Technical Information

Got a "bright white" T-shirt—does it truly look white, or is it just "cheating" with fluorescent agents? When matching colors for your products, do your dyes contain any fluorescence? After all, that directly affects the accuracy of color measurements! Today, we’ll teach you a professional yet practical method: use a spectrophotometer to precisely determine whether your product contains fluorescent components.

In the field of color measurement, there’s a crucial point that’s often overlooked: fluorescent components. For ordinary colored products, it’s usually not necessary to pay much attention to the UV content or spectral power distribution (SPD) of the measurement light source. However, once a product contains optical brightening agents (OBAs), fluorescent dyes, or pigments, the situation changes dramatically!

Why? Because these fluorescent substances absorb ultraviolet light (or other specific wavelengths) from the light source and re-emit it as visible light, making objects appear brighter and more vibrant.

Measurement Challenge: If the UV content of the measurement light source isn’t controlled (by using UVcal mode), the color data obtained will be severely distorted! Even more complex, some fluorescent pigments can actually be excited by visible light, meaning both the intensity of the light source and its overall Spectral Power Distribution (SPD) can significantly influence the measurement results. Therefore, accurately determining in advance whether a product contains fluorescent components is the first critical step toward achieving precise color measurements!

The most straightforward method is to place the sample under a UV (ultraviolet) lamp and observe whether fluorescence is emitted. As shown in the figure below:

Figure 1: Appearance of the sample under conventional lighting

Figure 2 Under the UV Light Source

This method is simple and quick, allowing you to immediately spot samples with strong fluorescence (such as the three in the upper-right corner of the image). However, it has significant limitations: when dealing with products containing trace amounts of fluorescent substances, the human eye often struggles to detect even faint fluorescence, leading to potential misjudgments. That’s precisely why we need more precise quantitative methods in such cases.

Professional Quantitative Assessment Method: Spectrophotometer "Dual-Mode" Comparison

1. A spectrophotometer provides objective data and is an excellent tool for determining fluorescence. The core idea is to compare the measurement results of the same sample under two illumination modes—one with UV light and one without. The specific operational steps are as follows:

One is UVin, which means fully releasing the UV energy inside the measurement lamp for use; the other is UVex, which involves filtering out the UV energy inside the measurement lamp using a 400nm filter.

Figure 3: Predefined UVin and UVex Measurement Modes in the Software

2. Instrument Calibration: Perform black-and-white board calibration separately for each of the two preset modes. Please note carefully: The orientation and support condition of the calibration board must be kept strictly consistent to ensure a reliable calibration foundation.

Figure 4: Two Preset Modes After Calibration Is Complete

3. Sample Measurement: Secure the sample to be tested in the measurement hole, ensuring its position remains unchanged.

Switch to UVin mode and measure the data twice (with an interval of about 20 seconds).

Switch to UVex mode and measure the data twice again (with an interval of about 20 seconds).

Figure 5: The four samples to be tested—Yellow 1, Orange 1, Yellow 2, Orange 2

Figure 6: Schematic Diagram of Sample Placement and Measurement

4. Interpreting the Results: Fluorescence leaves no room for concealment—take the measurement results of the "Yellow 1" sample as an example:

Figure 7: Reflectance Curve of Sample Yellow 1 – in UVin and UVex Modes

Key Signal 1: Reflectance > 100%! In UVin mode, you’ll notice that at certain wavelengths—particularly in the green light region—the reflectance exceeds 100%. This is a hallmark characteristic of fluorescent materials: after absorbing UV light, they emit additional visible light, resulting in total reflected light surpassing the amount of incident light.

Key Signal 2: Still Over 100% Under UVex? Even when the UVex mode filters out UV light, its reflectance in certain areas still exceeds 100%. This indicates that the fluorescent yellow isn’t just excited by UV light—it can also be triggered by visible light.

Fluorescence-based core logic: Difference comparison

Perform a detailed comparison of the measurement data from the four samples (specifically the L*a*b* values):

*(Table 1-4: Comparison of Data Differences Between the Four Samples in UVin and UVex Modes)*

Judgment Principle:

If the difference (ΔE*ab) in color data measured under UVin mode and UVex mode is significantly greater than the difference observed between the two measurements taken within the UVin mode itself (or within the UVex mode itself), then it can be confirmed that the UV content of the measurement light source has a notable impact on the results—indicating that the sample contains fluorescent components!

5. Conclusion: According to the data analysis (ΔE* comparison), samples Yellow 1 and Orange 1 contain fluorescent components, while Yellow 2 and Orange 2 do not. This aligns with their behavior under a pure UV light source in a darkroom (Yellow 1 and Orange 1 exhibit fluorescence, whereas Yellow 2 and Orange 2 show no fluorescence).

Figure 8: Measurement Results of Sample Orange 1

Figure 9: Measurement Results of Sample Yellow 2

Real-world challenge: Does this dark brown sample exhibit fluorescence?

Now, let's use this method to analyze the following dark brown sample:

Figure 10: Reflectance Curve of Sample "Deep Brown" – in UVin and UVex Modes

 

Observe the curve:

In UVin mode, the reflectance is significantly higher overall compared to UVex mode.

Key area: In the short-wave region (such as near 400–500 nm blue-violet light), the reflectance in UVin mode is significantly higher.

Does it exceed 100%? In this dark-colored sample, although the overall reflectance is relatively low (less than 100%), the UVin mode exhibits significantly higher reflectance in the short-wave region compared to the UVex mode.

6. Finally, place the four sets of data into an Excel spreadsheet for comparison, as follows:

Table 1: Comparative Analysis, Reflections, and Interactions on Sample Yellow 1

Table 2: Comparison Results for Sample Orange 1

Table 3: Comparison Results for Sample Huang 2

Table 4: Comparison Results for Sample Orange 2

7. Fluorescence Determination: If the difference in data (ΔE*) measured between the UVin and UVex states of the sample is significantly greater than the difference in data obtained from two separate measurements taken within either the UVin or UVex state, it can be concluded that the UV content of the measuring lamp affects the measurement results. In other words, the sample contains fluorescent components.

8. Therefore, we can conclude that Yellow 1 and Orange 1 contain fluorescent components, while Yellow 2 and Orange 2 do not.

9. Place these four samples in the darkroom—simply turning on the UV light will also reveal the differences:

Fluorescence Identification

Now, let's use this method to determine whether the following sample contains fluorescence:

10. The measurement results are as follows:

Figure 10: Measurement Results of the Sample Dark Brown

May I ask if you think this sample contains fluorescent components?

Mastering this "dual-mode" comparison method will enable you to more professionally and precisely identify the "fluorescent secrets" within products—whether in quality inspection, R&D, or procurement—ensuring reliable color measurement and quality control. Next time you encounter a "strikingly vibrant" color, don’t hesitate to use a tool for an objective measurement; let the data do the talking!