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IT8 Calibration Targets

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Does cost really make a difference?

By: Ian Lyons

 

 

A Computer Darkroom Review

 

This article was written to complement my Optimising the Scanner Response tutorial. It also discusses some of the quality issues surrounding IT8 targets.

In the following discussion I compare  IT8 targets from three different sources (Kodak, Coloraid.de and FujiFilm). These targets cost from around $40 to one costing significantly more (read hundreds of $'s). Clearly with such a wide variance in cost we would expect to see significant differences in the quality of the final ICC profile. However, expectation and reality don't always behave in the way we would anticipate.

IT8 targets come in two forms: transmissive (IT8.7/1) and reflective (IT8.7/2), and are commonly used to calibrate film and reflective type scanners. They are available in three different sizes and can be purchased from any number of different sources. That said there are only a few companies who actually produce them. The targets themselves are based upon an internationally agreed standard which requires that they provide the following characteristics :

  • A set of  colour patches that fall within most device-colour gamut's.

  • A neutral "lightness" scale comprising 24 patches ranging from white through to black. The first and last patches denoting the Dmin and Dmax values of the target.

  • A series of Target-specific colour patches that have been selected to define the gamut limit of the colorants used to produce the target

  • The target will also contain vendor-optional material such as additional colour/neutral patches or images.

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Generic IT8 Target

Apart from cost the main difference between the three IT8 targets that I'm comparing here is the manner in which they and the associated reference data file were produced. The Kodak target is mass produced and even targets with the same batch number seem show a degree of variation. Strictly speaking the differences between targets within the same batch and the reference should be within a certain error limit (CIE Lab dE 5) and the typical error must be reported by the manufacturer for each patch of the target. The reference data files for Coloraid.de and FujiFilm targets compare very favourably with the standard. In Table 1 we see the aim values for the neutral scale lying along the bottom of the IT8 target. The neutral scale for transmissive targets should have the following L* aim values, based on the measurement conditions defined in the standard, read from left to right across the target:

GS1

GS2

GS3

GS4

GS5

GS6

GS7

GS8

GS9

GS10

GS11

GS12

GS13

GS14

GS15

GS16

GS17

GS18

GS19

GS20

GS21

GS22

82

78

74

70

66

62

58

54

50

46

42

38

34

30

26

22

18

14

10

6

4

2

Table 1 - Lab aim values for grayscale as defined in the IT8 7/1 standard

Table 2 below shows the batch average grayscale values for the three IT8 targets being reviewed here.

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GS1

GS2

GS3

GS4

GS5

GS6

GS7

GS8

GS9

GS10

GS11

GS12

GS13

GS14

GS15

GS16

GS17

GS18

GS19

GS20

GS21

GS22

Kodak

81.42

76.94

73.26

68.78

64.22

59.89

55.68

51.98

48.08

44.72

39.95

35.73

31.75

27.16

23.59

19.59

15.70

11.61

7.48

4.74

3.08

1.76

Coloraid

82.39

78.69

74.68

70.76

66.72

62.91

58.84

54.77

50.81

46.32

42.39

38.60

34.55

30.59

26.42

22.40

18.35

14.32

10.07

6.52

4.38

2.02

FujiFilm

81.43

77.50

73.50

69.41

65.34

61.74

57.59

53.63

49.71

44.79

41.98

37.42

33.99

29.88

25.85

21.72

17.79

14.19

9.96

5.67

3.68

1.65

Table 2 - Batch average Lab values for grayscale

The density of the targets Dmin and Dmax patches will also have a significant bearing on the targets ability to enable the production of an accurate ICC profile. If the targets Dmin has a luminance of say 85 (we really want something in the order of 90 plus) and we scan an image with brighter highlights (e.g. luminance of 90) these highlight details will be clipped to maximum white. Good profiling applications will usually allow a very small amount of tolerance knowing that making the IT8 targets is not easy, but there are limits. The goal is to make sure that the Dmin/Dmax of the IT8 target really reflects the maximum possible density range that users will experience when the film is developed. In order to achieve this the IT8 vendor needs to ensure that their targets actually do include the maximum density range and that the targets are processed to the highest standards.

Another area of scanner profiling that causes confusion is that of the film characteristics. Many folk are of the view that because each film will tend to have a different colour bias then we need to build a profile for that type of the film. However, whilst images will tend to vary from shot to shot depending upon exposure and ambient lighting the film's colour bias will within certain limits (the exception being reciprocity law failure with long exposures) remain fairly constant. So in effect it's not the film that we're profiling, but the scanner response to it. Again, the technical information provided by Kodak has the following to say on this matter:

"It is important to note that any colour photographic product contains both image capture (sensitizing) dyes as well as image-forming dyes. While the image sensitizing dyes are a vital part of the film performance, it is only the image forming dyes that affect scanner calibration or characterisation. Fortunately, only a small number of image forming dyes are used. Specifically, one set of image forming dyes can be used for EKTACHROME film, and one for EKTACOLOR paper. This means that only one Q-60 target is required to represent the complete EKTACHROME film family and one for EKTACOLOR paper"

Nevertheless, when I set about this comparison of IT8's the jury was still out on whether we should always match a Fuji target to Fuji films and a Kodak Ektachrome target with Kodak Ektachrome films, etc. To be honest I've not yet come across a real world situation where scanning an image captured on Fuji film whilst using an ICC profile based on a Kodak Ektachrome target has caused me any great problems; at least not for anything other than precise colour matching. However, the same cannot be said of Kodachrome type film as it really does require a matched IT8 target. Likewise it's very likely that we'll need different targets for Velvia RVP 50 and the new Velvia 100F as these two films are  in many respects very different from each other and the rest of the Fuji range.

The following 3D gamut plot compares the gamut of Kodak Ektachrome and FujiFilm Provia and should give the viewer some indication of the differences between these two films. The solid orange area defines the gamut of the Provia and the yellow "wire frame" area the Kodak Ektachrome.

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3D Gamut Plot - Kodak Ektachrome Vs Fuji Provia

Building a Scanner Profile

The first step in building a scanner profile is to capture the IT8 target as an RGB image using your preferred scanning application. This RGB image is then imported into colour-management software that has been specially written to read the target and compare the values of each patch with a reference data file. The reference data file will usually be in text format and comprises rows and columns of Lab values for each patch making up the IT8 target. By comparing the Lab values of the scanned patches with the known Lab values of the patches on the target itself, the software calculates the scanner colour error for each patch. The software then uses this information to build a colour table that describes how the scanner reproduces the colour and neutral patches. This table along with its supporting information is then saved as an ICC colour profile.

As mentioned above, depending upon the type, size and source of the actual IT8 calibration target the reference data file will either be unique to an individually measured target or a batch of targets. Clearly an individually measured IT8 will be more accurate than batch produced targets but this accuracy comes at greater cost (sometimes significantly greater). The question is - does the additional cost of a uniquely measured IT8 target  really convert into more accurate colour matching?

The three IT8 target slides that I compare are:

  1. Kodak Ektachrome Q60 supplied by Kodak

  2. Fuji Velvia supplied to me by Wolf Faust at Coloraid.de

  3. Fuji Provia supplied to me by Fuji Germany (no sales)

The following series of scans were captured by my Nikon LS-4000ED using LaserSoft SilverFast 6 with the gamma-gradation value set to 2.4 rather than my usual gamma value of 2.2 Using the higher gamma ensured that shadows were open and that the final scan is relatively low in contrast. The actual ICC profiles were created using GretagMacbeth ProfileMaker Pro 4.1.5

The sample image (Fuji Sensia II 100 ISO) contains saturated red and blue. Please note that the animations are NOT intended to demonstrate that one target is better than another but to show the quite significant improvements that can be achieved by using a colour managed workflow when scanning.

Kodak Q60E3

The Kodak IT8 otherwise known as the Kodak Q60 is probably the most widely used of all the available targets. Since it is produced in large batches it's relatively inexpensive and easy to obtain. However, quality and consistency have never been Kodak's strongest attributes when it comes to creating batch IT8's. I have four Kodak Q60's all with the same batch number and yet they are visually different.

Note: the mouse-over animations may be slow on dial-up internet connections.

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Before and after comparison of Kodak Ektachrome Target

(Roll mouse over image to see result of applying the ICC profile)

Notice that greyscale patches remain relatively unchanged in terms of neutrality but after applying the ICC profile the shadows become more compressed. The dmin/dmax patches also alter in value quite significantly for the better. The sky in the Before scan is nowhere near the original image, likewise the red boat. Once the ICC profile is applied we see a substantial improvement throughout the image. The blue sky and reds are vastly superior to the raw scan.

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Before and after comparison of real world image

(Roll mouse over image to see result of applying the ICC profile)

FujiFilm: - Fujichrome Velvia

This IT8 is one of the most recent supplied by Wolf Faust and is based upon a relatively new production technique that he developed to maximise the density range of his targets. In developing this process he spent some considerable time and effort ensuring that the target was as flexible as possible without jeopardising overall quality. Velvia film with normal Fuji development usually allows luminance values from 0.2 to 90. When developed with Kodak chemistry, this typically changes to 0.8 to 92. Wolf made the Velvia targets with a density range of 0.5 to 92. So the target does not cut-off highlights and the 0.3 above the normal Fuji dmax is far from visible and below the typical data resolution of ICC profiles. So while not a perfect dmax in the Fuji sense it is a lot better than anything Kodak or Agfa currently supply.

Note: the mouse-over animations may be slow on dial-up internet connections.

Before and after comparison of Coloraid.de Target

(Roll mouse over image to see result of applying the ICC profile)

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Before and after comparison of real world image

(Roll mouse over image to see result of applying the ICC profile)

FujiFilm - Fujichrome Provia

This target slide was supplied to me as a sample by Fuji Germany. It was one of a small batch (12 targets) specially produced for quality assessment. I'm not sure if this target would fit the bill as one of Fuji's hand measured targets but it surely comes close. I've also been told that original supplier has made further refinements and the most recent version of this target is even better.

Note: the mouse-over animations may be slow on dial-up internet connections.

Before and after comparison of FujiFilm Target

(Roll mouse over image to see result of applying the ICC profile)

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Before and after comparison of real world image

(Roll mouse over image to see result of applying the ICC profile)

Conclusion

Comparing the batch average data for each target it is clear that those from Coloraid.de and FujiFilm are much closer to the specification than the Kodak target. Actually, other than patch GS22 the FujiFilm target values are very close to the standard. Whether my sample targets are better or worse than the batch average I can't say but my four Kodak targets certainly do vary from each other.

As mentioned above the FujiFilm Provia target was supplied to me by Fuji Germany as a sample of their IT8 targets. I'm advised that it was created for FujiFilm for quality assessment and is one of a batch of 12 targets created from a single sheet of Fuji Provia film.  It's the closest I'm likely to come to an individually measured IT8 target for a 35mm film. Yet there appears little to choose between it and the Fuji based Coloraid.de target in terms of how each reproduces the sample scans. Given the cost differential between the two I'll admit that it was quite a surprise to find the final scans from the Coloraid.de target apparently matching those from the FujiFilm target. So, at around $40 including shipping it certainly looks like a no-brainer as far as which target supplier offers the better cost/benefit ratio. On the other hand none of my Kodak targets are quite as good as the Coloraid.de target and I would be hard pressed to recommend anyone spending $'s on one. That said the Kodak still offers substantial improvements over using no profile at all.

It's worth mentioning that Wolf Faust is of the view that matching your film and target is important and recommends that Kodak Ektachrome film users do purchase his Ektachrome targets in preference to the Fuji targets, and Fuji targets for Fuji film. Given that the test image was shot on Fuji Sensia II this may well explain my preference for the Fuji based targets, and effectively contradicts my earlier view that matching targets to film stock isn't really necessary. Further details of  IT8 targets based on both Kodak and Fuji film can be obtained from Wolf Faust at Coloraid.de.

September 2003

Another target that appears to have gathered a following is the HCT target by Don Hutcheson of HutchColor. Don  has also produced a very informative guide to scanning to complement his HCT target. Shortly after publishing this article Don kindly sent me one of his HCT targets. Whilst the 35mm versions are not true hand measured targets they are still very accurate. This accuracy is achieved by including approximately twenty-five 35mm and two larger format targets on a single 10 by 8 inch sheet of film. The larger targets are then measured and averaged so as to obtain a very accurate batch-average.  At  $155 the HCT targets are expensive but they definitely produce very high quality results. Unfortunately they don't follow the normal IT8 format and because of this not all profiling applications can read the reference data file. So you should check that your profiling application is compatible with the HCT format before ordering one.

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HutchColor (HCT) Target

 

 

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