Their contribution included:

Light Unit ---- foot-candle-second (now changed into Metric System, meter-candle-second),candle is made of 1 pound wax for a length of 1 foot. Though they had later used pentane light, a English standard in 1898, to replace candle, and today we have some other light source, unit of candle remains a de facto standard.

Unit of grain reduced --- density, which is the log value of opacity, and opacity is the reciprocal of transparency. Transparency is the ratio of light passing through.

Film Characteristic Curve --- using the above mentioned Density and log value of foot-candle-second (exposure), representing the performance of sensitive material after development. It proves that sensitive material has three stages of change. The first stage, shown as the toe of curve, is passively reacted, the second stage, the middle straight line part of curve, which is the most important part with a stable reaction, the third stage, shown as the shoulder of the curve, which is the beginning of a negative reaction, less density for higher exposure, which is the basic principle of Solarisation.

They called it Characteristic Curve, afterward, some used H & D Curve as a meroiral to their contribution, or D logE curve to remind the basic units. However, all kind of measurement of sensitive material are based on this curve.