This is a test method to determine the color strength of a dye sample relative to a reference dye by means of spectrophotometric transmittance measurements on solutions of each. It is a comparison of the color strength of the dyes in a chosen solvent which may or may not correspond to strengths when applied to a textile substrate. 1.2 To be valid as a measurement of the relative color strength of a dye for a particular textile end-use, additional tests must be performed to demonstrate that the relative strength in solution is equal to the relative strength when applied to the textile substrate. 1.3 The test is often used by dye manufacturers and users because of its speed, repeatability, and reproducibility. 2. Principle 2.1 Solutions of the test dye and the reference dye are prepared in known concentrations and their transmittance values are determined using a spectrophotometer. The relative color strength for the test dye is then calculated using absorbance and concentration values.