This application brief explains the unique challenges associated with color measurement of recycled PET (rPET) materials, including variability caused by heterogeneous recycled streams, sample presentation inconsistencies, and unknown optical brightening agents (OBAs). It demonstrates how the Agera L2 provides visually relevant, repeatable, and production-ready color measurement through true 0°/45° geometry, calibrated UV control, and large-area sample averaging.

 

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The rapid growth of recycled PET (rPET) usage in packaging, consumer goods, and sustainable manufacturing has introduced new challenges in color quality control. Unlike virgin PET, rPET streams often contain uncontrolled material variability including contamination, degradation, inconsistent flake color, directional texture effects, and unknown levels of optical brightening agents (OBAs).

Traditional color measurement systems frequently struggle with:

• Small-area sampling errors
• Poor visual correlation on textured or translucent materials
• Inconsistent readings caused by directional variation
• Inability to properly characterize UV-reactive optical brighteners
• False pass/fail conditions due to inadequate sample averaging

This article explains the scientific and operational challenges associated with rPET color measurement and demonstrates how the Agera L2 addresses these issues through:

• True circumferential 0°/45° measurement geometry
• Large Area of View (LAOV) measurement capability
• True D65 illumination with calibrated UV control
• Improved measurement repeatability on heterogeneous materials
• Enhanced visual correlation with human perception

 

1. Why rPET Is Difficult to Measure

Virgin PET resin is generally uniform in:

• Color
• Transparency
• Optical behavior
• UV response

Recycled PET is fundamentally different. rPET streams can contain:

• Multiple source materials
• Unknown additive histories
• Variable thermal degradation
• Residual dyes and pigments
• Contaminants
• Optical brighteners from previous applications

As recycled content percentages increase, color variability becomes more difficult to control.

Typical manufacturing challenges include:

Challenge	Operational Impact
Mixed feedstock variability	Inconsistent bottle or preform appearance
Yellowing/degradation	Consumer rejection
Unknown optical brighteners	Supplier mismatch under retail lighting
Sample nonuniformity	Poor QC repeatability
Directional texture effects	Measurement disagreement between labs

 

2. The Optical Brightener Problem in rPET

What Are Optical Brighteners?

Optical Brightening Agents (OBAs), also called Fluorescent Whitening Agents (FWAs), absorb ultraviolet energy and re-emit visible blue light. This creates the visual perception of:

• Increased whiteness
• Cleaner appearance
• Reduced yellowing

OBAs are common in:

• Textile packaging
• Consumer containers
• Labels
• Films
• Household products

When these materials enter the recycling stream, OBAs may unknowingly become incorporated into rPET feedstock.

 

Why OBAs Create Measurement Problems

The appearance of OBA-containing material changes dramatically depending on UV content in the illumination source. Without controlled UV measurement:

• Two materials may numerically match but visually mismatch
• Color measurements become lighting-dependent
• Suppliers and customers disagree under retail lighting

UV Included vs UV Excluded Measurement Behavior Example: rPET Flake Containing Unknown Optical Brighteners

 

Measurement Condition	L*	b*
UV Included	72.8	-3.4
UV Excluded	69.9	+0.8

 

Interpretation

• UV Included → material appears brighter and bluer
• UV Excluded → material appears duller and yellower

This difference indicates fluorescence activity from optical brighteners embedded within the recycled stream.

Graph 1: UV Included vs UV Excluded Response - Negative b* values indicate increased blue appearance caused by fluorescence.

 

3. Sample Presentation Challenges in rPET

Heterogeneity

rPET materials are rarely optically uniform. Variation may include:

• Mixed flake coloration
• Particle size differences
• Surface gloss variation
• Directional texture
• Localized contamination

Small-area measurements can therefore produce dramatically different readings depending on exactly where the instrument measures.

 

The Small Area View Problem

Traditional small-aperture measurements may capture only a tiny portion of the sample surface. This creates:

• High reading-to-reading variability
• False reject conditions
• Poor correlation to human visual averaging

Example: Small vs Large Area Measurements

 

Measurement Method	Avg ΔE Repeatability
Small Area View	0.82
Large Area View	0.19

Graph 2: rPET Pellet Blend — 10 Repositioned Measurements

The larger measurement area averages across material variability more effectively, better representing how the human eye perceives the material.

 

4. Why Traditional Sphere Instruments Can Struggle

Diffuse sphere instruments are excellent for many applications, but rPET presents unique challenges. Sphere geometry may:

• Reduce sensitivity to texture and directional effects
• Over-average gloss behavior
• Decrease visual correlation for certain packaging materials

In many rPET applications, manufacturers care most about “Does this material visually match what the customer sees?” This is where 0°/45° geometry becomes extremely valuable. 

 

5. The HunterLab Agera L2 Solution

The HunterLab Agera L2 was specifically designed to address visually critical color applications where texture, directional effects, and optical variability matter:

Circumferential 0°/45° Geometry

The system uses:

• Circumferential illumination
• 15-direction optical pickup design
• True 0°/45° measurement geometry

This configuration closely matches human visual perception of color and appearance. Benefits include:

• Improved visual correlation
• Better agreement between operators and instrument
• Reduced directional bias
• Improved supplier-to-customer consistency

 

True CIE D65 Illumination with UV Control

The Agera L2 includes true D65-calibrated illumination with controlled UV characteristics. This enables:

• Detection of optical brighteners
• Reliable fluorescent material evaluation
• Improved agreement with retail lighting environments
• Better control of recycled feedstock variability

This capability is especially important in rPET streams where OBAs may be present but unknown.

 

Extra Large Area of View (XLAV)

The industry-leading 2-inch area of view allows the instrument to average across larger portions of heterogeneous samples. Advantages include:

• Improved repeatability
• Better representation of bulk appearance
• Reduced impact of localized contamination
• More stable production tolerances

For rPET pellets, flakes, and irregular materials, this dramatically improves measurement robustness.

 

Improved Operational Confidence

By combining:

• Large-area averaging
• Visual-correlation geometry
• Controlled UV measurement
• High repeatability

the Agera L2 helps manufacturers:

• Reduce false rejects
• Improve supplier communication
• Detect recycled-stream variability earlier
• Tighten color tolerances confidently
• Improve consistency between plants

 

6. Recommended Best Practices for rPET Color QC

Sample Preparation

• Use glass sample cup
• Use consistent sample depth
• Minimize voids and layering artifacts
• Standardize backing materials

Measurement Methodology

• Use Large Area of View whenever possible
• Average multiple measurements for highly variable materials
• Evaluate UV Included vs UV Excluded behavior for fluorescent detection
• Establish retained reference standards

Process Control

Monitor:

• L*, a*, b*
• Fluorescence response (UV included vs UV excluded) using the Agera L2 D65 UV Compare Mode (Agera® L2 D65 UV400 Compare Mode — Detecting Optical Brighteners and Fluorescent Additives)
• ΔE variation trends
• Lot-to-lot repeatability

Trend analysis is often more valuable than single-point measurements in recycled streams.

 

7. Conclusion

Recycled PET introduces significant color measurement complexity compared to virgin materials. Unknown optical brighteners, heterogeneous sample presentation, directional texture effects, and material variability can create major challenges for traditional QC workflows. Accurate characterization requires:

• Controlled UV measurement
• Large-area averaging
• Strong visual correlation
• High repeatability on heterogeneous materials

The Agera L2 addresses these challenges through its:

• Circumferential 0°/45° geometry
• True D65 illumination
• Controlled UV performance
• Large Area of View capability
• Enhanced repeatability on variable materials

As recycled content requirements continue increasing globally, robust color measurement systems will become increasingly critical for maintaining product consistency, customer confidence, and sustainable manufacturing success.

Download the Application Brief below

To learn more about Color and Color Science in industrial QC applications, click here: Fundamentals of Color and Appearance

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