OSDx measures the Radius of Curvature (RoC) of the lower tear meniscus (LTM) by reflectometry. The operating principle is measurement of the reflection image of a known sized object and using concave mirror formulae to calculate the RoC. Note that RoC is not tear meniscus height (TMH) – the latter can only be measured using anterior segment OCT. However, RoC has got good correlation with TMH and can be used interchangeably. Unit of measurement is millimetres.
OSDx gives the user the freedom to choose the site of LTM measurement - but ideally it should be measured at the junction with the inferior cornea.