The values provided by the scanner devices measure the flux transmitted through the photographic plate. Since the photographic plate is not a linear detector, the plate blackening is not directly proportional to the flux of solar radiation incident during the plate exposure. As presented by Dainty and Shaw (1974), it is common to convert the data numbers from photographic measurements to photo graphic density values D based on the blackening of the scan beam by the exposed plate.

The density is defined as Log10 of the opposite of the plate transparency (or transmission) T , which in turn is given by the relationship between the incident flux Fi and that transmitted by the plate

Ft: D = Log10(1/ T) = Log10 (Fi/ Ft).

Dedicated software was developed to provide automatic reduction of the produced digital images.

This reduction includes:

1) correction for the scanner response to a flat-field,

2) separation of each solar observation,

3) extraction of pixel values in the step-wedge exposures.

This software was developed in IDL language. The flat-field response of the scanner has been obtained at the beginning of each digitization session by taking an average image of the scanner fluorescent screen. Then the sub-image corresponding to each plate has been singled out; couples of images for each individual spec troheliogram, both in FITS (1020×1360 pixel and 2kx2k pixel) and JPG formats (510×680 pixel) have been stored.

Whenever the calibration exposures are included in the plate, the developed procedure also provides from 7 to 21 triplets of average and median values of the transparency T , and the related standard deviations for each step of the calibration wedge. These values were used to evaluate the calibration curve as described in the following.

Given the number of digitized plates we first tried to fully automate the step-wedge identification procedure. However, the plates are very different one from the other: the solar observation and the step-wedge show different shades and positions against the plate background, as well as different dimensions. These differences together with the presence of plates defects, such as lines, large-scale lack of homogeneity, scratches, and over-exposition, hampered the running of a completely automated procedure. Therefore the procedure developed for the identification of the sub-array (3500×3500 pixel) containing the solar disk observation in the image, and for the calculation of median, average and standard deviation of transparency values in each step of the calibration wedge required a check control.