Color Evaluation of Structurally Colored Films by Angle of Incidence

Significance of the Topic

Structural coloration, as seen in nature (e.g., Morpho butterfly wings), relies on nanoscale interference rather than pigments. Such effects offer durable, angle‐dependent color without fading. Biomimetic multilayer films replicate these properties, opening innovative applications in optical coatings, sensors, anti‐counterfeiting, and decorative materials.

Objectives and Study Overview

This study evaluates the color changes of three multilayer films (MLF-13.0, MLF-16.5, MLF-19.0) under varying angles of incident light. By measuring transmission spectra and converting them into L*a*b* coordinates, the research aims to characterize angle‐dependent structural coloration and demonstrate the utility of variable‐angle spectrophotometry for biomimetic material analysis.

Methodology and Instrumentation

Measurements were performed using a SolidSpec-3700DUV spectrophotometer equipped with:

• Variable angle absolute reflectance attachment
• Large polarizer assembly (set to 45°, approximating unpolarized light)
• Integrating sphere detector

Key parameters:

• Wavelength range: 300–800 nm
• Scanning speed: medium
• Sampling pitch: 1 nm
• Slit width: 20 nm
• Incident angles: 0°, 15°, 30°, 45°, 60°

Main Results and Discussion

MLF-13.0:

• At 0°, absorption peaked at 400–500 nm, yielding a yellow appearance.
• Increasing angle shifted absorption to shorter wavelengths, reducing colorfulness and producing near‐white transmission at 60°.

MLF-16.5:

• At 0°, green wavelengths (500–600 nm) were absorbed, producing violet color.
• As incidence increased, the absorption band moved toward shorter wavelengths; at 60°, blue absorption dominated (400–500 nm), yielding yellow.

MLF-19.0:

• At 0°, red absorption (600–700 nm) created pale blue appearance.
• With increasing angle, absorption shifted toward green (500–600 nm), resulting in violet at 60°.

For each film, L*a*b* coordinates confirmed complementary color shifts and decreasing chroma with angle, illustrating tunable structural coloration.

Benefits and Practical Applications of the Method

The variable‐angle spectrophotometric approach provides:

• Quantitative analysis of angle‐dependent optical properties
• Non‐destructive evaluation of multilayer interference structures
• Rapid color assessment for material design and quality control

This method supports development of advanced optical films for automotive coatings, packaging, security printing, and smart sensors.

Future Trends and Potential Applications

Advances may include:

• Integration of active tuning elements (e.g., electrochromic layers) for dynamic color control
• Nanofabrication of films with optimized layer thickness for wider color gamut
• Applications in flexible displays and wearable optical devices
• Environmental sensors leveraging angle‐dependent reflectance changes

Conclusion

This work demonstrates the effective use of SolidSpec-3700DUV and variable angle reflectance attachment to characterize structural coloration in biomimetic multilayer films. Angle‐dependent shifts in transmission spectra and L*a*b* coordinates confirm tunable optical responses, underscoring the value of this analytical approach for material innovation.

References

1. Teijin Laboratories. "Morphotex" structurally colored fibers created with advanced nanotechnology.
2. H. Hirose; Polymers, 60(5), 298–301 (2011).
3. Teijin DuPont Films Japan Limited. PET Film MLF Product Overview.
4. Konica Minolta. IROIROZATSUGAKU.