A spectrophotometer calibrated accurately on a relative scale cannot be used for
measuring total ozone amounts correctly since it is necessary to know the "extra-terrestrial
constant" (ETC) for the instrument, i.e., the value of log (I0/I'0) + K which would be found for each
Dobson instrument wavelength pair if the measurements were made on sunlight outside the earth's
atmosphere. Fixation of the "extra-terrestrial constant" to the optical wedge density table for A, B,
C, and D wavelengths constitutes calibrating the spectrophotometer on an absolute scale. The
modified wedge density tables are then called "N tables.”* Values of N are simply values of