Application 10 -
Keep your color under control with color-guide sphere

In today’s world color consistency is an important criterion in the purchasing decision. Therefore, reliable color control according to ISO 9000 is the key to a good customer – supplier relationship.

Whatever the application may be, the requirements for portable color measurement are the same:

• Repeatable results today and tomorrow
• Temperature independent results for measurements inside or outside the plant
• Excellent agreement between instruments for good color communication
• Precise and accurate results comparable to other available spectrophotometers
• Professional documentation for ISO 9000
• Easy to use and to handle

This note explores how the BYK-Gardner color-guide sphere instrument meets and even exceeds all these requirements.
For more details on the new LED technology used in the color-guide sphere please refer to Application 9.

Control the Color

In order to deliver consistent color on the final product several steps of the production process need to be controlled

• The color of the incoming raw material.
• Weathering stability, temperature influence and process compatibility of the raw material.
• The agreement from batch to batch to a defined standard.

Color instruments using the sphere geometry with the specular component included objectively measure the color hue. The sample is diffusely illuminated by means of a white coated integrated sphere (Fig. 1.)

Fig. 1 Sphere Geometry specular included

Baffles in the inside prevent the incoming light from directly illuminating the sample surface.  Measurement is done using an 8° viewing angle and the total reflected light is reported:

Total reflection = diffuse reflection (color) + direct reflection (gloss)

Therefore, differences in gloss or texture will not cause color differences. To ensure accurate and repeatable results a reference principle (dual beam principle) is used. The interior wall of the sphere is measured before each sample measurement to monitor any aging of the sphere wall and light output variance of the lamp due to voltage and temperature changes.

Analytical Studies

The color-guide sphere instrument was compared to several competitive commercially available, portable instruments:

Minolta CM-508d, X-Rite SP88, Datacolor Microflash 200d and GretagMacbeth Color-Eye 580.

Test materials for the comparison included standard tiles from the British Ceramic Research Association (BCRA), glossy/matte paint chips from the Federal Institute for Materials Research and Testing of Germany (BAM), and paint and plastic materials obtained from the automotive interior, appliance and trade sales industries.

All samples were measured using the 10° CIE 1964 standard observer under illuminant D65 (daylight). All results are the average of 10 measurements for specular component included mode.

Repeatable Results

The reliability of an instrument to produce repeatable results can be quickly determined by consecutively measuring a sample 10 times. The white calibration tile of the color-guide sphere, a dark blue paint draw dawn, and an orange plastic chip were evaluated. Dark and high chromatic specimens can clearly determine the capability of a color instrument to give repeatable results as very small differences in the measured values cause large differences in the color results.

Average and standard deviation were calculated. The differences of each individual measurement compared to the average are listed in the following table.

For all three samples, differences were well below ÆE* of 0.1. This guarantees reliable results for the user today and tomorrow.

Temperature Independent

For portable color instruments the temperature independence is an important quality criterion. Color standards are measured and stored in the instrument very often in the laboratory. In practice, color differences are evaluated at the production line or outside in the field at a different temperature. For reliable color control the instrument must not show any difference relative to ambient temperature.

To test the temperature influence while measuring, the white standard of each instrument was measured at 10°C. Temperature was then increased using 5°C steps with the resulting ÆE* (Step – Original temperature) value reported.  Step measurements were made without calibrating the instrument at the higher temperature. The instruments Microflash 200d and Color-Eye 580 did not participate in this test because they automatically ask for calibration after a certain time period.

Fig. 2 shows the results of a temperature range from 10–45°C. Even for the highest temperatures, color-guide sphere shows only small differences of ÆE* 0.3. This guarantees stable results without calibration, whether measurements are taken inside or outside. Competitive instruments showed larger drifts and therefore require calibration with temperature changes.

Fig 2

Excellent Inter-Instrument Agreement

Inter-instrument agreement is used to express the capability of two or more identical instruments to measure the same. Generally interpreted the larger the difference the poorer the agreement. Interinstrument agreement becomes important when multiple instruments are in use at a single site, or when a supplier uses the same instrument as his customer.

If the agreement is poor, color communication becomes confused and trouble begins when tolerance values are set lower than the inter-instrument agreement value. Inter-instrument agreement of the color-guide sphere was determined by measuring a complete set of BCRA tiles with 25 instruments.  Individual instrument results were subtracted from the average of the 25 results and ÆE* was then computed for each of the tiles.

As shown in Fig. 3 the average ÆE* across the entire spectrum is well below a practical tolerance of ÆE*= 0.5. Reliable and precise color communication is guaranteed when using the color-guide sphere.

fig 3

Precise and Accurate Results

For reliable customer – supplier communication it is very important that the used spectrophotometer gives comparable results to other color instruments. As there is no "true value", the BYK-Gardner color-sphere, a bench-top instrument with proven accuracy and precision, was used in this test as the bench mark.

Averages for a range of samples spanning the entire visible spectrum were compared to averages taken by the color-sphere, and the total color difference ÆE* for each sample was computed from the resulting differences.  Fig. 4 shows that all instruments give values in a realistic order, but also clearly determines that deviations will occur when comparing absolute values of different color instruments. The main reasons for these differences are:

• Traceability to different institutes like NPL (National Physical Institute) or NIST (National Institute of Standards and Technology)
• Different instrument design (optical principle, size of sphere, light source etc.)
• Different spectral resolution (10 or 20 nm); has a big influence when colors with sharp reflection curves are measured

Because of this, it is highly recommended not to communicate absolute color values. By comparing relative the color differences to a physical standard all available instruments give comparable results.

Fig. 4 Differences between Instruments

Color Regardless of Gloss

As described earlier the sphere geometry, specular component included, measures the complete reflection of a sample surface:

Total reflection = diffuse reflection (color) + direct reflection (gloss)

The influence of gloss was evaluated by comparing the average color difference values of identically pigmented matte and glossy paint samples (Fig. 5).

Fig. 5 Influence of Gloss

color-guide sphere and traditional spectrophotometers showed the same small differences between the two gloss levels. Therefore, all instruments can be used for reliable color measurement regardless of the sample surface condition like it is necessary for incoming QC of raw materials or weathering and temperature influence studies of pigments.

No Influence of Structure and Directionality

Color instruments with sphere geometry illuminate the specimens diffusely by means of a white coated, integrating sphere. The advantage of the diffuse illumination is to achieve more repeatable measurement results on highly directional and structured surfaces, regardless of the measurement direction.

For the following test a structured plastic standard from the automotive interior industry was used. Measurement was done at 0° position. Then the instrument was turned by 90°, 180° and 270° and the ÆE* to the 0° position was calculated (Fig. 6).

color-guide sphere showed no influence of structure on the measurement results. ÆE* compared to the 0° position was always below 0.1. Highly repeatable results are guaranteed no matter in which direction the readings are taken.

Most of the other traditional spectrophotometers also gave repeatable results. Two instruments showed a bigger influence of the structure with ÆE* > 0.2. These instruments require averaging when evaluating structured samples to get a representative value.

Conclusion

The study proves that measurement results for the color-guide sphere rival traditional spectrophotometers guaranteeing high reliability and accuracy. Color control is possible for any specification: all relevant color scales, illuminants and observers are available.

color-guide sphere fulfills all the requirements for portable color control and provides additional benefits never seen in portable instruments before:

• Easy to use and handle:
  Four-cursor button operation
  Customization of the display to your needs
  Easy to measure difficult to access areas
  ergonomic design, weight only 500g
• Low maintenance:
  long-lasting batteries – up to 5,000 readings with one set
  Long-term stable calibration – only every three months
  10 year warranty on LEDs
• Professional documentation for ISO 9000:
  Easy and direct data transfer to WindowsTM programs
  Predefined Excel TM templates for QC reports
  Up/Download of standards with tolerances
  Interface with BYK-Gardner’s complete color control software auto-QC
• Reliable and accurate:
  Excellent short and longterm stability
  Temperature independent results
  Superior inter-instrument agreement

In addition, color-guide sphere was also evaluated by the BAM (Test Report S1E0546B).

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