Application 9 The new generation in color measurement

color-guideTM 45/0

 

The market for color instruments is changing with the need for precise, simple to use, low cost devices. This note explores the new technology of BYK-Gardner´s color-guide instrument in comparison to BYK-Gardner´s traditional spectrophotmeters, "handyspec" and "colorview".

Traditional Technology versus color-guide

The dual beam principle has been the traditional approach for color measurement. The substance of the dual beam principle (fig.1) is to split the polychromatic illumination beam: one part of the beam illuminates the specimen and the other part monitors light output variance due to voltage and temperature changes. Reflected ligth from the specimen is dispersed into the spectrum (using a diffraction grating or filters) and measured with a detection system (using a photodiode array or multiple detectors).


Fig. 1: Dual Beam Principle

Some attempts have been made to use monochromatic illumination, but until this time, have produced mediocre or poor results. The difficulty was to adequately cover the visible spectrum with LEDs (light emitting diodes) and to control the light source.

New technology used in the color-guide has overcome the initial shortcomings of LED based color instruments. Highly accurate and reliable readings comparable to traditional spectrophotometers can now be achieved.

The monochromatic illumination is done using 30 LEDs with distinct colors to cover the entire visible spectrum. To minimize the influence of surface structure, each LED appears 3 times, 120° apart. The LED triplets illuminate the specimen successively, while one detector measures the reflected light; all ocurring within 0.3 seconds (fig. 2). LEDs have specific curves within the visible spectrum. Their emission curves are overlapping and therefore, provide additional spectral information (fig. 3). color-guide’s highly automated calibration routine (patent pending) and the information from the overlapping emission curves allow calculation of spectral data every 20 nm. Superior repeatability and inter-instrument agreement are provided during each measurement by a LED temperature reference principle (patent pending).

An analytical approach was used by comparing popular traditional technology to that of the color-guide instrument.


Fig. 2: Circumferential Illumination of LEDs

Fig. 3: LED Emission Curves

Analytical Studies

For comparison, the color-guide instrument was evaluated relative to BYK-Gardner´s traditional portable spectrophotometer handy-spec. For accuracy testing, the bench mark or standard instrument used was the BYK-Gardner color-view, 45°/0° spectrophotometer: a bench-top instrument with high accuracy and precision. Test materials for these comparisons included BCRA standard tiles (British Ceramic Research Association), glossy/matte paint chips from the BAM (Federal Institute for Material Testing of Germany), metameric samples from CTS (Collaborative Testing Service), and paint and plastic materials obtained from the automotive, container, appliance and trade sales industries.

All samples were measured using CIE 1964 10° standard observer under illuminant D65 (day-light). All results are the average of 10 measurements. Instrument capability without the use of a computer program was evaluated for all portable instruments.

Instrument Repeatability

The reliabilty of an instrument to produce repeatable results can be quickly determined by consecutively measuring a sample 10 times. In this test, color-guide´s white calibration tile, a green glossy paint draw down, and an orange plastic chip were evaluated. Average and standard deviation were calculated and the difference of each individual measurement compared to the average are listed in the following table.

For all three samples, differences were well below ĈE* of 0.1. This guarantees reliable results for the user comparable to traditional spectrophotometers, today and tomorrow at any time.

Inter-Instrument Agreement

Inter-instrument agreement is used to express the capability of two or more identical instruments to measure the same. The larger the value the poorer the agreement. Inter-instrument agreement becomes important when multiple instruments are in use at one 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 was determined by measuring a complete set of BCRA tiles with 50 instruments. Individual instrument results were subtracted from the average of the 50 results and ĈE* was then computed for each of the tiles.

As shown in figure 4 the average ĈE* across the entire spectrum is well below a practical tolerance of ĈE* 0.5. Reliable and precise color communication is guaranteed when using the color-guide.


Fig. 4: Inter-Instrument Agreement on BCRA-Tiles

Color Difference Comparison

The accuracy of an instrument can be best measured by comparing it to a known standard instrument. The BYK-Gardner color-view 45°/0° spectrophotometer, a bench-top instrument with high accuracy and precision was used in this test as a bench mark. Averages for a range of samples spanning the visible spectrum were compared to averages taken by the color-view spectrophotometer, and the total color difference (CIE ĈE*) for each sample was computed from the resulting differences (fig. 5).


Fig. 5: Total Color Difference ĈE*

color-guide gives the same excellent results for total color difference ĈE* when compared to the bench mark instrument color-view. This is true for the entire spectrum.

Influence of Gloss on Color Measurement

It is known that the 45°/0° instrument geometry excludes gloss from color measurement just as the eye excludes gloss from color judgment. To test for instrument ability to exclude gloss during measurement, average color difference values were computed between identically pigmented matte and glossy paint samples (fig. 6).

Results of the color-guide agreed with the visually noticed differences between the glossy and matte samples. Therefore, color-guide excludes gloss during measurement to the same extend as the traditional spectrophotometers, handy-spec and color-view.


Fig. 6: Influence of Gloss

Metamerism Comparison

The capability of an instrument to determine metamerism (two samples matching under multiple light sources) can be evaluated by computing the Metamerism-Index (MI).

Metamerism is the ability of a pair of samples to match under one light source, but not under a second. Metamerism occurs when the colorants used to color the pair of samples are not the same.

For determination of MI a spectrophotometer is necessary. Based on the spectral data color values for all illuminants can be calculated and therefore also MI. A metameric sample pair from Collaborative Testing Service (CTS) was used for this test. CTS is an independent, international interlaboratory and quality assurance test agency for color instruments. CTS performs this test quarterly with participating subscribers (fig. 7).


Fig. 7: Metamerism-Index

color-guide as well as the traditional spectrophotometers handy-spec and color-view could clearly determine the mismatch between illuminant D65 (daylight) and A (tungsten light).

Conclusion

The color-guide’s LED technology combined with complex mathematics provide performance of a true spectrophotometer: color control is possible for any specification; all color scales, all illuminants and observers are available. The study proves that measurement results for the color-guide rival traditional spectrophotometers guaranteeing high reliability and accuracy.

In addition, the new LED technology provides benefits never seen before in portable instruments.

colorguide - your QC solution

color-guide, "The New Generation" in color measurement, makes color control easy and secure.