Powders and granules are bulk, non-uniform samples. Their measured color depends not only on composition but also on particle size, moisture, packing, depth, surface levelling, cup design and how many particles fall inside the viewed area.
Why loose materials are difficult to measure
A loose sample contains particles, gaps and many small facets. Those features create shadowing, light trapping and directional reflection. A single reading of a few large granules can describe the particles that happened to face the detector rather than the batch. Preparation must make the optical presentation reproducible without changing the material being controlled.
| Variable | Possible color effect | Control |
|---|---|---|
| Fill depth | Backing or transmitted light affects a shallow sample. | Use a defined depth demonstrated to be optically opaque. |
| Packing density | Gaps, shadows and surface height change. | Define filling, tapping, compression and levelling. |
| Particle size | Scattering and the number of particles viewed change. | Control size distribution and do not grind unless the method requires it. |
| Moisture and temperature | Surface refractive behaviour, clumping and chemistry can change. | Condition and measure within a defined time window. |
| Cup or window | Window reflection, color and scratches enter the optical path. | Use one cell design, orientation and cleaning procedure. |
Choose and lock the sample presentation
Open-port measurement can view the prepared top surface directly but needs a light-excluding cover and careful control of surface height. Measurement through an optically clear cup or window contains the material and presents a flatter boundary, but the window becomes part of the method. Use the same accessory for standards and samples and check it for scratches, residue and static-held particles.
Fill by mass, volume or a marked line as specified. If tapping is used, define the number and force; if a plunger is used, define the load or stop. Excessive compression can change the color of powders by altering density, surface smoothness or particle structure. Levelling should remove large voids without polishing or segregating the sample.
Prove that the sample is deep enough
Determine optical opacity during method development by measuring increasing fill depths with the proposed cup and backing. When additional material no longer changes the result beyond the method’s repeatability, the sample has reached a practical infinite-thickness condition for that setup. Record a fill depth safely above that point.
Do not assume an opaque-looking powder is optically opaque
Gaps between particles and translucent granules can allow the backing or cup base to influence the reading. Changing cup depth or backing can then shift L*a*b* even when the batch is unchanged.
Use a viewed area that represents the particles
A large aperture includes more particles and usually reduces sensitivity to one granule, void or dark speck. A smaller aperture can be necessary for limited sample quantities, but it increases sampling uncertainty and may require more independent preparations. The cup must cover the required illuminated area as well as the detector’s viewed area.
For coarse granules, rotate or refill the cup between readings according to the approved plan. Repeatedly triggering the instrument on one unchanged presentation tests short-term instrument variation; it does not sample the bulk material.
Take independent preparations
Draw a representative laboratory sample from the batch before preparing the measurement cup. Segregation during transport can separate large, small, heavy or pigmented particles, so define mixing and sampling without creating attrition. Prepare several independent cup fills during validation and compare their spread with the process tolerance.
Keep individual readings and investigate outliers rather than automatically deleting them. A dark pellet, contaminant or agglomerate may be a real quality defect. Decide separately whether the report controls average bulk color, within-sample uniformity, or both.
Loose granules, ground powder and moulded plaques are different tests
Grinding changes particle size and exposed surfaces. Moulding or pressing changes density, optical interfaces, orientation and sometimes chemistry through heat and pressure. A resin pellet result, ground-powder result and processed plaque result should not share one target unless correlation has been demonstrated. Measure the form that answers the quality question: incoming raw material, blended bulk or final processed product.
For translucent or optically brightened granules, also define backing and UV condition. A difference caused by fluorescence or internal light spread cannot be corrected by simply selecting a different calculation illuminant.
A routine powder and granule workflow
01
Condition and mix
Control moisture, temperature, settling time and the representative subsample procedure.
02
Prepare the cup
Fill to the defined mass or line, apply the stated packing action and level consistently.
03
Measure independently
Empty and refill or otherwise re-present the material between readings to sample bulk variation.
04
Clean and record
Prevent cross-contamination and retain the preparation details with each result.
Minimum method record
Record material form and particle-size range, conditioning, mixing and subsampling, cup/window and backing, fill depth or mass, tapping or compression, levelling, geometry, aperture, UV condition where relevant, illuminant, observer, formula and the number of independent preparations. Include dust-control and cleaning requirements for safe operation and cross-contamination prevention.
References and further reading
- ASTM E1164: spectrometric data for object-color evaluation.
- HunterLab: sample cups, opacity and independent powder preparations.
- Datacolor: sample preparation and presentation for powders.
Continue with sample preparation, sampling and averaging, aperture size, reflectance vs transmittance, measuring plastics and UV control.
Last reviewed 21/09/2026 · Level: Intermediate
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