The symbol ' refers to the longer wavelength.
- I/I': ratio of intensities measured by the Dobson spectrophotometer at Earth's surface.
For meaningful measurements a Dobson spectrophotometer needs a wedge calibration.
- Io/Io': ratio of intensities outside the atmosphere, as it would be measured by the specific Dobson
spectrophotometer (also called extraterrestrial constants (ETC)). This ratio can be directly
determined by the Langley Plot method (see below) or indirectly by transfer from another (reference)
instrument.