How accurate is your color measurement device? Some thoughts about instruments meant for relative versus absolute readings.
Many printing companies believe that they can save money by using an inexpensive spectrophotometer for higher-order color measurement tasks such as quality assurance or pressroom process control. Such printers are ‘penny-wise and pound foolish’, risking significant downtime and running the chance of alienating high-profile customers. The following paragraphs discuss why this practice is not such a good idea.
It is important to distinguish between instrument that were designed to be used in a relative way, and those that can be used effectively in an absolute manner. Typically, instrument designed for prepress applications work well when used in a relative manner, and these instruments are normally less expensive those instruments that are designed to be more accurate and repeatable, and are also more expensive.
The accuracy of the many prepress spectrophotometers is typically sufficient if your color measurement workflow involves one and only one instrument (not one and only one model of instrument, one and only one instrument itself...one measurement device with one serial number). For their intended purpose, these spectrophotometers will measure a calibration or profiling target, and that same instrument would measure all subsequent readings (i.e.: tomorrow's calibration or profiling target, and the next day's etc.). This is what is meant by using the instrument in a "relative" way, the inter-instrument agreement is not especially relevant as the user is utilizing only one instrument. Higher end instrument have tighter inter-instrument agreement specifications, and therefore will fare better when used in an "absolute" way...that is, when readings are being taken with and shared among several instruments.
Think of it this way: suppose that you purchase a cheap ruler that is 'slightly irregular': on that package it states that the ruler is traceable to national standards +/- 1/8" per inch. When compared to a expensive, finely tuned steel ruler the one inch mark on your 'irregular' may read 7/8", or maybe read 1 1/8" or anything in between. Let's say it reads 7/8"; it met the advertised tolerance, so you should have no complaint with the manufacturer, as they told you going in that this was the tolerance and you made a value judgment based on the (seemingly) sweet price of the ruler.
Now, let's say that you use that 'irregular' ruler and that specific 'irregular' ruler only, as your sole measurement device to build a birdhouse. As the birdhouse is a 'stand alone' device (it does not have to 'fit' into anything else) and since you are only using one ruler for each and every measurement, the fact that each inch was off by 1/8" is likely not very relevant in the scope of things, because you are using this ruler in a relative way. If you were careful cutting, all of the pieces would fit and the house itself would generally be the size that you would need at the end.
Alternatively, let's say you are manufacturing several dozen birdhouses on an assembly line. Someone makes the right side walls, someone else makes the left side walls, a third individual else makes the roofs. Then, let us assume that it is your job to assemble these parts into a birdhouse. If you were using three 'irregular' rulers, and the right side wall person was using one where an inch was actually 7/8", and the right side wall person was using a ruler where an inch was actually 1 1/8" and the roof person was using one where an inch was 1 1/16", you would likely be in trouble at the assembly stage, even though each ruler was within the published specification of the ruler manufacturer. These issues occurred because you were using several rulers in an 'absolute' way. Would it be worth it to spend a bit more on a more accurate ruler, rather than risk wasting the cost of the raw materials, or more importantly, the cost of the labor? I would think so. Even worse, if a lucrative customer was purchasing the birdhouses from you, and there were dozens of other birdhouse manufacturing companies hungry for this customers' business, would you risk that relationship on risks involved with purchasing a cheap ruler? I hope not.
I guess one could get the 'irregular' rulers and make some sort of convoluted operating procedure where rulers are shared among the employees and the parts are marked in bins as to which ruler measured which to aid in the assembly, but this would likely lead to mistakes and misunderstandings, again wasting raw materials and time. This type of action seems eerily familiar. I've seen companies take a similar "workaround" position, in an analogous manner to the one described above. To me, the risks involved here represent an excessive price to pay to justify a cheap ruler.
Others would take the approach of using "plastic wood filler" to accommodate the poorly fitting pieces...I would think that this process would only add costs and degrade quality, but this is the type of response we often see in manufacturing (that is, until they go out of business). This approach represents the classic ‘inspect quality into the product’ methods that nearly drove all American manufacturing companies out of business in the 1970’s. The ‘plastic wood filler jury rig’ approach is another hallmark of the approach taken by some manufacturing managers, but typically not one used in successful companies. Again, to me these workaround methods are likely to lead to excessive production costs to justify a cheap measurement device.
So what instrument should a printing company purchase? Like any purchase, the decision should be thoughtfully considered around the below criteria involving instrument accuracy:
1. What are the published specifications for the measurement device (inter-instrument agreement and repeatability?)
2. What are the factory recertification procedures, costs and logistics?
3. What training resources are available, so that every pertinent employee understands the procedures involved with instrument calibration and operation?
4. What are the other relevant parties in my color communication workflow (i.e.: vendors like the ink company, and customers) using?
Other criteria should also be considered, like software interface, ease of use, warranty, etc.
As always, variables like instrument geometry, illuminant, observer, color space, tolerancing method, user-defined parametric values, and instrument condition together with measurement technique represent the majority of variation present in many color communication workflows: if any of these are uncontrolled and poorly understood by any parties in the process there will likely be errors that are frequently blamed on inter-instrument agreement. Printers should develop standard operating procedures and invest in training to avoid the chance of poor procedures and uncontrolled variables resulting in excessive waste and dissatisfied customers.
Recognizing what is at stake when printers run the risk of shipping non-conforming product to an increasingly critical, brand-oriented customer base with aggressive competitors salivating for the chance to steal the business, the costs of trying to make a prepress instrument work for quality assurance and process control applications is simply too high to even entertain the thought.
©Bruce Leigh Myers, Ph.D. November 05, 2010
This is a good post on this topic.
Regards
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