G7 Numbers Conversion

[Bill W]

Greetings,

Would someone be so kind as to convert the G7 Lab numbers under 50/2° to what they would be under 65/2°.

I am guessing that since the illumination is different between 50 and 65, there would also a difference in the hue and chroma numbers to compensate for the difference. True?

Thanks.

-Bill-

 

[meddington]

Greetings,

Would someone be so kind as to convert the G7 Lab numbers under 50/2° to what they would be under 65/2°.

I am guessing that since the illumination is different between 50 and 65, there would also a difference in the hue and chroma numbers to compensate for the difference. True?

Thanks.

-Bill-

You'd need spectral data to perform that, which isn't available for the published data sets.

To get an idea though, here's the values of a real world sample

D50
Cyan: 56.8, -36.9, --49.3
Mag: 48.8, 74.2, -3.3
Yel: 88.4, -2.8, 91.8
Black: 16, -0.4, -0.1

D65
Cyan: 57.5, -36.3, -47.9
Mag: 47.3, 72.8, -6.5
Yel: 88.3, -3.5, 94.7
Black: 16, -0.5, 0.0

Comparing a full IT8/7.4 spectral measurement file exported from Measuretool at both D50 and D65, I get the following results:


dE Report

Number of Samples: 1617

Delta-E Formula dE76

Overall - (1617 colors)
--------------------------------------------------
Average dE: 1.49
Max dE: 3.70
Min dE: 0.00
StdDev dE: 0.82

Best 90% - (1454 colors)
--------------------------------------------------
Average dE: 1.32
Max dE: 2.61
Min dE: 0.00
StdDev dE: 0.69

Worst 10% - (163 colors)
--------------------------------------------------
Average dE: 2.95
Max dE: 3.70
Min dE: 2.61
StdDev dE: 0.27

 

[michaelejahn]

I have a silly question - what spectral instrument has both a D50 light source and a D65 light source to illuminate the section of the IT8 target when you measure it with you spectral instrument ?

 

[Bill W]

G7 Conversion

G7 Conversion

I have a silly question - what spectral instrument has both a D50 light source and a D65 light source to illuminate the section of the IT8 target when you measure it with you spectral instrument ?

Greetings Michael,

X-rite's spectrodensitometers have many choices for illuminant and angles. In fact if one is not careful of what buttons they push, one can accidentally change an illuminant /angle with little effort. One can also input the G7 values as references and when measuring, the device will calculate the dE to the G7 CMYK spec'd values, as well as how far off each of the L a b channels are from the spec.

Some software, such as MeasureTool, will allow one to change the illuminant and angle when evaluating spectral data measurement files.

-Bill-

 

[rich apollo]

I have a silly question - what spectral instrument has both a D50 light source and a D65 light source to illuminate the section of the IT8 target when you measure it with you spectral instrument ?

No instrument has either. D50 and D65 are mathematical models - tables "of spectrophotometric data." When measuring a color you have to define white and the illuminant under which you want to know the color.

 

[meddington]

X-rite's spectrodensitometers have many choices for illuminant and angles. In fact if one is not careful of what buttons they push, one can accidentally change an illuminant /angle with little effort.-

Note that the "change" in the illuminant is purely mathematical, rather than a separate illuminant. Many spectros are using an incadescent bulb whose spectral response is closer to Illuminant A. The angle refers to the CIE Standard Observer angle (rather than an angle of illumination, which for graphic arts is primarily 0/45 degree devices). The 2 degree observer referes to the angular field of view that was used in initial studies to determine the color matching functions.

Correction to my own post, its not actually required to have spectral data to change the illuminant used in calucluating Lab values. Chromatic adaptation to D65 can be performed on XYZ values. With spectral data, Lab values are calculated from the spectral values of the sample, the illuminant spectral data (D50, D65, etc), and the color matching functions (observer angle). It remains to be seen how close the two methods correlate, but there is a difference.

Oddly enough, apparently MeasureTool is deriving D65 data from chromatic adaptation of XYZ data...despite that fact that this option is only enabled when spectral data is present. Hence Lab values derived at D65 from Measuretool will not match measurements from a handheld spectrophotometer with the same settings. Kind of eye opening. Luckily (for everyone but Bill) this is something that doesn't rear its head too much.

 

[tmiller_iluvprinting]

Rich,
Why then, does my Spectrolino come with a D65 filter? If the D65 filter is installed wouldn't this be like using a D65 light source, or is it filtering out the D65 part of the spectrum?
Best regards,
Todd

 

[meddington]

Rich,
Why then, does my Spectrolino come with a D65 filter? If the D65 filter is installed wouldn't this be like using a D65 light source, or is it filtering out the D65 part of the spectrum?
Best regards,
Todd

As I understand it, the D65 filter is essentially a "blue" filter the attempts to change the shape of the spectrum from the original illumination to be closer to the D65 spectrum. This could be done mathematically, but the point of the filter would be to improve the signal to noise ratio during measurement...perhaps helping with optical brightener issues. I would doubt that measurements with the D65 filter would match a handheld device set up to D65, but haven't ever tested this.

 

[rich apollo]

tmiller,
That's a question for someone whose propeller spins a bit faster than mine, but I imagine that Mike Eddington is correct - he usually is.

Run a search on CIE Illuminants and you can find all kind of information.

Oh, and as Mike Eddington stated (and I left out), when measuring a color you also have to define the observer angle.