One of the most common appearance changes in white, near-white, colorless, or transparent materials is yellowing.

A product that initially appears clear or white may gradually develop a yellow tint due to heat, ultraviolet exposure, oxidation, aging, chemical reactions, processing conditions, contamination, or material degradation.

Visual inspection can identify severe yellowing, but small changes are often difficult to evaluate consistently with the human eye.

This is where Yellowness Index (YI) becomes valuable.

Yellowness Index provides a numerical method for evaluating the degree to which a material’s color shifts toward yellow relative to specified measurement conditions.

It is widely used in quality control, R&D, material evaluation, weathering studies, process optimization, and product-development applications.

What Is Yellowness Index?

Yellowness Index, commonly abbreviated as YI, is a calculated colorimetric value used to describe the degree of yellow coloration of a material under specified measurement conditions.

Rather than describing a sample as:

“slightly yellow”

“more yellow than the previous batch”

or

“almost white”

instrumental color measurement provides numerical information that can be recorded and compared.

This makes Yellowness Index particularly useful when small color changes matter.

The concept is straightforward:

Reference material → Measurement → Exposure or processing → New measurement → Compare Yellowness Index

If the YI changes significantly, the material has undergone a measurable appearance change according to the selected method.

Why Do Materials Become Yellow?

Yellowing can occur for many reasons.

The mechanism depends on the material, environment, manufacturing process, and application.

Common contributors can include:

• UV exposure
• Thermal aging
• Oxidation
• Polymer degradation
• Processing temperature
• Chemical reactions
• Raw-material variation
• Additive changes
• Contamination
• Long-term storage
• Environmental exposure
• Weathering
• Repeated heat cycles

For example, a transparent polymer component may initially be nearly colorless.

After extended UV exposure, its molecular structure or additives may undergo changes that cause visible yellowing.

Similarly, a white plastic processed at excessive temperature may develop a yellow tint.

A Yellowness Index measurement can help quantify these changes.

Why Visual Inspection Is Not Enough

Human vision is excellent at recognizing color, but it is not an ideal measurement system.

Perceived color can change depending on:

Lighting

Background

Viewing angle

Observer

Surrounding colors

Sample thickness

Surface condition

Time of day

Two quality inspectors may look at the same sample and disagree about whether it is noticeably yellow.

Instrumental color measurement helps reduce this subjectivity.

A properly configured color-measurement instrument evaluates the sample under defined conditions and reports numerical results.

This enables manufacturers to establish tolerances and monitor trends.

What Does a Higher Yellowness Index Mean?

In general, a higher Yellowness Index indicates a stronger yellow coloration according to the selected YI calculation and measurement conditions.

However, YI values should not be interpreted without context.

The exact numerical result depends on factors including:

The YI equation or standard used

Illuminant

Standard observer

Instrument geometry

Sample presentation

Transmission or reflectance measurement

Sample thickness

Material characteristics

For this reason, Yellowness Index values should only be compared when measurements are performed under equivalent and properly documented conditions.

Yellowness Index ASTM E313

One commonly referenced standard in searches related to Yellowness Index is ASTM E313.

ASTM E313 addresses the calculation of whiteness and yellowness indices from instrumentally measured color coordinates.

When a company specifies an ASTM-based Yellowness Index, the measurement procedure, sample conditions, instrument settings, and current applicable standard requirements should be followed.

It is important not to assume that every Yellowness Index value generated by every instrument is directly interchangeable.

When reporting YI, the laboratory should document the method used.

Yellowness Index and CIELAB b*

Another common question is:

Is Yellowness Index the same as b?*

No.

In the CIELAB color space:

L* represents lightness.

a* represents the red-green direction.

b* represents the yellow-blue direction.

A positive b* value indicates movement toward yellow, while a negative b* value indicates movement toward blue.

Therefore, b* can provide useful information about yellow-blue color changes.

However, b* and Yellowness Index are not the same parameter.

Yellowness Index is calculated according to a specified formula or method, while b* is a coordinate in the CIELAB color space.

Both can be useful, but they should not be treated as interchangeable.

Why Measure Yellowness Index?

Manufacturers measure YI for several reasons.

Quality Control

Production batches can be compared against approved specifications.

Aging Studies

Materials can be measured before and after environmental or accelerated aging.

UV Testing

YI can help quantify yellowing after ultraviolet exposure.

Heat Stability Testing

Materials can be measured before and after thermal exposure.

Raw-Material Comparison

Different suppliers or material grades can be compared.

Product Development

R&D teams can evaluate how formulations influence resistance to yellowing.

Process Optimization

Processing conditions can be adjusted while monitoring changes in YI.

Supplier Quality

Incoming materials can be tested against established acceptance criteria.

Yellowness Index as a Quality-Control Parameter

Consider a manufacturer producing transparent polymer components.

An approved reference material is measured before production.

The company then measures each production batch under identical conditions.

If YI begins increasing over several batches, this trend can prompt an investigation into:

Raw materials

Processing temperature

Residence time

Additives

Storage

Contamination

Equipment conditions

YI therefore provides more than a pass/fail result.

It can act as a process-monitoring indicator.

How to Measure Yellowness Index

Accurate Yellowness Index measurement requires appropriate instrumentation, controlled sample preparation, standardized settings, and consistent procedures.

Which Instrument Measures Yellowness Index?

Depending on the application and method, Yellowness Index can be calculated from colorimetric measurements obtained using suitable color-measurement instrumentation such as a spectrophotometer.

The instrument measures the optical properties of the sample and software calculates YI according to the selected method.

Depending on the material, measurements may be performed in:

Reflectance mode

or

Transmission mode

Reflectance is commonly relevant for opaque samples.

Transmission may be relevant for transparent or translucent samples when the instrument and test method support it.

The appropriate measurement mode depends on the material and application.

Step 1: Determine the Applicable YI Method

Before measurement, determine which standard or calculation should be used.

Do not simply select a YI function because it appears in the instrument software.

The selected method should match:

Customer specifications

Industry requirements

Internal QC procedures

Applicable standards

Historical data

Product-development requirements

Using the same method consistently is essential for meaningful comparison.

Step 2: Prepare the Sample

Sample preparation can significantly influence color measurements.

For solid materials, important variables may include:

Surface condition

Thickness

Texture

Opacity

Orientation

Cleanliness

For powders, additional factors can include:

Sample depth

Packing density

Particle distribution

Surface leveling

Sample container

For liquids, the optical path length and sample cell can influence results.

The preparation procedure should therefore be standardized.

Step 3: Calibrate the Instrument

The instrument should be calibrated according to the manufacturer’s procedure before routine measurements.

Calibration helps establish reliable reference conditions for subsequent measurements.

Laboratories should also maintain appropriate verification and maintenance practices.

Step 4: Select Measurement Conditions

Important settings can include:

Illuminant

Observer

Measurement geometry

Aperture

Color scale

Yellowness Index calculation

Specular settings where applicable

Transmission or reflectance mode

These settings should be documented and kept consistent when comparing samples.

Step 5: Measure the Reference

A reference sample provides a baseline.

For example, a polymer manufacturer may measure a newly produced, approved material and store its YI value.

The sample can then be compared with material after:

100 hours of exposure

500 hours

1,000 hours

or another defined test period.

This provides quantitative information about yellowing over time.

Step 6: Measure the Test Sample

The test sample should be positioned consistently.

Multiple measurements may be appropriate when:

The sample is heterogeneous.

The surface is textured.

Color varies across the sample.

A laboratory procedure requires averaging.

If several measurements are taken, the measurement locations and averaging procedure should be standardized.

Step 7: Record Yellowness Index

The instrument software can calculate the Yellowness Index according to the selected method.

For example, a hypothetical aging study might produce:

Initial sample: YI = 2.1

After exposure: YI = 3.4

After extended exposure: YI = 6.2

These values indicate increasing yellow coloration according to the selected measurement conditions and YI calculation.

Delta YI – Change in Yellowness Index

In many applications, the change in Yellowness Index is more useful than an isolated YI value.

This can be expressed conceptually as:

ΔYI = YI after test – YI before test

For example:

Initial YI = 2.0

Final YI = 5.5

ΔYI = +3.5

This indicates an increase in measured yellowness.

Delta YI can be particularly useful for:

Accelerated aging tests

UV exposure studies

Weathering studies

Thermal stability tests

Material comparisons

Formulation development

Measuring Transparent Materials

Transparent plastics, films, sheets, resins, and similar materials may require transmission-based color measurement depending on the method.

Sample thickness becomes especially important.

A 1 mm sheet and a 10 mm sample may not produce directly comparable results.

Therefore, transparent-material YI testing should carefully control:

Sample thickness

Optical path

Surface condition

Measurement mode

Instrument settings

Measuring Opaque Materials

Opaque plastics, powders, coatings, paper, and other non-transparent materials may be evaluated using reflected light when appropriate.

Surface characteristics can influence the measurement.

Gloss, texture, orientation, and sample presentation should therefore be controlled.

Measuring Powder Yellowness Index

Powders require special attention because their physical presentation can affect optical measurements.

A repeatable procedure should define:

Sample cup

Sample quantity

Sample depth

Packing

Surface leveling

Measurement area

Number of readings

Powder samples should be prepared consistently when comparing production batches.

Importance of Measurement Geometry

Color-measurement instruments can use different optical geometries.

Different geometries may interact differently with surface appearance.

Therefore, measurements obtained from different instrument configurations should not automatically be assumed to be interchangeable.

For long-term quality control, maintaining consistent instrumentation and measurement conditions helps improve comparability.

Yellowness Index vs Color Difference

YI specifically focuses on yellow coloration according to the selected calculation.

Color difference metrics such as ΔE describe broader differences between two measured colors.

A material can potentially undergo changes in lightness or another color direction without those changes being fully represented by YI alone.

Therefore, manufacturers may monitor:

YI

L*

a*

b*

ΔE

depending on the application.

Applications of Yellowness Index Across Industries

Yellowness Index testing is relevant wherever yellowing can affect appearance, quality, performance expectations, or product acceptance.

Yellowness Index of Plastics

Plastic and polymer industries are among the major users of YI measurement.

Plastic materials may yellow due to:

Heat

UV radiation

Oxidation

Processing

Aging

Additive degradation

Recycled material content

Environmental exposure

YI measurement allows manufacturers to quantify these changes.

Applications include:

Transparent plastics

White plastics

Plastic sheets

Films

Molded components

Engineering polymers

Packaging materials

Yellowness Index of Polymers

Polymer development often involves studying resistance to degradation.

Researchers may prepare several formulations containing different stabilizers or additives.

Each formulation can be subjected to controlled aging.

YI is measured before and after exposure.

The results provide quantitative information that can be used alongside other performance tests when comparing formulations.

Yellowness Index of Polycarbonate

Polycarbonate is widely used where transparency, strength, and appearance can be important.

Yellowing may become a concern following extended heat or UV exposure depending on formulation and environment.

YI testing can support:

Material qualification

Accelerated aging

UV resistance studies

Supplier comparison

Process optimization

Quality control

Yellowness Index of PET

PET and related materials can also be evaluated for color consistency and yellowing.

Applications can include packaging, films, sheets, resins, and molded products.

Manufacturers can monitor YI to identify color shifts associated with raw materials or processing.

Yellowness Index of Resin

Resins used in coatings, adhesives, composites, polymers, and other applications may need consistent appearance.

YI measurement can help evaluate whether resin has undergone yellowing during:

Production

Storage

Heating

Curing

UV exposure

Aging

Yellowness Index of Transparent Materials

Transparent materials can be particularly sensitive to visible yellowing.

Even a small tint may be noticeable because consumers expect clarity.

Applications can include:

Transparent polymer sheets

Protective covers

Optical components where the method is suitable

Packaging

Films

Clear resins

Transparent molded parts

A transmission-capable measurement configuration may be appropriate depending on the sample and standard.

Yellowness Index of White Materials

White products can also develop yellow tones.

A white product does not need to become strongly yellow before the change becomes important.

Small shifts may affect:

Visual consistency

Customer perception

Batch matching

Brand appearance

Product acceptance

For these materials, manufacturers may evaluate YI together with Lab* values and other relevant indices.

Yellowness Index of Powder

Powders are used in numerous industries, including:

Chemicals

Pharmaceuticals

Food ingredients

Minerals

Pigments

Polymers

Cosmetics

Powder coatings

YI can provide an objective method for monitoring yellow tint when an appropriate measurement method is established.

Yellowness Index of Paper

Paper and paper-related products may be evaluated for changes in appearance associated with aging, storage, light exposure, or material composition.

Whiteness, brightness, color coordinates, and yellowness may all be relevant depending on the product specification.

Yellowness Index in Textiles

White and light-colored textiles can yellow due to heat, chemical treatments, storage, aging, or environmental exposure.

Instrumental measurement can help compare samples before and after treatment.

Yellowness Index of Coatings

Clear and white coatings may undergo yellowing because of curing, aging, heat, UV exposure, or formulation changes.

YI can help coating manufacturers compare:

Resin systems

Additives

Curing conditions

Weathering performance

Production batches

Pharmaceutical Applications

Where color is a relevant quality attribute, pharmaceutical and related materials may be evaluated instrumentally according to appropriate procedures.

Powders, packaging materials, and other products may require objective appearance measurements.

YI should be used only where appropriate to the validated quality method and product specification.

Chemical Industry

Chemical manufacturers often produce clear, white, or near-white products.

Color changes can sometimes indicate differences associated with processing, storage, raw materials, or degradation.

YI measurement provides a numerical way to track such appearance changes.

Automotive Applications

Automotive materials experience demanding environmental conditions.

Interior and exterior polymers, coatings, transparent materials, and other components may be exposed to:

Sunlight

Heat

Humidity

Weathering

Chemical contact

Accelerated aging studies can include YI measurements to quantify changes in yellow coloration.

Building Materials

Polymer panels, coatings, sealants, adhesives, laminates, and other building materials may need long-term appearance stability.

YI testing can contribute to weathering and aging evaluations.

Why Yellowness Index Is Important in R&D

R&D teams often need to compare several formulations.

Suppose five polymer formulations are being developed.

Each is subjected to the same accelerated aging test.

The laboratory measures initial and final YI.

Rather than relying on photographs alone, researchers obtain numerical data showing how much yellowing occurred under the defined test conditions.

This makes formulation comparison more objective.

Yellowness Index for Quality Control, Best Practices and FAQs

Reliable Yellowness Index testing requires consistency.

The numerical result is only useful when the measurement method is controlled.

Best Practices for Yellowness Index Measurement

1. Use the Correct Standard

Identify the appropriate YI calculation or standard before testing.

ASTM E313 is commonly associated with Yellowness Index calculations, but laboratories should verify the appropriate current method for their material and application.

2. Keep Sample Preparation Consistent

Changes in thickness, surface texture, powder packing, orientation, or sample holder can influence measurements.

3. Calibrate the Instrument

Follow the manufacturer’s recommended calibration procedure.

4. Maintain Consistent Measurement Settings

Do not compare results generated using different illuminants, observers, geometries, or YI calculations without understanding the differences.

5. Control Sample Thickness

This can be especially important for transparent and translucent materials.

6. Keep Samples Clean

Dust, fingerprints, residues, scratches, or contamination can affect optical measurements.

7. Use Multiple Measurements Where Appropriate

If the sample is heterogeneous, measurements at multiple locations can provide a better representation.

8. Record Measurement Conditions

A QC report should ideally include enough information to reproduce the test.

Yellowness Index for Batch-to-Batch Quality Control

One of the most practical applications of YI is production monitoring.

A company can establish an approved reference range based on product requirements.

Each new batch is then measured under the same conditions.

This enables:

Fast batch comparison

Trend monitoring

Supplier evaluation

Process troubleshooting

Quality documentation

Out-of-trend investigation

Instead of waiting until yellowing becomes obvious visually, manufacturers can identify smaller numerical changes.

Yellowness Index for UV Aging

UV radiation can contribute to color changes in many polymeric and organic materials.

A typical study may involve:

Measure initial YI.

Expose the sample under specified UV/weathering conditions.

Measure YI after defined intervals.

Calculate the change.

Plot YI against exposure time.

This provides a quantitative representation of yellowing behavior under the test conditions.

Yellowness Index for Thermal Aging

Heat can also cause yellowing in some materials.

Samples can be measured before and after controlled thermal exposure.

This is useful for evaluating:

Processing stability

Long-term heat aging

Material formulations

Stabilizer performance

Curing effects

The results should always be interpreted alongside the actual thermal exposure conditions.

Yellowness Index and Weathering Tests

Outdoor products experience combinations of UV radiation, temperature, moisture, and other environmental factors.

Accelerated weathering tests attempt to evaluate material changes under controlled conditions.

YI can be one of the measured appearance parameters.

For example:

Initial measurement → Weathering exposure → Intermediate YI → Additional exposure → Final YI

This produces a measurable yellowing trend.

Yellowness Index vs Whiteness Index

Yellowness Index and Whiteness Index serve different purposes.

Yellowness Index (YI) evaluates yellow coloration according to a defined calculation.

Whiteness Index (WI) describes whiteness according to a specified calculation.

A material may be evaluated using one or both indices depending on its intended appearance and applicable test method.

For white materials, laboratories may also monitor L*, a*, and b* values.

Yellowness Index vs b*

This distinction is important for anyone searching for how to measure yellowness.

b* represents the yellow-blue coordinate within CIELAB.

Positive b* indicates the yellow direction.

Negative b* indicates the blue direction.

YI is a separately calculated index.

Therefore:

b = color-space coordinate*

YI = calculated yellowness index

Both can indicate useful information about yellowing, but they should not be substituted for each other without technical justification.

Yellowness Index vs ΔE

ΔE describes overall color difference between two measured colors according to the selected color-difference formula.

YI focuses specifically on yellowness.

Suppose a sample becomes darker without becoming significantly more yellow.

The overall ΔE could increase while YI changes differently.

For comprehensive color quality control, laboratories may therefore evaluate multiple parameters.

How to Choose an Instrument for Yellowness Index Measurement

When selecting a color measurement system for YI testing, consider:

Measurement Mode

Does the application require reflectance, transmission, or both?

Sample Type

Will you measure:

Solids?

Powders?

Films?

Liquids?

Transparent plastics?

Opaque components?

Measurement Geometry

Select a geometry appropriate for the material and required standard.

Supported YI Calculations

Confirm that the system supports the required Yellowness Index method.

Repeatability

Small changes in YI may require a system capable of consistent measurements.

Sample Accessories

Powders, liquids, films, and small samples may require suitable holders or accessories.

Software

Quality-control software can help store:

Standards

Tolerances

Measurements

Batch data

Color differences

YI results

Historical trends

Frequently Asked Questions About Yellowness Index

What is Yellowness Index?

Yellowness Index is a numerical colorimetric parameter used to describe the degree of yellow coloration of a material according to specified measurement conditions and a defined calculation.

What is YI in color measurement?

YI is the abbreviation for Yellowness Index.

What does a high Yellowness Index mean?

Generally, a higher YI indicates stronger yellow coloration according to the selected YI calculation. Results should always be interpreted using consistent measurement conditions.

Which instrument measures Yellowness Index?

Suitable color-measurement instrumentation, commonly including spectrophotometric systems, can measure the required optical/color data and calculate YI when the appropriate method is supported.

Is Yellowness Index the same as b*?

No. b* is the yellow-blue coordinate of CIELAB, while Yellowness Index is a separately calculated parameter.

What is ASTM E313 Yellowness Index?

ASTM E313 is a commonly referenced standard concerning the calculation of whiteness and yellowness indices from instrumentally measured color coordinates. Laboratories should follow the applicable current edition and specified measurement conditions.

Can Yellowness Index be measured in plastic?

Yes. YI measurement is widely applicable to suitable plastic and polymer materials when the correct measurement method is used.

Can YI be measured in transparent samples?

Yes, when the instrument, measurement mode, sample preparation, and applicable method are suitable for transparent materials.

Can Yellowness Index be measured in powder?

Powder yellowness can be evaluated instrumentally when a repeatable sample-presentation method and appropriate YI calculation are used.

What causes yellowing in polymers?

Depending on the polymer system, factors can include UV exposure, oxidation, heat, aging, processing conditions, chemical reactions, additives, and environmental exposure.

What is ΔYI?

ΔYI represents the change in Yellowness Index between two defined measurements, such as before and after aging.

Conclusion

Yellowness Index (YI) is an important numerical tool for evaluating yellow coloration and monitoring yellowing in materials.

It is particularly valuable for plastics, polymers, transparent materials, white products, powders, resins, coatings, chemicals, paper, textiles, packaging, automotive materials, and other industrial products where appearance stability matters.

Instead of relying only on subjective visual descriptions, instrumental Yellowness Index measurement enables manufacturers and laboratories to quantify changes and create traceable quality data.

YI can be especially useful for:

Batch-to-batch quality control

UV aging studies

Thermal aging studies

Weathering tests

Raw-material inspection

Supplier comparison

Process optimization

Product development

Material stability testing

Research and development

For reliable results, Yellowness Index measurements should always be performed using a defined method with controlled sample preparation and consistent measurement conditions.

Parameters such as measurement geometry, illuminant, observer, reflectance or transmission mode, sample thickness, surface condition, sample presentation, and YI calculation can influence results.

It is also important to distinguish Yellowness Index from other color parameters.

CIELAB b* provides information about the yellow-blue color direction, while YI is a calculated yellowness parameter. ΔE provides information about overall color difference, while Whiteness Index evaluates whiteness according to its specified calculation.

Used together where appropriate, these measurements provide manufacturers with a much more complete understanding of material appearance.

Ultimately, Yellowness Index transforms a subjective observation—“this material looks more yellow”—into numerical information that can be measured, compared, documented, and monitored.

For industries where product appearance and color stability are important, Yellowness Index testing can therefore become an essential component of modern color quality control.