Where is the value of seeing at zenith. Equation 3.1 is used to correct DIMM data from different altitudes and is essential so the data to be self-consistent.

The seeing also varies with the wavelength according to the fowllowing equation :

Where is the wavelength. Seeing is defined as the image size (FWHM) in arcsec on a long exposure limited by the atmosphere (ie. telescope without optical aberrations nor dome/mirror seeing) as observed at zenith and at wavelength 0.5 micron. So when DIMM data are collected a correction of these two effects must be done. An extapolation to the zenith and to micron wavelenqth.