How to Measure Small Parts and Small Color Samples

Small-part color measurement succeeds only when the instrument sees the intended material—and nothing else. The method must distinguish the port opening, illuminated area and detector’s viewed area, then control targeting, backing, curvature and aperture-specific standardisation.

Aperture is not one universal diameter

Instrument specifications may list a port or mask opening, an illumination area and a measurement or area-of-view diameter. These can be different. A part that covers the viewed spot but leaves the wider illuminated region exposed can admit background or stray-light effects on some designs. Ask for the optical diagram and the minimum sample area required for the selected aperture.

Choose the measurement approach

Small-port contact instruments suit flat, opaque samples that can cover the required area and seal against the port. Use the largest aperture that fits entirely within a uniform feature; very small apertures collect less light and make positioning and local variation more influential.

Imaging or non-contact spectrophotometers can select a virtual measurement region inside a larger illuminated field and show the operator exactly what is targeted. This is useful for tiny components, printed details, cosmetics and irregular objects. Confirm the minimum spot, working-distance tolerance and how the design handles surrounding colors rather than assuming every camera-equipped instrument behaves the same way.

Purpose-built fixtures can make a standard instrument viable when they locate the same feature, height and angle every time. A fixture is part of the method and should be controlled like any other measurement accessory.

Prevent edges, gaps and backing from becoming the color

Aperture edges, gaps around a component, holes, chamfers and curved shoulders can expose the fixture or allow ambient light into the measurement. Use a repeatable, non-reflective holder that does not intrude into the measured field. If a backing is visible through a translucent part, define its material and orientation; changing black, white or metal backing changes the reported result.

Control curvature and orientation

Measure a stable tangent area where the part can sit square to the optics. Tilting changes the geometry; rocking changes it from reading to reading. For domed buttons, caps, moulded ribs and automotive trim, use a nest or jig that holds the same surface normal and prevents the instrument from seeing an edge. Flexible samples should be held flat without stretching, whitening or embossing them.

Directional mould flow, brushed finish and print screening also require a marked orientation. If rotation averaging is used, define the angles and calculate the result in the same way at every site.

Standardise for the aperture and accessory

Changing the mask, aperture, lens position or sample holder can change the optical configuration. Follow the manufacturer’s aperture-specific black and white standardisation routine after each relevant change. Do not calibrate with one aperture and measure production with another unless the instrument procedure explicitly supports it.

Before releasing the method, use a stable check sample to quantify repeated-read variation and repositioning variation. The second value is often larger on small parts because targeting and fixture placement are part of the measurement system.

A reproducible small-part workflow

Record enough to reproduce the measurement

Record instrument model, geometry, mask or virtual aperture, measured and illuminated dimensions, fixture ID, backing, part orientation, feature location, SCI/SCE where relevant, illuminant, observer, formula and reading sequence. Store a targeting image when the instrument supports it. A result labelled only “4 mm aperture” is incomplete when different models illuminate or view different areas.

A smaller spot is not automatically a more accurate result. It is the correct choice only when it isolates the required feature with acceptable signal, repeatability and positioning uncertainty. If the usable feature is smaller than the validated optical area, change the instrument or specimen preparation—do not allow the background to become an unreported part of the measurement.

References and further reading

• ASTM E1164: spectrometric data and required measurement parameters.
• X-Rite: imaging, target area and over-illumination in small-spot measurement.
• X-Rite CiF3200: an example of imaging and virtual-aperture measurement.

Continue with aperture size, sample preparation, averaging and repeatability, measuring plastics and instrument selection.

Last reviewed 21/09/2026 · Level: Intermediate

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