The color of liquid is one of the most difficult things to control during production. But it’s important. Would you choose a bottle of juice or cleaner that is lighter in color than the other bottles on the shelf? What about cough syrup?
Liquid is hard to measure because it can range in transparency from translucent to opaque. It’s also hard to hold, and the measurement device can’t touch it or the optics and the sample will both be contaminated.
Today’s blog explains how to choose the right spectrophotometer to measure liquids, how to present a liquid sample, and what to do with the data once the measurement is complete.
Choosing a Device for Liquid Color Measurement
When choosing a spectrophotometer, the chemical makeup of the liquid is not as important as the transparency. Take apple products, for example.
Apple juice has a light color but is mostly translucent so light passes through easily. Apple cider is fairly opaque due to the pulp. Similar to orange juice, light passes through and can illuminate it from behind, but it’s not translucent like apple juice. Applesauce is almost completely opaque. Although all three of these products are made from apples, the color of each is measured differently.
Beer is another example. A dark stout beer will be measured differently than a light, transparent beer.
This thinking extends into liquid dyes, chemicals, and cleaning solutions. A clear blue liquid window cleaner will be measured differently than an opaque blue laundry soap.
The bottom line is that the chemical makeup of the liquid is not important. To determine how to measure a liquid, you must look at opacity and translucency.
Measuring a Liquid with Opacity
Light doesn’t completely pass through an opaque liquid, so it must be measured in reflectance mode. To take the measurement, the device shines a light into the sample and records the type and percentage of light that is reflected off of the sample.
Ci7000 Benchtop Sphere Spectrophotometer
Here the Ci7520 is fitted with a test tube holder to measure orange juice. The holder fits right onto the aperture plate of a Ci7000 benchtop and holds the test tube at the right position to the reflectance color of the liquid. Since most liquids are not completely opaque, the background will play a role in the final color. This holder has a black surround, but X-Rite also manufactures one with a white surround.
All of the models in the Ci7000 series measure in both reflectance and transmission mode, with the exception of the Ci7500 and Ci7520 which both measure in reflectance mode only. This makes them an economical choice for labs that do not need to measure translucent liquids.
MetaVue VS3200 Imaging Spectrophotometer
To measure a thicker liquid such as applesauce, the MetaVue VS3200 is able to take color measurements on liquids, pastes, and gels, as well as other non-liquid items like powders and non-planar items.
The MetaVue VS3200 can be fitted with a stand and different types of trays to hold the sample. The trays slides into the instrument stand holder under the device in to position for a reflectance measurement.
Since it is a non-contact instrument, there is no risk of contamination for the sample or the device.
Measuring a Liquid with Transparency
Since light passes through a transparent liquid, it must be measured in transparent mode. To take the measurement, the spectrophotometer shines a light through the sample and measures the type and amount of the light that passes through.
The Ci7600 can measure in reflectance mode, but also has a top door that slides open to allow for transmission measurements, as shown here. It’s holding a square glass cuvette that fits into the optional Ci7000 Series Transmission Kit and seals into place over the measurement aperture.
How to Use Data from Liquid Color Measurement
Each measurement captures the type and percentage of light that passes through or reflects off the sample. When entered into a quality control program like Color iQC, this data can provide insight about the color and transparency of the liquid. This data works the same way for both reflectance and transmission measurements.
Pass/Fail Data
When using standard CIE color metrics and tolerances, quality control software can plot where each sample falls compared to the standard and indicate whether it falls within the defined tolerance. This data can help determine what to add or subtract in the recipe to adjust the color so it passes tolerance.
Index Data
Quality control software can also determine whether a measurement meets a specific index, such as the Orange Juice Index, to earn a USDA grade. Other products like chemicals are graded, too. A higher grade has a higher value, which makes measuring liquid color important to determine accurate pricing.
Sometimes the data can be used to adjust the recipe, and sometimes it can’t. For example, if it’s a naturally occurring product such as a fruit or vegetable juice, the spectrophotometer is measuring what the recipe created using the raw ingredients like oranges or tomatoes. Additives cannot be used to adjust the final color, but the results can still help grade the product.
Both the Ci7520 and Ci7600 paired with Color iQC software are USDA-approved for color grading orange juice. This solution calculates color scores for frozen, concentrated, pasteurized, and canned orange juice to deliver verified results and earn a USDA grade shield. Learn more in our Fresh Perspective on Orange Juice Color blog.
Clarity Data
A spectrophotometer can also quantify the clarity of a liquid. Back to our apple juice example, if the transmission measurement shows not enough light is passing through, the juice may need to be filtered. The same can be true for other liquids like chemicals and dyes.
Which Device is Right for Your Liquid Color Measurement Needs?
To learn more, check out these on-demand webinars:
If you’re still not sure or have questions about your specific application, get in touch. We’re happy to help you talk through the options to choose the best device for your needs.
