Contact person
Hans Christiansson
Application engineer
Contact Hans
Mottle evaluation
Mottle is without doubt one of the most important factors regarding visual impression of print quality. Investigations in our perception laboratory show that the calculated mottle values correlate very well to the human perception both for full tone prints, screened prints and unprinted samples (e.g. white-top mottle)
Purpose
It is still common to use visual assessments to quantify the amount of mottle. However, the drawback of using visual evaluation is high cost and time consumption. Instrumental
evaluation using the STFI-Mottling tool solves this problem being both less time consuming and easier to compare with other evaluations.
The evaluation can be used for:
- Measure if the mottle level is acceptable and stable over time
- Benchmark to competitors
- Monitor how new settings in the productions affect the mottle level
- Check if new products are better or not when it comes to mottle
Method
A flatbed scanner is used to collect the images and the data is calibrated to reflectance using a calibration set.
Small- and large-scale variations are removed using frequency analysis. Only variations in the range 1 to 8 mm are measured since these have been found to be the most disturbing range in visual assessments. The software is very flexible, and it is possible to measure areas of size 25x25 mm up to 200x200 mm. Normally, 4-8 areas are evaluated for better accuracy.
Deliveries
The mottle value, which is the reflectance variations as Coefficient of Variation in size interval (wavelengths) 1-8 mm proved to correlate to the perceived mottle at a normal reading distance of 400 mm. Also, detailed mottle values divided in spatial octave bands, ex. 0.25-0.5 mm, 0.5-1 mm, 1-2 mm, 2-4 mm, 4-8 mm and 8-16 mm revealing the mottle in fine scale, medium scale and large scale. The results are saved in an Excel file.
