Working Principle of Color Fastness to Light

The influence of light on the fading of dyes is a complex phenomenon influenced by many variables, making predictive tests for color fastness to light difficult to establish. The depth of shade, presence of unwanted chemicals, humidity, temperature, presence of atmospheric impurities and spectral quality and intensity of the incident light all have a bearing on the end result. There are a large number of different light fastness tests available on the market. Each has its advantages and disadvantages. The most commonly used are the xenon arc and MBTF lamp; however, carbon arc and natural sunlight are also used.

Purpose and Scope:
The purpose of color fastness to light test is to determine how much the color will fade when exposed to a known light source. It is an off line quality assurance system. Generally people wear the fabric and goes outside of the home for doing their job. In day; sun light fall on the fabric surface. So it needs to know how much protection ability have a fabric to sun light. It is determined by an experiment called color fastness to light. To measure the color fastness a blue scale is used. After completing the test, sample is compared with the blue scale.

Color fastness to light refers to the ability of the fabric to withstand the sun light. The resistance to sunlight depends on the following:

  1. The intensity of light
  2. Inherent properties of the fabric
  3. Season
  4. Altitude
  5. Distance from the equator

This test method provides the general principles and procedures which are currently in use for determining the colorfastness to light of textile materials. The test options described are applicable to textile materials of all kinds and for colorants, finishes and treatments applied to textile materials.

Test options included are:

1—Enclosed Carbon-Arc Lamp, Continuous Light

2—Enclosed Carbon-Arc Lamp, Alternate Light and Dark

3—Xenon-Arc Lamp, Continuous Light, Black Panel Option

4—Xenon-Arc Lamp, Alternate Light and Dark

5—Xenon-Arc Lamp, Continuous Light, Black Standard Option

6—Daylight Behind Glass

Light Fastness Measurement Methods:

1. Direct method: Sunlight method:
Samples are placed in a glass cabinet and exposed to sunlight from 9 am to 3 pm.

2. Laboratory instrument method:

  • Fade-o-meter
  • Weather-o-meter

These machines have either a carbon-arc lamp or xenon-arc lamp as the light source.

Working Principle of Color Fastness to Light:
This test measures the resistance to fading of dyed textile when exposed to day light. The test sample is exposed to light for a certain time which is about 24 hours to 72 hours or by customer/buyer demand and compare the change with original unexposed sample the changes are assessed by Blue Scales.

Example of light fastness testing of an orange coloured fabric (top) and blue wool standards
Figure 1: Example of light fastness testing of an orange colored fabric (top) and blue wool standards

For testing light fastness, samples are exposed to an intense artificial light generated by a Xenon arc lamp. The light passes through a series of filters to ensure that its spectrum (wavelength) closely matches the spectrum of natural daylight coming through the glass. The humidity and temperature of the test atmosphere are controlled. A small sample of material, together with a series of eight blue wool standard fabrics (reference 1–8), are simultaneously exposed to high-intensity light. Two exposure times are used to determine whether fading progresses steadily or initially at a different rate from the longer-term exposure. The light fastness rating system is based on the rate of fading of eight blue-dyed wool samples (blue chips) which are rated from 1 (poor) to 8 (excellent) are given in next section.

The test involves assessing samples once during the test and again at the end. The assessment is completed by comparing the degree of fading shown on the sample with the fading shown by the blue wool standard fabrics. Each sample is assigned a light fastness rating based on the number of the blue wool standard which shows the equivalent degree of color change.

Light Fastness Scale / Grade / Rating:

Grade Degree of Fading Light Fastness Type
8 No fading Outstanding
7 Very slight fading Excellent
6 Slight fading Very good
5 Moderate fading Good
4 Appreciable fading Moderate
3 Significant fading Fair
2 Extensive fading Poor
1 Very extensive fading Very poor

Depth of Shade Effect on Light Fastness:
The depth of shade of a color has a significant effect on the light fastness of a product. Light fastness is the degradation of a fixed number of dye molecules per exposure to a fixed intensity of light. A deeper shade is affected less than a pale shade, as a smaller percentage of the overall dye molecules are degraded per light exposure. Figure 2 represents this.

Depth of shade effect on light fastness
Figure 2: Depth of shade effect on light fastness

Color Fastness to Light with the Microsol Light Fastness Tester:
The testing is done step by step. Following step is maintained during measure the color fatness to light.

  1. Cut the four pieces of test specimens according to the length and width wise and attached with the specimen holder.
  2. Then the holder set in to the Microsol light fastness tester.
  3. Then the experiment continued at 72 hours according to the buyer’s requirement.
  4. After 72 hours later the specimen taken from the light fastness tester.
  5. Then the test specimen compare with the Blue scale or computer color matching system (CCMS).

You may also like:

  1. Light Fastness of Textiles: Factors Affecting and Control Measures
  2. What is Color Fastness | How to Improve Colorfastness | Importance of Color Fastness
  3. Different Types of Colour Fastness Test | Factors Affecting Colour Fastness
  4. Gray Scale for Color Change and Color Staining
  5. Colour Fastness to Washing Procedure (ISO 105 C06)
  6. Various Fastness Methods Given to the Dyed Material
  7. How to Determine colour Fastness to Wash
  8. Light Fastness of Textiles: Factors Affecting and Control Measures
  9. Colour Fastness to Laundering Test
  10. Factors Affecting the Rubbing Fastness of Textile Materials
  11. Colour Fastness to Water Test Method (ISO 105 EO1)
  12. Color Fastness to Sea Water (ISO 105 E02)

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