The following is a complete and practical judgment method and process:
I. Objective Instrument Measurement (Data-Driven Judgment)
This is the most scientific and impartial method, used to eliminate the subjective influence of human eyes and lighting.
Core Tool: Colorimeter
The colorimeter provides several key parameters to judge color differences:
1. ΔE - Total Color Difference Value
This is the most crucial indicator, representing the overall color difference between the standard sample and the test sample.
Judgment Criteria (Generally Used in the Industry, Specific Adjustments Needed Based on Customer Requirements):
ΔE < 0.5: Extremely small difference, almost imperceptible to the naked eye. Excellent match.
0.5 < ΔE < 1.0: Minor difference, possibly noticeable to experienced observers under specific lighting conditions. Good match, generally acceptable.
1.0 < ΔE < 2.0: Noticeable difference, possibly noticeable to ordinary consumers when comparing side-by-side.
ΔE > 2.0: Significant color difference, generally unacceptable. 2. ΔL, Δa, Δb - Individual Color Difference Values
Looking at ΔE alone is insufficient, as it doesn't tell you exactly where the color "off" its color. Individual color difference data provides precise directions for correction:
ΔL (Lightness Difference): `+` indicates a whiter/lighter tone, `-` indicates a darker/darker tone.
Δa (Red-Green Difference): `+` indicates a reddish tone, `-` indicates a greenish tone.
Δb (Yellow-Blue Difference): `+` indicates a yellowish tone, `-` indicates a bluish tone.
Application: If ΔE exceeds the limit, by analyzing ΔL, Δa, and Δb, colorists can determine which colorants to add or remove to correct the formula. For example, Δb of +2.5 indicates the sample is too yellow, requiring a reduction in yellow colorants or an increase in its complementary blue colorant.
Key Operational Points for Instrument Measurement:
Standardization of Sample Preparation: The preparation conditions of the test sample (injection temperature, pressure, time, thickness, etc.) must be completely consistent with the preparation conditions of the standard sample; otherwise, there is no comparability.
Instrument Calibration: The colorimeter must be calibrated before measurement.
Multi-Point Measurement: Measure 3-5 points at different locations on the sample and take the average value to avoid the influence of uneven material distribution or surface defects.
II. Subjective Visual Evaluation (Visual Confirmation)
Instrument measurement is fundamental, but the final product is for human viewing, so visual evaluation is indispensable.
Core Principle: Compare under a standard light source.
1. Use a Standard Light Source Box:
Purpose: To eliminate metamerism (i.e., colors match under one light source but not under another).
Must-Observe Light Source:
D65: Simulates average daylight and is the most important reference light source.
TL84: Commercial fluorescent lamps (cool white light) commonly used in Europe and Asia.
CWF: Commercial fluorescent lamps (warm white light) commonly used in the United States.
UV: Ultraviolet light, used to check fluorescent whitening agents or fluorescent pigments.
A Light Source: Household incandescent lamp, used to observe color performance under warm yellow light.
Method: Place the standard sample and the test sample side by side in a light box and observe under different light sources to see if the color remains consistent under all light sources. If there is a significant color difference under a certain light source, it indicates a metamerism problem, and the formula needs to be adjusted.
2. Observation Angle
For ordinary colors, use 45°/0° (light source incident at 45 degrees, human eye observed perpendicularly) or 0°/45° geometric conditions.
For metallic colors, pearlescent colors, and other effect colors, the observation angle needs to be changed to see if the angle-dependent color variation effect is consistent with the standard sample.
3. Sample Placement
Place the standard sample and the test sample side-by-side tightly, without any gaps, for direct comparison.
Swap their positions and observe repeatedly from different angles.
III. Comprehensive Judgment Process (Standard Operating Procedure)
A rigorous judgment of colorimetric accuracy should follow this process:
1. Sample Preparation: Prepare a sample using the colorant to be tested according to standard process conditions.
2. Condition Treatment: Place the test sample and the standard sample in the same environment (temperature, humidity) for a period of time to allow them to stabilize.
3. Preliminary Visual Inspection: Quickly inspect under natural light or a standard light source to check for significant differences.
4. Instrument Measurement: Use a colorimeter to perform multi-point measurements according to specifications, recording the ΔE, ΔL, Δa, and Δb values.
5. Data Analysis:
If ΔE and all differential values are within the customer-required tolerance range (e.g., ΔE < 1.0), proceed to the next step.
If the color deviation exceeds the acceptable range, analyze the direction of the color deviation based on the values of ΔL, Δa, and Δb, provide feedback to the colorist for formula adjustment, and then return to step 1.
6. Multi-light source visual evaluation: In a standard light source box, switch between different light sources to confirm the absence of color difference or metamerism.
7. Final judgment:
Instrument data qualified + Multi-light source visual evaluation qualified = Accurate color matching.
If either of these is unqualified, readjustment is required.
