Color matching and color consistency have become important quality control parameters in many markets. In the textile industry, a continuous swath of fabric moving through production equipment may be tested for variability, or different batches may be compared for consistency. In food quality control, the desired product color is carefully tuned to consumers’ tastes, and the product must look the same despite being produced at different factories. In commercial lighting, bulb-to-bulb consistency to within the limits of human detection is extremely important. This is particularly true for banks of white light or colored LEDs, or any type of lighting array.
To control color as a quality parameter, the measurement of color must be repeatable from one production location to another. A well-defined method of comparing color, either within a batch or to an acceptable standard, is also required. Ocean Optics color measurement systems offer both, providing the flexibility to configure the measurement system for a wide variety of sample types and needs, as well as the software to perform quantifiable color comparisons.
In this application note and the associated experiment, we’ll demonstrate the power of OceanView software schematic functions to perform advanced sample comparisons such as the calculation of color differences (Delta E). When used in the student lab as a teaching experiment, this experiment gives students an opportunity to take color measurements to the next level, while learning to work with prewritten schematics in OceanView for advanced data analysis.
Measuring color is challenging, as the way our eyes detect color is not directly related to easily measured parameters such as light intensity. Color is dependent on the sample, on the illuminant and on subjective human perception. Spectral sensing, however, makes color measurement quantifiable, repeatable and consistent. Measuring the amount of light across the entire visible wavelength range brings real advantages when compared to traditional colorimetry due to the higher resolution and wider bandwidth of the measurement.
There are many different color coordinate spaces used to measure color (xyz, XYZ, RGB, CMYK). In one of the most common, CIE L*a*b*, color differences between samples are quantified by a parameter called Delta E.
A Delta E value between 1 and 3 is typically undetectable by eye. A Delta E of 1 is often referred to as the lowest detectable limit for someone who is trained in color matching. For a perceptible color difference for the average person, a Delta E of 2 or 3 is often used.
OceanView allows you to generate, edit and export complete spectral process flow diagrams called “schematics.” Similar to working in LabVIEW, schematics are diagram-based process models where you can create, configure and connect nodes to read data from devices; transform and combine that data through a library of built-in spectroscopic functions; and then output the results to visual graphs and Excel-ready CSV files.
Not only are schematics useful for customizing your data collection, but they can also be saved and shared with others to allow them to replicate the same data collection process. Schematics are specific to the spectrometer used for development, but OceanView will walk you through the steps required to use them with a different spectrometer when you load the project.
Modular color measurement systems are easily adapted to a wide range of sample types, from flat to rough or curved, and even particulate. Many textiles, for example, have enough texture to their surface to justify use of an integrating sphere as the sampling accessory, particularly for materials that appear to be a different color when viewed from a different angle or direction.
In this experiment, the samples are flat, allowing use of a basic reflection probe (QR400-7-VIS-NIR) in an RPH-1 holder. This samples an area a few millimeters across, looking at diffusely scattered reflected light. An HL-2000 tungsten halogen light source provides illumination at all visible wavelengths, spanning the range of interest for color (380-780 nm). A Flame-S-VIS-NIR spectrometer is adequate for many color measurements and for teaching applications, but when looking for very fine color differences, the Maya LSL may be a better choice.
The Maya LSL is an excellent spectrometer for color measurements, as it has low stray light and very good response in the blue region of the spectrum, allowing even small color differences to be quantified accurately. It also has a back-thinned detector, enhancing its sensitivity for high-throughput applications like process lines. It offers speed and precision, beneficial for high throughput testing in production environments.
Color Measurement System
Flame-S-VIS-NIR (350-1000 nm) or Maya LSL (360-825 nm)
|Light source:||HL-2000 tungsten halogen light source|
|Optical fibers/probe:||QR400-7-VIS-NIR reflection probe|
|Accessories:||WS-1 diffuse reflectance standard; RPH-1 reflection probe holder|
|Software:||OceanView spectroscopy software|
|Sample:||Samples with similar colors (e.g., paint samples, textiles)|
This experiment combines basic color measurements with use of an OceanView schematic for advanced processing. The schematic to be uploaded calculates Delta E by defining and measuring a “color standard,” and then calculating Delta E for the sample by comparing the color coordinates measured to those for the color standard.
The first steps involve importing the OceanView schematic for Delta E, beginning by loading the Delta E calculation as a saved project. Opening the saved project results in two graph views – one for Sample Reflection and one for Color Standard Reflection. These must be updated by taking new Reference and Background spectra.
A reference spectrum can be collected by using the WS-1 white standard as the reference (adjusting acquisition parameters as needed). A WS-1 standard has very consistent reflectivity across all wavelengths, allowing the system to correct for variations in sensitivity at the different wavelengths.
A background spectrum can be collected by blocking light from entering the spectrometer at the light source, or by pointing the probe toward a dark location so that no light will be reflected back into the probe. When doing so, it is important not to turn off the light source. Turning the light source off and then on again will throw the light source out of thermal equilibrium and require a new reference measurement. A background measurement taken by pointing the reflection probe into a dark space is more accurate because it includes any scattered reference light that will be present in the sample measurements, allowing it to be subtracted properly.
Once new reference and background spectra have been saved, the spectrum for the color standard to be used for the comparison can be taken – this is done in the Color Standard Reflection view. The sample defined as the “Color Standard” will then become the sample to which all other samples will be compared.
With setup of the project complete, it is then possible to measure the L*a*b* and Delta E values for your samples. Looking at the spectra for a sample close in color to the chosen color standard, it becomes apparent how difficult it is to detect, let alone quantify, very small differences in color without advanced mathematical processing. Even a direct comparison of L*a*b* color coordinates can seem to yield little information or difference.
However, a difference of squares calculation like Delta E provides a method to quantify the difference between the two spectra in a manner approximating the complex process of visual color comparison performed by the human eye. Through use of a saved project schematic, even a novice can have access to advanced data processing for color comparisons.