Effective ways to measure beverages and liquids
There still seems to be confusion on when to measure liquids using Reflectance settings or when to measure using Transmittance settings. We have a blog that speaks to the technical explanations for this, but the topic continues to arise and we would like to show you how you can determine this in a practical way.
The technical explanation is clear enough, and if still not clear, we can show you how to check the difference in the real world.
When measuring opaque or solid objects, even if liquid, the reflectance setting is correct.
Objects that are translucent and liquids like tea may not return accurate values, but appear to give valid readings. So when is an object translucent enough to warrant a TRANSMITTANCE measurement?
Many of our customers start out measuring colour with entry-level Chroma Meters. While accurate, especially when returning comparative REFLECTANCE values, traditional portable chroma meters can NOT technically measure in TRANSMITTANCE mode.
Historically a work around was developed which appeared to be a transmittance reading, although in reality this is not the case.
A practical example of measuring a translucent product to determine whether to use Reflectance or Transmittance.
Here we see a commercial beverage being tested on a Konica Minolta Chroma Meter CR-410, one of the most sold Colorimeters globally. This instrument is setup to with a Cell holder and high-quality glass cell to contain the sample. In this case the device is set to perform a Reflectance measurement.
The user simply adds the sample to the glass cell and takes a measurement.
Using SpectraMagic NX Pro, the user takes a TARGET measurement of the liquid and then performs a SAMPLE measurement against the set TARGET. The defined tolerances report a colour difference judgement.
Under these conditions the result looks valid. We have received some colour data and as expected the same sample has returned a PASS judgement.
So how do we test if the measurement is really valid?
To check the validity of the data received, place a stable colour object (In this case a Yellow Ceramic Tile) and take another SAMPLE measurement to compare against the TARGET.
Here is the trick, if the Yellow Tile has an INFLUENCE on the VALUES, then it is clear that the liquid is TRANSLUCENT enough to INCLUDE colour data from the yellow tile or even background surroundings in the laboratory.
We can see that the TARGET data remains the same, as its held in memory. The “Yellow Tile” SAMPLE data is however RADICALLY DIFFERENT and the software reports a FAIL judgement.
As mentioned in the beginning of this blog, the old school workaround is to create a known repeatable error. Typically, the instruments White Calibration Tile can be used instead of a Colour Tile. Most of the escaping light will be reflected and at least you have a constant reduced error result. In practice this works for some applications, but there are more technically sound ways to measure a translucent liquid.
What is the optimal measurement practice for a translucent liquid?
When customers started upgrading to a newer bench-top Colorimeter model, they have the possibility to measure in Reflectance as seen with the CR-410 above or improve the process by performing a Transmittance measurement.
Although this device is not portable, it offers users a wider range of measurement possibilities including both Reflectance and Transmittance measurement. With the possibility to measure colour with true transmittance it is important to understand when to use Reflectance and when to use Transmittance.
To show this process on the CR-5, we can repeat a similar measurement setup to the CR-410 in reflectance using the same sample, same glass cell.
As the CR-410 uses FILTERS to sense colour and the CR-5 uses dual 40-element silicon photodiode arrays, the CR-5 returns more accurate but greater error.
The reflectance measurement shows a stable PASS/FAIL judgement although we can see that the colour difference reported is higher than with the CR-410.
Once again, we repeated the process of placing a stable Yellow Ceramic Colour Tile on top of the glass cell and as seen by the images below, the effect of the Yellow Tile in the background of the sample has a large effect on the colour of the sample that the CR-5 senses.
The target values remain the same as the data is held in memory. The effect that a colour (Yellow) tile has on the sample values is evident.
This provides an immediate indication that the Reflectance measurement procedure is not optimal for this sample, however instead of the old school workaround of a constand error, the possibility to measure in transmittance is available.
Measuring Liquids and Beverages in Transmittance mode:
Transmittance is the best method to measure a translucent liquid to attain accurate and repeatable colour measurement data.
The CR-5 has a separate TRANSMITTANCE bay, which requires a different setup. Instead of flashing the light onto the object and reflecting it back into the sphere, Transmittance settings allow the light to pass THROUGH the OBJECT and to the detector.
Values returned will not corelate to Reflectance values as the measurement procedure has changed.
Note that the values are now much HIGHER compared to reflectance values on both the CR-410 and the CR-5. These are True Colour Values with stable repeatability. The L*a*b* values correlate more to a humans perception of the beverage which further indicates that the measurement procedure is correct.
Changes in the liquid colour will now be easily seen in the colour difference values
A few drops of water were added to the above beverage sample and measured again against the target data held in memory. The water was enough to change the translucent beverage quite a lot, as a drop of a colour in a white paint would do.
For more resources or assistance on these measurement practices, visit The Narich Academy or contact us.
** We thank one of our customers who produce Energade for this very palatable sample and object**