competing products, so that an increasing number of modified semi-Scheiner-based systems started to spread, which no longer followed Scheiner's original procedures and thereby defeated the idea of comparability.
What eliminated comparability?
modified semi-Scheiner-based systems
Still, it remained difficult for manufactures to reliably determine film speeds, often only by comparing with competing products, so that an increasing number of modified semi-Scheiner-based systems started to spread, which no longer followed Scheiner's original procedures and thereby defeated the idea of comparability.
comparability