Print is exposed to daylight in specified conditions with Blue Wool standards and the light fastness is evaluated by noting the rating of the standard which has undergone a similar change to that of the test print. There are blue wools from 1-8 increasing in light fastness and the measure of light fastness is:
- Blue Wool 1, 4-6 hours
- Blue Wool 2, 6-12 hours
- Blue Wool 3, 40 hours
- Blue Wool 4, 80 hours
- Blue Wool 5, 160 hours, doubling in value to Blue Wool 8
8 7
6 5
4 3
2 1
These figures are based on the time to see visible changes (i.e. fade)
when the particular wool is exposed to a Xenon UV light source (that
approximates to the same wavelengths emitted by the Sun) through a cycle
of 50% light and 50% darkness.
Additional Information
Under "real" conditions a given blue
wool would generally give enhanced light fastness and typically BW1 can
give 4 days, BW2 - 10 days, BW3 - 2 weeks etc. These "typical"
figures are based on UK summer conditions with the exposure in a
southerly direction, to trap daytime sunshine. However results in other
countries will vary considerably related to UV intensity, sunshine hours
etc. but are unlikely to be worse than hours obtained from a Xenon test.
Many factors have to be taken into consideration to determine what blue
wool value is necessary to have for a commercial print. Print life cycle
needs to be taken into consideration and for example in a supermarket
with fluorescent lighting, there is very little UV emitted and fade can
be up to 100 times slower compared to a Xenon test. For prints that will
be used for packaging of materials stored externally, fade in desert
latitudes may equate to Xenon test results. Also for print stored
externally, atmospheric pollution may cause a faster fade rate than that
would be obtained by only exposure to UV.
As a consequence the blue wool number for a commercial print has to be
considered on the basis of "fitness for purpose". As a general
guide, print that has a short life cycle and will be primarily kept in
an internal environment may be adequate with a blue wool light fastness
of 2-3. For external applications of longer life cycles (e.g. fertiliser
or building material sacks) a blue wool light fastness of 6+ is usually
necessary.