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FAQ - Why do two instruments disagree?

Few situations create more frustration in color quality control than when two instruments measure the same sample and produce different results. One laboratory reports the product is within tolerance. Another reports it is out of tolerance. A supplier and customer disagree on color acceptance. A new instrument does not match the historical data from an older instrument. When this happens, the first reaction is often to assume that one of the instruments is wrong.

In reality, two instruments can produce different color measurements for many reasons, and instrument malfunction is often one of the least common causes. Before concluding that an instrument is inaccurate, it is important to understand the many factors that can influence color measurement results.

The first question should be: "Are the instruments measuring the sample under the same conditions?"

Color measurement is not defined solely by the instrument. It is also defined by the measurement geometry, illuminant, observer, aperture size, sample presentation method, color scale, and color difference equation being used. Differences in any of these parameters can produce different results even when both instruments are functioning perfectly.

Common Causes of Instrument Disagreement

1. Different Measurement Geometries

One of the most common causes of disagreement is geometry. Examples include:

  • Sphere vs. directional geometry
  • d/8 vs. d/0 geometry
  • 45°/0° vs. 0°/30° geometry

Because each geometry evaluates appearance differently, measurements may not correlate directly.

2. Different Measurement Conditions

Differences in measurement conditions can affect reported color values. These differences include:

  • Illuminant
  • Observer
  • Aperture size
  • SCI vs. SCE settings
  • UV included vs. UV excluded settings


Even small configuration differences may create noticeable measurement discrepancies.

3. Sample Presentation Differences

Often the instruments agree more closely than the samples being presented. Examples include:

  • Different sample locations
  • Different orientations
  • Different packing densities
  • Different sample thicknesses
  • Different amounts of material
This is especially common with:
  • Plastic pellets
  • Powders
  • Recycled materials
  • Food products
  • Textured surfaces

4. Instrument Standardization and Calibration

A properly standardized instrument is more likely to produce consistent results. Issues may arise when:

  • Standardization or Calibration is overdue
  • Reference standards are contaminated
  • Standards are damaged
  • Operators follow different procedures

5. Material Non-Uniformity

Many products are not perfectly uniform. Two instruments may measure different areas of the same sample and obtain legitimately different results because the sample itself varies. This is often mistaken for instrument disagreement when the real issue is material variability.

6. Instrument Technology Differences

Different instruments may use:

  • Different optical designs
  • Different detector technologies
  • Different light sources
  • Different spectral bandwidths
  • Different processing algorithms

Modern instruments are generally designed to minimize these effects, but small differences may still occur, particularly with highly saturated, fluorescent, textured, or optically complex materials.

Instrument Agreement Is Not Perfect Agreement

A common misconception is that two instruments should always produce identical measurements. In reality:

  • Every measurement system has some level of uncertainty.
  • Even instruments of the same model measuring the same standard will exhibit small differences.

This is why instrument-to-instrument agreement specifications exist and why color tolerances are typically larger than the expected instrument variation.

The important question is not: "are the measurements identical?" but rather: "are the measurements close enough to support the same quality decision?"

In many cases, small measurement differences do not affect pass/fail outcomes and are therefore operationally insignificant.

Start With the Basics

When investigating instrument disagreement, the following questions should be answered first:

  • Are both instruments standardized?
  • Are both instruments measuring the same sample?
  • Are measurement conditions identical?
  • Is the sample presentation consistent?
  • Are the same illuminant and observer being used?
  • Is the same geometry being used?
  • Is the sample uniform?

Only after these factors have been verified should instrument performance become the primary focus.

Ultimately, two instruments rarely disagree for a single reason. The most successful investigations approach the issue systematically, evaluating the entire measurement process rather than assuming the instrument itself is at fault.

Different Does Not Mean Wrong

Two instruments can produce different measurements and still both be functioning correctly. Before investigating the instrument, verify that the sample, method, geometry, and measurement conditions are truly identical.

HunterLab Perspective

One of the most common assumptions in color quality control is that measurement disagreement indicates an instrument problem. In practice, HunterLab often finds that differences originate from sample presentation, measurement conditions, geometry changes, or product variability rather than instrument performance.

For example, a customer may compare measurements from a legacy instrument and a new instrument without realizing that:

  • The aperture sizes differ.
  • The geometry differs.
  • The observer setting differs.
  • The sample preparation method differs.

In these situations, the instruments are often reporting exactly what they are designed to measure.

This is particularly important during instrument upgrades and migrations. A newer instrument may provide more representative measurements than an older system, yet the values may differ because the measurement approach itself has changed.

A Practical Example

Consider a manufacturer upgrading from a legacy spectrophotometer to a newer instrument. Measurements on a recycled plastic pellet sample show a consistent shift in L* and b* values compared to historical data.

Initial concerns focus on instrument performance. However, investigation reveals that:

  • The new instrument uses a larger aperture.
  • The larger aperture measures more pellets simultaneously.
  • The resulting measurement is more representative of the bulk material.

The material did not change. The instrument did not malfunction. The measurement became more representative. This example highlights an important principle:

When two instruments disagree, the first objective is not determining which instrument is right—it is understanding what each instrument is actually measuring.

Once that question is answered, the reason for the disagreement often becomes much clearer.

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

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