Usually we use densitometers and spectrophotometers to measure various types of prints, proofs, and substrates. When reporting these values, the instrument's specified tolerances (technically referred to as measurement uncertainty errors) or restrictions are generally not considered. In fact, the values ​​we report are only approximate estimates of true values, which contain many errors introduced by uncertainty, which are often overlooked.
There is a certain difference between the specified tolerance and the uncertainty error, the former is the allowable error range, and the latter is the error introduced in the measurement process due to the lack of accuracy. The relationship between them is to reduce the tolerance by limiting the uncertainty of the measurement.
In many cases, there is no problem in doing so, but sometimes people arguing over some subtle differences in the measurement process with the same parameters, conditions, and knowledge. When they understand the uncertainty of the measurement, they will find it. Such arguments are in fact meaningless, and it is quite possible that they are equal.
CGATS co-published an ANSI standard in collaboration with PIMA IT2, titled CGATS.11/PIMA IT2.11-1999 Image Technology and Photography - Measurement of Reflection and Transmission Density - Standard Reference Materials - Documents and Procedures Used Including the characterization of the standard uncertainty of the combination.
(PIMA is the Imaging Imaging Manufacturers Association, and IT2 is a group within PIMA that develops standards relating to density measurement. The standard is sponsored by the Joint Working Group on ISO/TC42 (Photography) and ISO/TC130 (Image Technology). Voted through.)
The title of the standard is long and attractive, but it does not include actual information and some useful guidelines. The standard is divided into two parts. Firstly, the standard reference materials used in the printing and photography process are defined. Secondly, there are special measurement steps and guidelines on how to determine the reasonable uncertainty error. These uncertainty errors are in the Parameters such as density, dot value, and color are introduced when measuring.
Standard Reference Material (CRM)
What is standard reference material (CRM)? Simply put, CRM refers to any material used for measurement and calibration. Its value can be obtained through a set of standard processes, and this standard process establishes the traceability of reference materials. These values ​​themselves also contain uncertainties. T-Ref is a CRM that contains some calibration information. Calibration swatches used in densitometers or spectrophotometers can also be CRMs, but they should have proper traceability calibration information.
CGATS.11 clearly defines the traceability and file type of standard reference materials used in the printing industry for measuring the transmission or reflection properties of materials. The standard also defines values ​​for a set of parameters that encourage equipment manufacturers to provide calibration swatches and reference materials that are close to the standard values ​​during production.
The definition of these parameters also makes it easier for manufacturers and calibration laboratories to work, making it easier to obtain third-party reference materials.
As a user, what should we do?
If you have a calibration swatch, you should ensure that there is a set of data associated with it and a list of the uncertainties associated with each sample area. The meaning of these uncertain errors should be described in detail and exactly describe how these values ​​and uncertainty errors are generated.
In addition to these, consult with the supplier whether these standard samples meet the parameter criteria of CGATS.11. You can also request a CGATS.11 parameter standard and compare the differences in detail.
Combined Standard Uncertainty (Combined Standard Uncertainty)
The exact term describing the difference between the true and actual measured values ​​is the combined standard uncertainty error, which is sometimes also expressed using a “measurement system†because the uncertainty error is usually the result of a combination of several factors. , Such as including the problems of the equipment itself, the problems of the operators and process steps, the problems of the used CRM, and the like. Usually these are attributed to the measurement system.
How can these uncertainties be reasonably assessed and used effectively?
From the perspective of most practices, there are two main factors. One is the repetitiveness of the results that often appear within the company. The second is the uncertainty error associated with standard reference materials. It may be that the standard reference materials are used as independent The result of the inspection of the element.
If you have a standard reference material, it should be used as part of the equipment calibration procedure. If the parameters are invalid, it is also important to look at the steps of the experiment.
The uncertainty caused by the reproducibility of results may be broken down into several factors such as the operator, the material being measured, the equipment used, the time after calibration, and so on. But in most companies, so many reasons are usually attributed to a simple numerical value.
The method for obtaining this estimate is to select a uniform, stable test sample of the tested material, and count the number of measurements. The measurement results should be representative of the wide variety of conditions encountered in normal practice. That is, it should include a variety of changes, such as operator-related changes (dayshift/nightshift), humidity and other environmental factors, equipment warm-up time, and recalibration.
Record all available variable values ​​and then use spreadsheets or other methods to determine the deviation of these results from the standard. (In the Microsoft Excel spreadsheet software called STDEV function), the standard deviation is the uncertainty error introduced by the reproducibility of the results.
The solid cyan block is measured using a common densitometer. Measurements are made 30-40 times in consecutive days. Typical results are obtained with a density deviation of at least 0.007.
If the T-Ref reference material is applied to the calibration process, the T-Ref uncertainty error is also introduced. T-Ref's uncertainty error is usually 0.012. These two values ​​are not simply added, but the square root of the sum is the final result.
That is 0.007 × 0.007 + 0.012 × 0.012, and then this open squared. This is very easy to calculate in the Excel table, the result is 0.014.
If we assume that the two uncertainties are normally distributed, the probability that the true value appears is 67%. When used, it is also multiplied by a factor of 2 or 3 to achieve a probability of 95% or 99%. Call coverage factor. In the printing field, the use of 2 is sufficient, and the final uncertainty value is 0.028.
How to use these values?
If the measured density of solid green in front is 1.33, then the actual value is between 1.302 and 1.358. The probability of appearing in this interval is 99%. It can also be said that if the blue density value is 1.33, then its uncertainty error is 0.028.
If we compare the measured values ​​of the same cyan patch of two printing companies, assuming that the uncertainty values ​​of both factories are 0.028, then their difference is between 0.056, which may indicate that the measurement result is identical.
Also ensure that the specified tolerances are met. For example, it can be said that the densities are between 1.29+ and 0.07. Then in order to ensure that there is a 99% probability of appearing in the expected error segment, the uncertainty factor must also be considered, so the final range may be 1.29+-0.42 instead of 1.29+-0.07, resulting in a reduction of the specified tolerance range.
The same calculation can be applied to the measurement of parameters such as dot area and dot gain. In any case, uncertainty has an important relationship with the company's measurement system.
There is a certain difference between the specified tolerance and the uncertainty error, the former is the allowable error range, and the latter is the error introduced in the measurement process due to the lack of accuracy. The relationship between them is to reduce the tolerance by limiting the uncertainty of the measurement.
In many cases, there is no problem in doing so, but sometimes people arguing over some subtle differences in the measurement process with the same parameters, conditions, and knowledge. When they understand the uncertainty of the measurement, they will find it. Such arguments are in fact meaningless, and it is quite possible that they are equal.
CGATS co-published an ANSI standard in collaboration with PIMA IT2, titled CGATS.11/PIMA IT2.11-1999 Image Technology and Photography - Measurement of Reflection and Transmission Density - Standard Reference Materials - Documents and Procedures Used Including the characterization of the standard uncertainty of the combination.
(PIMA is the Imaging Imaging Manufacturers Association, and IT2 is a group within PIMA that develops standards relating to density measurement. The standard is sponsored by the Joint Working Group on ISO/TC42 (Photography) and ISO/TC130 (Image Technology). Voted through.)
The title of the standard is long and attractive, but it does not include actual information and some useful guidelines. The standard is divided into two parts. Firstly, the standard reference materials used in the printing and photography process are defined. Secondly, there are special measurement steps and guidelines on how to determine the reasonable uncertainty error. These uncertainty errors are in the Parameters such as density, dot value, and color are introduced when measuring.
Standard Reference Material (CRM)
What is standard reference material (CRM)? Simply put, CRM refers to any material used for measurement and calibration. Its value can be obtained through a set of standard processes, and this standard process establishes the traceability of reference materials. These values ​​themselves also contain uncertainties. T-Ref is a CRM that contains some calibration information. Calibration swatches used in densitometers or spectrophotometers can also be CRMs, but they should have proper traceability calibration information.
CGATS.11 clearly defines the traceability and file type of standard reference materials used in the printing industry for measuring the transmission or reflection properties of materials. The standard also defines values ​​for a set of parameters that encourage equipment manufacturers to provide calibration swatches and reference materials that are close to the standard values ​​during production.
The definition of these parameters also makes it easier for manufacturers and calibration laboratories to work, making it easier to obtain third-party reference materials.
As a user, what should we do?
If you have a calibration swatch, you should ensure that there is a set of data associated with it and a list of the uncertainties associated with each sample area. The meaning of these uncertain errors should be described in detail and exactly describe how these values ​​and uncertainty errors are generated.
In addition to these, consult with the supplier whether these standard samples meet the parameter criteria of CGATS.11. You can also request a CGATS.11 parameter standard and compare the differences in detail.
Combined Standard Uncertainty (Combined Standard Uncertainty)
The exact term describing the difference between the true and actual measured values ​​is the combined standard uncertainty error, which is sometimes also expressed using a “measurement system†because the uncertainty error is usually the result of a combination of several factors. , Such as including the problems of the equipment itself, the problems of the operators and process steps, the problems of the used CRM, and the like. Usually these are attributed to the measurement system.
How can these uncertainties be reasonably assessed and used effectively?
From the perspective of most practices, there are two main factors. One is the repetitiveness of the results that often appear within the company. The second is the uncertainty error associated with standard reference materials. It may be that the standard reference materials are used as independent The result of the inspection of the element.
If you have a standard reference material, it should be used as part of the equipment calibration procedure. If the parameters are invalid, it is also important to look at the steps of the experiment.
The uncertainty caused by the reproducibility of results may be broken down into several factors such as the operator, the material being measured, the equipment used, the time after calibration, and so on. But in most companies, so many reasons are usually attributed to a simple numerical value.
The method for obtaining this estimate is to select a uniform, stable test sample of the tested material, and count the number of measurements. The measurement results should be representative of the wide variety of conditions encountered in normal practice. That is, it should include a variety of changes, such as operator-related changes (dayshift/nightshift), humidity and other environmental factors, equipment warm-up time, and recalibration.
Record all available variable values ​​and then use spreadsheets or other methods to determine the deviation of these results from the standard. (In the Microsoft Excel spreadsheet software called STDEV function), the standard deviation is the uncertainty error introduced by the reproducibility of the results.
The solid cyan block is measured using a common densitometer. Measurements are made 30-40 times in consecutive days. Typical results are obtained with a density deviation of at least 0.007.
If the T-Ref reference material is applied to the calibration process, the T-Ref uncertainty error is also introduced. T-Ref's uncertainty error is usually 0.012. These two values ​​are not simply added, but the square root of the sum is the final result.
That is 0.007 × 0.007 + 0.012 × 0.012, and then this open squared. This is very easy to calculate in the Excel table, the result is 0.014.
If we assume that the two uncertainties are normally distributed, the probability that the true value appears is 67%. When used, it is also multiplied by a factor of 2 or 3 to achieve a probability of 95% or 99%. Call coverage factor. In the printing field, the use of 2 is sufficient, and the final uncertainty value is 0.028.
How to use these values?
If the measured density of solid green in front is 1.33, then the actual value is between 1.302 and 1.358. The probability of appearing in this interval is 99%. It can also be said that if the blue density value is 1.33, then its uncertainty error is 0.028.
If we compare the measured values ​​of the same cyan patch of two printing companies, assuming that the uncertainty values ​​of both factories are 0.028, then their difference is between 0.056, which may indicate that the measurement result is identical.
Also ensure that the specified tolerances are met. For example, it can be said that the densities are between 1.29+ and 0.07. Then in order to ensure that there is a 99% probability of appearing in the expected error segment, the uncertainty factor must also be considered, so the final range may be 1.29+-0.42 instead of 1.29+-0.07, resulting in a reduction of the specified tolerance range.
The same calculation can be applied to the measurement of parameters such as dot area and dot gain. In any case, uncertainty has an important relationship with the company's measurement system.
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