If you were to choose which one are you going to use, SpectroEye or EyeOne Pro?
When using these two instruments, it gives slightly different readings. However these slightly different readings especially in densities gives significant difference in the computation of TVI using murray-davis. At 40% with plus or minus 6 tolerance, usually EyeOne Pro's result gives a higher CMY and CMYK midtone spread. If you are in my shoes, which one are you going to trust?
Completely different instruments designed for completely different uses in my opinion. I have both...and use both regularly.
The EyeOne Pro is more a general purpose spectro that is designed more for "color management" applications. It's not intended to be used as a "pressroom" instrument as it requires being tethered to a host computer and, of course, has no measurement display of it's own. On the other hand, it is one of the most widely supported spectros on the market. There's LOTS of 3rd party software products that support the EyeOne, including several apps designed for use at the press (more on that later).
The SpectroEye is a great spectro-densitometer and is an excellent instrument for use at the press. Extremely flexible and accurate...as somebody mentioned, it is one of the few instruments that can be "certified" via the X-Rite NetProfiler product. The SpectroEye has it's own display so can be used un-tethered as well as tethered. I've used a SpectroEye for many years as my primary instrument for doing press-side measurements for G7/GRACoL/SWOP press calibrations/verifications.
Back to the EyeOne...what's changed for me in the past year or so is the introduction of SpotOn! software. The combination of the EyeOne and SpotOn!'s direct support for the EyeOne has completely changed my routine for doing press-side G7 press calibrations. Out of habit I continue to have my SpectroEye with me press-side but I can honestly say I haven't used it for the last several G7 calibrations that I've performed. The amount of data that the EyeOne+SpotOn! can collect in a single swipe of a short color bar is amazing and "liberating".
OK, enough of the SpotOn! plug.
My guess is that your problem is that you have a mis-match between the type of measurements you're taking with the EyeOne vs. the SpectroEye. This has nothing to with the formula used (Murray-Davies is fine) but with the "raw" measurement or "colorimetry" that you're using. My guess is that with the SpectroEye you're using Status T densitometry and feeding that into the Murray-Davies formula while the EyeOne is using XYZ colorimetry being fed into Murray-Davies. You'll likely get very different dot gain/TVI measurements using these different types of data input. In my experience, cyan in particular can measure quite different TVIs depending on whether you're using density vs. colorimetry (XYZ or L*a*b*); cyan will tend to measure with a lower TVI when using XYZ TVI as compared to Status T TVI values.
As far as WHAT measurement to use, it depends. If you're interested in comparing your dot gain/TVI numbers with legacy values or you intend to adjust your plate curves using legacy values as targets, then you'd best stick with density-based TVI (Status T in the USA) as opposed to "colorimetry"-based TVI values. However, if you're simply interested in setting and establishing internal TVI standards for your shop, you could use colorimetry-based TVI if you like...just be careful in trying to compare your TVI values with "outside" values as you'll be right back to the same discrepancy you're having currently.
If you want to compare if there is a REAL difference between the EyeOne and the SpectroEye, use something like X-Rite's Measure Tool to measure a step wedge with both the EyeOne and the SpectroEye (Measure Tool will support the SpectroEye directly on the Windows OS), that way you're comparing apples-to-apples (or nanometers-to-nanometers!).
Hope this helps,
Terry Wyse
