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- Photography using Digital SLRs

 

Color Management for Photographers #009

Tone Stability for Epson and HP Inks
under Different Illuminants

review by John Hollenberg (9/18/2005)

 
 

 

Background

The color of inkjet prints varies depending on the color of the light used to illuminate the prints. Printer profiles are generally made for “CIE Illuminant D50”. This defines the spectral distribution of daylight that has a correlated color temperature of 5000 Kelvin. In nature, you would see this color of sunlight sometime during the “golden hour” (one hour after sunrise or before sunset). However, prints are generally viewed under other kinds of lighting, such as incandescent and fluorescent bulbs. I became aware of this issue when I took my carefully crafted prints to hang in my office and immediately noticed color shifts. The purpose of this brief note is to provide a quantitative answer to the question, “How much will the color of my prints vary under [fill in the blank] type of lighting?” If the difference is great enough and the source of illumination under which prints will be displayed is known, it may be worthwhile getting a custom printer profile built for that specific light source. See the Links section at the end of the article for background reading.

Methods

Profiling targets (Atkinson 918 patch Eye One RGB target) were printed using the following printers, inksets and papers:

1) HP Designjet 130 with Vivera Inks (dye inks) on HP Premium Plus Photo Satin Paper
2) Epson 2200 with Ultrachrome Inks (pigment inks) on Epson Premium Luster Paper

After the measurements were taken, the patches were sorted by RGB numbers to put similar tones together (the Eye One target has color patches mixed up so that the scanning spectrophotometer will be able to distinguish adjacent patches). Note that the measurements of the raw targets are normally used to build custom profiles. However, in this case we are instead using the raw measurements of the target obtained from the spectrophotometer as a palette of colors that can be analyzed to see how the colors vary with different light sources. The only reason for using these specific colors is that I already had the measurements on hand since they were used to build custom profiles. Any other set of colors could have been printed and analyzed instead, provided there was a reference file (required by Measure Tool application).


Tone stability [Red Box in Figure 1 above] under different light sources was analyzed using the the “Comparing” Function of the Measure Tool application, which is part of the Profilemaker Pro 5 suite from Gretagmacbeth. The reference illuminant (Illuminant 1, which in our case is D50) and the comparison illuminant [Green box in Figure 1] can be selected. Illuminants were either standard ones supplied with the Profilemaker application:

  • D65
  • CWF2 – Cool White Fluorescent
  • Incandescent Illuminant A

or measured with the Eye One Pro spectrophotometer with Ambient Light Head:

  • Office East Wall – the illumination (fluorescent, unknown bulbs) in my office
  • Just Colormaster – the Just Colormaster Duo viewing box (specially made fluorescent bulbs to try to match D50)
  • Solux 4308 Lux 4351 Kelvin – Solux clip-on lamp with 4700 degree Kelvin bulb (36 degree beam spread), at 8-10 inches away

    [Note: 4351 Kelvin was the actual correlated color temperature measured by the Eye One Pro for the 4700 Kelvin bulb.Measurements with a Color Temperature meter showed that the color temperature varied with the distance from the Solux lamp. At 3 inches from the lamp, the value was 4670. The lower value was measured at 8-10 inches.]

The ambient light sources were measured for emission of light every 10 nm between 380 nm and 730 nm. Figure 2 shows graphs (produced in a spreadsheet with the data from the CXF file) of two measured light sources. Note the spikes in the fluorescent lighting between 540 and 610 nm.

Figure 3 shows spectral emission of various light sources compared to D50 (gray line):

Interpretation

The difference between the colors under the two illuminants is described quantitatively by a measurement called Delta E [Blue box in Figure 1]. Delta E interpretation:

A color difference of 1 Delta E corresponds to a barely noticeable difference by 50% of the persons comparing the two patches. In general, Delta E of greater than 3 may be observed in real world images.

There are different formulas to calculate delta E. The original formula overestimates the degree of difference for large differences in color, so I decided to compare using delta E 2000. According to color management guru Bruce Fraser, Delta E 2000 “provides a more accurate gauge of perceived color differences than older systems”.

The worst 10% of patches (most perceived change) are outlined in yellow, the worst match is outlined in red. The average, standard deviation (sigma) and maximum are provided for the total (all patches), the best 90% and the worst 10%.

Below are screen captures for Solux and CWF compared to D50:

Note that the worst 10% of patches have a Delta E 2000 of only 1 for the Solux Lamp, but over 8 for the fluorescent. Also, my office fluorescent lighting is even worse than CWF F2 standard. So that's why those photos didn't look right!

Please note that the images are screen captures (SRGB) and thus will not accurately reflect the actual colors, but probably give some idea of the magnitude of the differences. Also note that some of the colors are outside the gamut of the display.

To put all of this data in perspective, I created two graphs summarizing the most important Delta E 2000 averages, one for Ultrachrome and the other for Vivera Inks. Each shows comparisons among different illuminants for the Average Delta E 2000, the Average Delta E 2000 for the worst 10% of patches, and the Maximum Delta E 2000 (the very worst patch):

In order to allow you to study these results at your leisure, I have put all of the results for both ink sets in a single TIFF file (Tone Stability Comparison) on separate layers. You can open the TIFF files in Photoshop and zoom in to 300-400% to see the comparisons more clearly. Clicking a layer on and off will give you a good visual comparison of what colors are most affected by using a particular light source. Please note that Epson results cannot be compared directly with HP results as far as actual color is concerned. This is because the files were made by sending the same numbers to the printers (untagged RGB), so the workflow was NOT color managed. There is also a summary file for the HP Designjet (DJ 130 Tone Stability Summary), which shows all of the averages, standard deviations, and other figures on one layer. There is one layer showing Delta E and another layer showing Delta E 2000. No summary file is included for the Epson 2200. I have included a file showing spectral comparison of different illuminants to D50. Finally there are graphs of Delta E 2000 Average, Average Worst 10%, and Maximum for both Ultrachrome and Vivera Inks. The Zip file can be downloaded here [link to zip file].

Summary

  • The Solux clip-on task lamp ($70 including shipping) was the closest match to D50, beating out the much more expensive viewing box from Just Normlicht.

    However, the Just Normlicht has a dimmer control to match the light intensity to the monitor. To use the Solux for monitor to print match you would have to adjust the distance of the Solux from the print.

  • For display of prints under Solux 4700 degree Kelvin lamp, no compensation needed in profile to achieve the same look.
  • For display of prints under Incandescent lighting, a custom profile to match the illuminant is only needed if the match is critical. If you are using Gretagmacbeth software, you need Profilemaker Pro for this capability (Eye One Match won't do it).
  • For display of prints under fluorescent lighting, a custom profile to match the illuminant is strongly recommended.
  • Designjet 130 dye inks had slightly greater tone stability under different illuminants than Epson pigment Ultrachrome inks, but this was only enough to be significant for fluorescent (office) lighting.

I found all this information quite “illuminating” :-)


Links/Background Reading

Information on different illuminants:

Information on Delta E:


Disclosure

The author has a strong relationship with the D50 and D65 light sources (i.e., he spends as much time outdoors as possible). The remaining light sources were borrowed or purchased with his own funds. He has no relationship with any manufacturer other than using their hardware or software.

 
 
 
   

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