Hitch standardization, also known as transfer standardization, is a process by which two instruments of similar design can be made to read the “same” values on a group of specimens. The process involves assigning one instrument to be the reference, or master, unit and mathematically adjusting the secondary, or slave, unit(s) to read the “same” values. In this way, two or more instruments can be hitched together.
Choosing the Best Instrumentation
The best hitch results are obtained when the instrument set contains units of similar geometry. This means that hitched instruments should all have either diffuse illumination and 8° viewing or 45° illumination and 0° viewing. The two geometries should not be combined in a hitch.
When using a combination of colorimeters and spectrophotometers (sometimes referred to as spectrocolorimeters) in the hitch network, it is recommended that a spectrophotometer (such as a ColorQuest, LabScan, or UltraScan), rather than a colorimeter (ColorTrend HT or D25), serve as the reference unit. This makes the most of the spectrophotometer’s ability to measure across the entire visual spectrum and takes advantage of the tightness in readings that can be achieved within a population of spectrophotometers. In the case where all spectrophotometers or all colorimeters are being used for the hitch, the reference unit can be selected at random. It is recommended that the reference unit be placed in a central laboratory where performance can be monitored on a regular basis.
Other instrument capabilities, such as varying port sizes, UV filters, and light sources, should be well-defined and as standardized as possible for all the instruments. Similarities between the instruments should be maximized for the best results
The Basic Technique
Hitching a secondary unit to a reference instrument requires that a specimen be read on both units and the values compared and adjusted accordingly. This specimen, known as the hitch standard, is first read on the reference instrument and its values recorded as spectral data or colorimetric (tristrimulus) data. The hitch standard is then physically moved to the secondary instrument where it is reread and the values from the reference unit are input into the secondary instrument’s processor. Through the software, the secondary unit is then “biased” to the reference unit using either an additive or a ratio calculation, as shown below.
Small differences in the hitch calculations are due to rounding of the final results.
In general, the bias type of hitch is preferable for dark colors and when the samples to be measured are very close in color to the hitch standard. Bias hitches are convenient because once the instrument is hitched, it rarely needs to be rehitched. On the other hand, ratio hitches are acceptable for a wider range of colors (including colors less similar to the hitch standard), but, at least with the HunterLab D25-9, must be re-established each time you standardize the instrument.
When the hitch has been implemented, the values obtained on a hitched instrument would ideally be the same as those obtained on the reference unit. The unhitched values represent data read relative to the secondary instrument’s white tile. The hitched values represent data read relative to the reference instrument’s white tile.
The capability of establishing a hitch standardization varies among HunterLab products. Most of the instruments which offer hitch standardization offer the ability to hitch on tristimulus data. Some of the spectrocolorimeters have the additional ability to perform a hitch based on each point of spectral data, which means you can then view the hitched data under any color scale, and observer. Tristimulus hitches limit your data viewing capabilities to the illuminant and observer conditions under which the hitch was created. For a general overview of instrument and software capabilities, refer to the chart below. For specific instrument operation instructions, refer to your instrument user’s manual.
Selection of a Primary Hitch Standard
Due to the visual non-uniformity of the color scales, it is recommended that each distinct color be assigned a separate hitch standard. This will improve the reproducibility of the measurements between the reference and secondary units. Ideally, hitch standards should be made of the same material and colorants as the samples to be analyzed and should differ by no more than about 5 ∆E units from the usual samples to be measured. The use of hitch standards that are spectrally different than the product can result in increased differences in the secondary units as compared to the reference. Each secondary unit requires a set of hitch standards and reference instrument values for initial setup.
There are many cases where multiple copies of the hitch standard are unavailable or the stability of the specimen is in question. For these cases, a material of the same color with the following characteristics would be ideal:
• Readily-available quantity
• Uniform surface
• Stable over time
• Non-metameric to the sample
A good example of a commonly-used and stable hitch standard is a ceramic tile. This type of standard can easily be sent to the various locations of the secondary instruments after being read on the reference unit.
Preparation of a Secondary Hitch Standard
In the case where the hitch standard’s color will change over time, care must be taken to ensure optimum shipping conditions and quick use of the hitch standard. Once the hitch standard is read on the secondary instrument to establish the initial hitch, a secondary standard should be chosen for use in routine product evaluation. This secondary standard should be a more stable material that can be used to re-establish the hitch as needed.
The secondary hitch standard procedure is used, for example, in the determination of United States Department of Agriculture (USDA) grade color for tomato products. The instrument network is composed of 45°/0° colorimeters that are hitched together to the reference instrument at USDA via an actual tomato paste sample. This “soft” standard is prepared and read on each secondary instrument, and the assigned values from the reference unit are entered into each unit. Since the standard is biodegradable, its shelf-life in an open container is relatively short. Therefore, the next step is to read a red ceramic tile, the “tomato tile,” relative to this hitch.
When the tomato tile is read on each secondary instrument with the soft standard hitch in place, the values obtained become the “new assigned” values from the reference instrument. The secondary instrument can then be rehitched using the tomato tile in the future rather than the soft standard. The tomato tile becomes the secondary hitch standard. It is checked periodically against a new soft standard.
Instrument Performance and Hitch Standardization
Each instrument should be standardized prior to working with a hitch standard to ensure robustness of the data. Periodic restandardization should be done per the individual instrument recommendations. Each hitch standard or product standard should be reread and compared to itself as a sample on a routine basis to ensure confidence in the hitch calculations.
In addition, a checking program should be implemented for evaluating the performance of the hitch standardization on each secondary unit for each color relative to the reference instrument. A set of stable specimens that could be read on each instrument and compared would serve as a check on the program.
(See attached pdf file for the complete article with Illustrations and formulas)