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Colour Science: About Meter Accuracy

 
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kal
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PostLink    Posted: Fri Jun 11, 2010 8:07 pm    Post subject: Colour Science: About Meter Accuracy Reply with quote


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Colour Science: About Meter Accuracy

In the area of video calibration there has been more nonsense written on the subject of color analyzers and what you need to get accurate readings than any other subject I can think of. This is probably because some ethically challenged calibrators only want people to believe that their high-end meters and services are worth using. I have dealt with some of these issues in the Video Calibration Myths article, but this subject is important enough to warrant its own article.

The Binary Fallacy

The most widely-repeated claim about what you need to obtain accuracy is what I will refer to as "The Binary Fallacy". This usually takes the form of simple, categorical statements about whether a particular meter is "accurate" and hence worth purchasing or relying on. Such statements are fallacious because accuracy is not a binary concept. Something cannot be simply accurate or inaccurate. Rather, accuracy is measured by the degree to which an instrument approaches some purely theoretical standard of perfection. Thus, an instrument's accuracy should always be understood incrementally. It is always a matter of degree.

Let's take one very popular front projector as an example to illustrate this point. The JVC RS1 was released in 2007, and in at least one way it was a revolutionary product.



It was the first 1080P display that offered a very high native contrast ratio and high brightness at the same time. Some DLP projectors could claim similarly high native contrast ratios, but only by employing a dual iris system in the light path that substantially lowered light output. However, despite all of the initial enthusiasm for the projector, it was something of a one-trick pony. Yes, it offered a great black level, but that was about it. The gamma was too low, it lacked adequate custom grayscale controls, the LCoS panel design and optics resulted in a visibly less sharp image than many competing products, and, worst of all, it was designed with the most inaccurate color gamut of just about any display on the market. All of the primary and secondary colors were wildly oversaturated, and the RS1 provided no tools to fix it.

This situation resulted in a couple of lines of thinking. One was an attempt by many proud RS1 owners to either deny that the problem existed, minimize the seriousness of the problem, or even to attempt to define the very concept of color error in such a way that the errors present on the JVC were immune from criticism. Another, more serious, reaction was from those RS1 owners who just wanted to solve the problem. These folks were willing to try any number of creative approaches to mitigate the oversaturated colors. However, until the arrival of the (expensive) Lumagen Radiance external video processor a few months later, unfortunately there was no effective means to remedy the JVC's exaggerated palette. Since then, much has changed. JVC now markets two updated versions of the original RS1 that include a very effective color management system and an affordable external color management solution is now within the means of anyone who can afford a $6,000 projector. What was one of the most flawed popular home theater projectors on the market is now among the very best.

What does all of this have to do with assessing the accuracy of color analyzers? Well, the errors of the RS1 have been fairly carefully documented. I have calibrated many of them myself. A common question among RS1 owners has been whether their meter was "accurate enough" to successfully fix the RS1's color problems. As perhaps you can see, this question falls prey to "The Binary Fallacy". It makes no sense to simply ask whether a meter is or is not accurate enough. There are only two answers to this question: "Yes" or "No". What one should be asking is how much of the color error exhibited by my display can be remediated by a given color analyzer? Once this question has been answered, then it is up to the consumer to decide whether to use that device or to invest in a more expensive, and presumably more accurate, instrument. That decision is not based on a simple yes or no answer to a question about accuracy. It is a contingent judgment: "Yes", if certain conditions obtain; "No", if they do not. The conditions in question are a price/performance ratio. Not everyone will come to the same conclusion.

Using the RS1 and a popular affordable colorimeter, the X-Rite Display 2, as examples, here's what we know. The RS1's most offensive color error was in the green primary. It measured in the neighborhood of x0.296, y0.680. Obviously, there was some unit-to-unit variation, but this figure is quite representative of what one was likely to find. This is an error of 0.080 in the y axis relative to the Rec. 709 HD standard. I have had a lot of experience with the Display 2 colorimeter and I also own a reference 5nm spectroradiometer, so I have a means to test the Display 2's accuracy against a known reference. It turns out that the Display 2 is generally least accurate when reading green. Measurements generally deviate from the reference in the neighborhood of 0.015. What this means is that even with the inherent inaccuracies of the Display 2, a calibrator can remove nearly 80% of the RS1's color errors. With respect to the red and blue primaries, where the Display 2 provides higher accuracy (generally below 0.010), the expected performance would be even better.



Could you do better? Sure. An i1Pro or a Chroma 5 (or Display 3) will have lower errors than the Display 2. Of course, they also cost significantly more. So the question is not so much one of accuracy, but rather "What you are willing to pay for?" Are you satisfied paying $200 for a meter that removes 80% of the errors or would your rather pay 2-3 times more to remove 90% of the errors? Of course, if you invest in a professional quality tool you can remove 95%-99% of the visible errors. Notice, however, that the price/performance curve is not particularly consumer friendly. You have to pay a significantly greater amount for what is really just an incremental improvement in accuracy. For casual amateurs, the Display 2 is fine. For prosumers and dedicated amateurs, or even the professional just starting out, the Chroma 5 PRO (or the newer Display 3 PRO) is probably the best choice.

Spectroradiometers vs. Colorimeters

Another area where a lot of misinformation has been disseminated concerns the relative value of spectroradiometers and colorimeters. Because of their design, spectroradiometers can be more accurate than colorimeters. They are certainly less subject to degradation over time. However, there are several practical reasons to prefer a tristimulus colorimeter for most work.

  • Colorimeters use much larger photo sensitive diodes than spectroradiometers. This results in a considerably higher signal-to-noise ratio and much greater ability to accurately read under low light conditions.
  • Colorimeters are generally less expensive, often MUCH less expensive than spectroradiometers.
  • Colorimeters are generally faster than spectroradiometers, often MUCH faster.
But, the argument goes, spectroradiometers are significantly more accurate over a wider range of displays than colorimeters. This certainly can be true, but it is not always so. For example, the i1Pro, which is a true spectroradiometer, is generally no more accurate than a Chroma 5 or Display 3 colorimeter, especially a new one. The reason for this is that to keep costs down the i1Pro is a 10nm design. This means that it samples the spectra of light with half the frequency as reference quality spectroradiometers. This clearly affects it accuracy.



And what about those reference 5nm spectroradiometers? First, they are very, very expensive—between 10 and 30 times as expensive as an i1Pro. Second, they are relatively slow. Individual readings take anywhere from several seconds to a couple of minutes, depending on the luminance level of the target. So for all their accuracy, they are not very practical as field instruments for day-to-day use. Their most valuable use is as a laboratory reference instrument used to calibrate other less-accurate, but more practical, instruments.

Furthermore, the two biggest problems with colorimeters—inaccuracy and degradation over time as you expose the filters to variations in heat and humidity—can be remedied fairly simply. All that is required is that the colorimeter has been individually calibrated for a variety of different displays and that it is periodically recalibrated to maintain its initial accuracy. With this in mind, we offer the Chroma 5 PRO (now replaced by the Display 3 PRO), a tristmulus colorimeter that offers much of the accuracy of a reference instrument without sacrificing affordability or practical utility.

For those who want to read more on this subject, see this white paper from X-Rite.

I now have enough data on the Chroma 5 colorimeter to provide a fairly clear picture of its level of accuracy relative to a known reference. The table below shows the errors of the Chroma 5 when reading white and then when reading an average of the primary and secondary colors for four different types of displays.

Code:
               White x  White y  Color x  Color y
Plasma          0.003    0.005    0.005    0.005
Sony LCD        0.005    0.003    0.006    0.007
Samsung LCD     0.002    0.005    0.005    0.005
Front Projector 0.004    0.004    0.004    0.005


This shows that the Chroma 5 is an excellent device, at least when factory-fresh. These errors will increase over time as the filters are exposed to changes in heat and moisture, which is why annual recalibration is recommended.


To order ChromaPure software & meter packages at CurtPalme.com exclusive prices, see our ChromaPure order page. Same product, same support - just more money in your pocket at the end of the day!


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Last edited by kal on Sat Apr 01, 2017 5:22 pm; edited 10 times in total
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Spanky Ham



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PostLink    Posted: Sun Jun 20, 2010 2:41 am    Post subject: Reply with quote

Very good article. I haven't really looked into this much, but having worked with a Datacolor spectrophotometer and Datacolor themselves I always think in terms of delta error. I just assumed most thought along the same lines regarding these instruments. Glad you are giving the less expensive crowd an alternative with less variance.
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kal
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PostLink    Posted: Sun Jun 27, 2010 1:39 pm    Post subject: Reply with quote

Added the following:

Quote:


I now have enough data on the Chroma 5 colorimeter to provide a fairly clear picture of its level of accuracy relative to a known reference. The table below shows the errors of the Chroma 5 when reading white and then when reading an average of the primary and secondary colors for four different types of displays.

Code:
               White x  White y  Color x  Color y
Plasma          0.003    0.005    0.005    0.005
Sony LCD        0.005    0.003    0.006    0.007
Samsung LCD     0.002    0.005    0.005    0.005
Front Projector 0.004    0.004    0.004    0.005


This shows that the Chroma 5 is an excellent device, at least when factory-fresh. These errors will increase over time as the filters are exposed to changes in heat and moisture, which is why annual recalibration is recommended.



Kal

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SecondaryColorsSent



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PostLink    Posted: Wed Feb 15, 2012 10:35 pm    Post subject: Re: Colour Science: About Meter Accuracy Reply with quote

[quote="kal"][

Furthermore, the two biggest problems with colorimeters—inaccuracy and degradation over time as you expose the filters to variations in heat and humidity—can be remedied fairly simply. All that is required is that the colorimeter has been individually calibrated for a variety of different displays and that it is periodically recalibrated to maintain its initial accuracy.


--------------

[u]How do you calibrate the colorimeters?[/u] I just ordered the i1 Display-2 But I might have to calibrate it, since it's been discontinued a long time ago.
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kal
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PostLink    Posted: Thu Feb 16, 2012 12:53 am    Post subject: Re: Colour Science: About Meter Accuracy Reply with quote

[quote="SecondaryColorsSent"]
kal wrote:
[

Furthermore, the two biggest problems with colorimeters—inaccuracy and degradation over time as you expose the filters to variations in heat and humidity—can be remedied fairly simply. All that is required is that the colorimeter has been individually calibrated for a variety of different displays and that it is periodically recalibrated to maintain its initial accuracy.


--------------

How do you calibrate the colorimeters? I just ordered the i1 Display-2 But I might have to calibrate it, since it's been discontinued a long time ago.


It's a service that we offer. In the case of the i1 Display 2 see: http://www.curtpalme.com/ChromaPure_EyeOneDisplay2.shtm

Kal

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PostLink    Posted: Thu Feb 16, 2012 2:59 pm    Post subject: Reply with quote

Kal - excellent post, very informative! Do you think you could add a section about meter accuracy with respect to different display types that use different types of lighting (i.e. plasma vs LED vs UHP vs CRT, etc)? In other words, how the spectral output of various lighting sources affects how well different types of meters can read them.
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PostLink    Posted: Sun Mar 02, 2014 12:36 am    Post subject: Reply with quote

Just noticed this error. Surprised no one has pointed it out in 3+ years...

"Colorimeters use much larger photo sensitive diodes than spectroradiometers. This results in a considerably lower signal-to-noise ratio and much greater ability to accurately read under low light conditions."


That should say that the larger photo-sensitive diodes result in a considerably lower noise, and thus a higher S/N ratio.

High S/N = Good. Low S/N = Bad. Very Happy

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kal
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PostLink    Posted: Sun Mar 09, 2014 6:09 pm    Post subject: Reply with quote

Oops - missed this older post of yours Tim. I've fixed the article. Good catch. Thanks!

Kal

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