Much of this blog series has been focused on the instrument and getting the best possible inter-instrument agreement. However, we can’t forget the basics of good practice in color measurement when it comes to sample presentation. Some types of samples can be very repeatable to present to the instrument, while others pose more challenges.
Sample presentation can contribute a significant amount of error to the total “error budget” in a color measurement system. A gauge R & R test should be performed to assess the amount of variance contributed by your current measurement procedures. To improve on this number, follow this list of basic principles to minimize the error contributed to a sample measurement from sample presentation.
- Sampling – Take a sample that is representative of the product by making sure that the batch or lot you prepare for testing is homogenous and mixed completely.
- Conditioning – Prepare the samples in the same way each time paying attention to heat and humidity. Samples should be allowed to equilibrate to room temperature before measurement
- Follow basic rules for good sample presentation
- Be consistent
- Free of dirt, grease, and fingerprints
- Flat area
- Free of scratches and imperfections
- If possible, mark the area that was measured on the back of the sample so that it can be duplicated if needed
Depending on the application, there can be certain things to note or to watch out for:
- Paint & Coatings – While these are usually nice, smooth, and easy samples to measure, the operator needs to be sure that the sample he is measuring is opaque. If not, then the sample film thickness needs to be measured and noted because opacity will definitely affect the reading. If steel or metal panels are sprayed with the coating and then measured, make sure the sample is flat. If not, then apply sufficient pressure to the sample back during the measurement to make the sample flat and in complete contact with the sample port.
- Plastics – The same guidelines given for coatings also apply to plastics. Opacity can be a particular concern as can sample bowing or lack of a planar surface. Whenever curved parts are measured, it is necessary to be very careful about the position on the sample being measured and the amount of pressure applied.
- Textiles – A typical problem with many textile samples is non-uniformity
- Use sample averaging
- Specify the procedure and areas to be measured and number of spots to be averaged
- Specify how many sample thicknesses are to be measured.
- Be aware of “pillowing” – this happens with soft and pliable samples when too much pressure is applied and the sample can actually protrude or “pillow” into the measuring port.
- Many textile samples exhibit directionality. Be consistent about the direction of the grain or texture and which way to orient the sample or else use a procedure that rotates the sample 90 after each reading (and be sure to include all 4 orientations in the average).
- Textured samples will require the use of sample averaging to minimize the affects of texture on the sample
- Ink and printed samples can exhibit some of the issues described above such as opacity, non-uniformity, and directionality. Film thickness can be inconsistent within a print or strike-off and therefore sample averaging is usually necessary.
- Food samples invariably require a customized procedure for consistent and repeatable measurement. Use of clear containers, cuvettes, and Petri dishes will help to provide for stable, consistent reading in food applications. Sampling can be very important too in making sure that the sample being read really represents the color of the whole batch.
Liquids and chemicals exhibit many of the same challenges as food samples. Finding the proper sample container can resolve many of the problems. With odd shaped items, a sample jig will most likely be needed for consistent placement.
Assessing the variability in your sample presentation can point out areas where you can improve your procedures and thereby increase the precision of your color assessments, and facilitate the exchange of reliable electronic color data.