Calculating Delta E using Reflective Color Measurements
Maintaining color consistency during manufacturing processes comes with many challenges. Manufacturers must match color from product lot to product lot and ensure color consistency for each component comprising a finished product. Because color perception is very subjective – everyone sees color a little differently – an objective, quantitative technique is needed for color matching during production. Spectroscopy combined with Delta E color difference calculations makes possible a quantitative assessment of color consistency.
We live in a world filled with color. While the average person may not give much thought to color perception, manufacturers work hard to ensure color consistency in their products. Since color perception is dependent on the sample, illuminant and subjective human perception, color matching can present a significant challenge for manufacturers. Enter spectral sensing.
Spectroscopy measurements alone are not enough to ensure color consistency. The calculation of Delta E (dE) from spectral data takes a subjective comparison of color and provides a quantitative value indicating how different the colors look to the human eye. A Delta E less than or equal to 1.0 indicates that the human eye will perceive two colors as the same. As the dE increases above 1.0, the perceived difference in color increases. A Delta E value between 1 and 3 is typically undetectable by the untrained eye. For this reason, a Delta E of 2 or 3 is often used as a perceptible color difference for the average person.
Delta E values are used in a wide range of quality assurance applications to ensure that final products look the same. The manufacturer determines Delta E limits. In some cases, where the perception of slight differences in color is acceptable, less rigorous limits are set.
There are many different color coordinate spaces used to measure color (xyz, XYZ, RGB, CMYK and others). CIELAB is one of the most common color spaces used to determine color differences. Delta E CIELAB is calculated using this equation:
About the Application
In this application note, we describe measurement of the color consistency of components to be assembled into a total security system package manufactured by Apollo Fire Detectors Ltd., one of the world’s largest manufacturers of smoke detectors, modules and notification devices. Apollo and Ocean Optics are both part of the Halma group of safety, health and environment companies.
The color matching application presented here was especially challenging. First, some of the components are used as the base and the lid of a fire detector, while the others comprise the rest of the security system (Figure 1). More significantly, although the goal is color consistency across all components, the parts are made in different parts of the world, come from different manufacturers, and use different materials.
Reflected color measurements were made using a modular spectrometer setup (Figure 2). We used a Flame model spectrometer configured for spectral measurements in the visible region (FLAME-S-VIS-NIR), a tungsten halogen VIS-NIR light source (HL-2000-HP), a reflection probe (QR400-7-VIS-NIR) with probe holder (RPH-1), and diffuse reflection standard (WS-1). Components were measured relative to a color reference piece provided by Apollo (part marked “Reference” in Figure 1).
Reflection spectra were measured at five locations on each piece. Measurements were spread across the piece to provide representative sampling. CIELAB and Delta E values were calculated at each location.
As seen in the sample reflectance spectra (Figure 3), reflectance from the reference piece and samples were similar in terms of reflected intensity and spectral shape, except for the Keypad Front Sample. The Keypad Front Sample has similar reflectance intensity to the other samples but shows a decrease in reflectance (increase in absorbance) in the region from 400-600 nm. Unlike the other pieces, the backside of the Keypad Front was a dark gray color, suggesting that it has a different composition than the other pieces. This difference in composition could have resulted in the dips in reflectance we observed.
Also, average CIELAB and Delta E values were calculated from the five locations measured on each sample (Table I). The similarity and consistency of the Delta E values – they are all below 2.5 -- indicate that these components are very close to the color of the Reference sample.
Values below 3 are not considered detectable by the untrained eye. Visual inspection of the pieces supports these Delta E findings. The pieces are all very similar in color, resulting in a finished product with good color consistency throughout.
Table I. CIELAB and Delta E Values for Plastic Components
|Keypad Back Plate|
Color matching and color consistency are important quality control parameters in many markets. Maintaining color consistency is especially important when several different components are assembled into a final product. To control color as a quality parameter, a well-defined method of comparing color, either within a batch or to an acceptable standard, is required. Ocean Optics color measurement systems provide the flexibility to configure the measurement system for a variety of sample types and needs, as well as the software to perform quantifiable color comparisons.
Apollo Fire Detectors Ltd. is one of the world's largest manufacturers of smoke detectors, modules and notification devices. Established in 1980, Apollo has become one of the top five manufacturers in the world specializing in the design and manufacture of high quality detection products. The company’s detectors have been installed at places such as the United Nations building, the Staples Center in Los Angeles, and the Statue of Liberty. Learn more at http://www.apollo-fire.com.