Light Fastness:
Light fastness, also called sunburnt fastness, refers to the ability to keep original color of dyed fabrics when illuminating. Light fastness divide into 8 grades, among which grade 8 is the best and grade 1 is the worst. Light fastness tester is usually used in light fastness test. When testing, tested sample and blue wool standard sample are illuminated at the same time, and the grade of light fastness depends on degree of discoloration of tested sample.
Blue wool standard sample is made from wool fabric which is dyed by 8 dyes of prescribed depth, and is divided into 8 grades, which represent 8 types of different degree of discoloration. It applies to ISO 105 B02: Europe exposure conditions prescribed in 1994. You can also adopt American blue wool standard sample L2~L9 when doing light fastness test, besides blue wool standard sample 1~8. Sample L2~L9 is used two dyes to dye raw fiber, formed by two dyed fibers in different proportions, applying to ISO 105 B02: America exposure conditions prescribed in 1994, AATCC TM 16. Sample 1~8 cannot be mix used with Sample L2~L9, and their test result can’t be swapped.
Light Color Fastness Test:
1.Preparation of Test Apparatus and Materials
Light Fastness Tester (Xenon Arc Lamp), Light Source Rating Box, Gray Cards Evaluating Discoloration, Blue Wool Standard Sample, Tested Samples.
2.Testing Method:
a) Sample Preparation:
Tested samples should be balanced in dark place at room temperature for 24h before evaluating light fastness.
The size of samples can be determined by amount of samples and shape and size of sample holder of Equipment. If adopt air-cooled equipment, the sample should be exposed to sun section by section in different periods. The size of sample should be used at no less than 45mm*10mm, and the exposure area is no less than 10mm*8mm. Tested sample is attached to hard card. If tested sample is yarn, yarn should be winded on the hard card or arranged parallel and fixed on the hard card. If it is disperse fiber, disperse fiber should be combed and finished into a uniform thin layer and fixed on the hard card.
In order to easy to operate, one or more tested sample should be arranged together with blue wool sample with same size according to Installation Chart 1 or Installation Chart 2, fixing on one or more hard cards.
b) Operation Procedure:
Sample clips which has been installed is set on sample holders of tester, arranged vertically. All empties on sample holders should be filled with sample clips with hard cards.
Turn on xenon lamp, and under pre-set conditions, tested sample and blue wool sample are exposed to light at the same time. The methods and time depend on whether each sample can be completely measured the light fastness comparing with blue wool standard samples.
Method 1:
This is the most accurate method, adopting when evaluation is controversial. The basic characteristic is to control exposure period by checking tested samples, so each samples should be equipped with a set of blue wool samples.
Tested samples and blue wool samples are arranged according to Installation Chart 1. The Covering AB is place on the 1/3 area of tested samples and blue wool samples. It is exposed under prescribed conditions, and taking the covering up from time to time to check lighting effect of tested sample until the color difference between exposed part and unexposed part of tested sample is up to grade 4 of gray cards. Using the Covering CD to cover the 1/3 area of left side of tested sample and blue wool standard sample, continuing to expose until the color difference between exposed part and unexposed part of tested sample is up to grade 3 of gray cards.
If the fade of blue wool standard sample 7 is up to grade 4 of gray cards ahead of tested sample, exposure can be ended at this time. This is because tested sample need to be exposed in a long time until it is up to grade 3of gray cards when its light fastness is equal or high to grade 7. What’s more, color difference can’t be measured when its light fastness is grade 8. Therefore, when the color difference of blue wool sample 7 is equal to grade 4 of gray cards, it can be measured in range of grade 7~8 of blue wool samples.
Method 2:
This method is suitable for a large amount of samples tested at the same time. The basic characteristic is to control exposure period by checking blue wool standard samples, which just requires a set of blue wool samples and tests a batch of tested samples with various light fastness which saves materials of blue wool samples.
Tested samples and blue wool samples are arranged according to Installation Chart 2. Using the Covering AB to cover 1/5 of total length of tested samples and blue wool samples, and exposed under prescribed conditions, taking Covering AB up from time to time to check light effect of blue wool samples. When the discoloration of blue wool sample 2 is up to grade 3 of gray cards, evaluation initially light fastness of tested sample comparing to the discoloration of blue wool sample 1,2,3.
Covering AB is placed again and precisely on the original area, continuing to expose until the discoloration of blue wool sample 4 is same as grade 4 of gray cards. Then, the Covering CD is placed on the area as Figure 6 shows, overlapping on the Covering AB and continuing to expose until the discoloration of blue wool sample 6 is up to grade 4 of gray cards. Next, the Covering EF is placed on the area as Figure 6 shows, and the other coverings are kept, then continuing to expose until any following situations occur: first, the discoloration of blue wool sample 7 is up to grade 4 of gray cards; second, the color difference of tested sample with the strongest light fastness is up to grade 3 of gray cards.
Method 3:
This method is suitable for checking whether the tested samples are consistent with some performance and specifications, allowing tested sample to expose together with two blue wool samples, and one blue wool sample of minimum allowable fastness and another of lower fastness. Continuing to expose until the color difference of the segmented part of blue wool sample with minimum allowable fastness is up to grade 4 of gray cards (first stage) and grade 3 (second stage).
Method 4:
This method is suitable for measuring whether the tested samples comply with an agreed reference sample, allowing tested sample to expose only together with the agreed reference sample. Exposing continually until it achieves to the specified amount of radiation, and evaluating after taking out together with blue wool sample.
Method 5:
This method is suitable for checking whether tested sample comply with recognized radiation energy value, allowing tested sample to expose alone, or to expose together with blue wool sample, until it achieves to the specified amount of radiation, then evaluating after taking out together with blue wool sample.
①、Stop the test and evaluate light fastness when the color difference of exposed part and unexposed part of tested sample is up to grade 3 of gray cards.
②、Move all coverings, and tested samples and blue wool samples show two or three segmented parts after testing, some of which have been exposed for several times, along with at least one area unexposed. Tested samples and blue wool samples are evaluated their discoloration in standard light box.
The light fastness of tested samples is number of blue wool sample which presents similar discoloration (color difference of exposed area and unexposed area evaluating through eyes). If discoloration of tested sample lies between two adjacent blue wool samples, but not get close to one of blue wool samples, judge it as middle grade, such as grade 4~5. If discoloration of different stages gets different evaluations, take arithmetic mean as its light fastness, presented by the closest half grade or a complete grade. When arithmetic mean of grade is 1/4 or 3/4, it should be judged as grade 0.5 or grade 1. If tested sample is easier to fade than blue wool sample 1, it should be judged as grade 1.
