Measuring Translucent Liquids Using the ColorFlex 45/0

Lot-to-lot color consistency is an important indicator of quality for many translucent liquids, such as citrus juice products. Translucent samples require special handling when being evaluated either visually or instrumentally. The color of a translucent sample changes when the path length is changed, so that path length must be fixed. Since some of the incident light will travel through the sample, its background must also be constant and, ideally, white. Ambient light may also affect the sample's appearance and should be minimized.

A HunterLab ColorFlex 45/0 spectrophotometer with the sample support plate can be used to measure the reflectance of translucent liquids in a glass sample cup with a ring and disk set and opaque cover. This is the most common method advocated by HunterLab for the measurement of translucent liquids.

The ColorFlex 45/0

The Application

Translucent liquids have several non-uniform characteristics that require compensating preparation and presentation techniques in order to ensure a repeatable sample measurement.

• Liquids must be measured through the bottom of a clear glass sample cup in order to be effectively made a solid.

• They are translucent — not opaque — and will be sensitive to ambient light, path length changes, background changes, and small differences in the optical configuration of the instrument. Using a constant sample thickness and background and an opaque cover will minimize these effects.

  Note: If inter-instrument agreement is a concern when measuring translucent liquids, all the instruments used for these measurements MUST be the same model to minimize measurement differences.

Recommended Color Scale	CIE L*a*b* or Hunter L, a, b as a full color descriptor
Recommended Illuminant/Observer	D65/10° or C/2°

Measurement Method

1. Configure your software to read using the desired color scale, illuminant, and observer.
2. Replace the regular port insert with the special port insert for the sample cup (HunterLab Part Number 04-6622-00).
3. Standardize the instrument using the black and calibrated white standards that come with the instrument.
4. Insert the 10-mm black ring (HunterLab Part Number 02-4579-00) into the 2.5-inch glass sample cup (HunterLab Part Number 04-7209-00) so that it settles flat on the bottom of the cup. The plastic ring fixes the internal path length of light through the liquid sample at 10-mm while excluding outside light that can cause measurement interference.
5. Fill the cup with the translucent liquid sample until the liquid is above the level of the ring.
6. Float the white ceramic disk (HunterLab Part Number 02-4579-00) down through the liquid sample until it rests firmly on top of the black ring. The disk provides a white background to direct light that has traveled through the liquid back to the detector.
7. Your goal is to have the sample appear smooth and opaque through the glass bottom of the sample cup.
8. Place the sample cup at the instrument port.
9. Cover the sample cup with the opaque cover (HunterLab Part Number 04-4000-00). The opaque cover prevents any ambient light from outside the instrument from leaking into the detector.
10. Take a single reading of the liquid through the bottom of the sample cup. Pour the sample out of the sample cup, refill it, and measure again at least three times from the same batch. Average the readings together for a single color measurement representing the color of the batch. Averaging multiple readings minimizes measurement variation associated with non-uniform samples.
11. Record the average color values for the sample batch.