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Chapter A: Colour Image Technology and Colour Management, Main part AECI

1. Introduction and goals.

An analog camera produces images on slide or negative film. The Kodak PhotoCD Process digitise the analog film materiel with 14bit/channel. The digital rgb data change with the exposure, the film materiel, the device values, the handling and the software for the production of the 8bit rgb PhotoCD output data. The rgb output data are coded in the digital range between 0 and 255 with 8 Bit. Between black and white the grey range is rarely used completely for the coding.

For the complete use of the range between 0 and 255 an rgb output linearization is required. For this the photo of a gray scale with 16 visual equidistant L* steps within the motif is successful. For an achromatic grey scale of equal difference the rgb values get equal by output linearization for 16 steps and equidistant between 0 and 255. In the range between 0 and 255 the values are 0, 17, 34, 51, ..., 255 (=15x17). These values are used in figure 1.


Figure 1 shows the In-Out relation for negative film nf with the standard exposure (+0,0 stops).
For the download of this figure in the VG-PDF format, see AGY11-4N.PDF.

By output linearization of the standard data of the PhotoCD the intended ergonomic relation was reached. The black dashed line data are approximately perfect, and therefore under or near the dashed black line. For the input range between L*=15 for black N and L*=90 for white the digital 8bit range between 0 and 255 is completely filled after the output linearization.

The ergonomic question arises, if the 8bit range is visually sufficient. For separate located grey samples on a grey background about 40 steps can be distinguished. If the samples are adjacent, then in an ideal case about 120 (40x3) steps can be distinguished with the visual system. The 8bit range is therefore visually sufficient and allows for scenes with much higher contrast.

The 8bit coding according to L* is defined in IEC 61966-2-1 (sRGB colour space). In applications usually a relative scala is used. The black N has always the value r=g=b=0 and white W has the value r=g=b=255. The output range of cameras and scanners shall therefore be normalized for this range. In addition in this value range there shall be a linear relation between the rgb data and the lightness L* of the grey samples. This is realized in figure 1.

The CIE lightness L* is approximated by IEC 61966-2-1 with the following formula:
L* = [Y/100] exp(n)

This formula uses the CIE tristimulus value Y with Y=0 for black N and Y=100 for white W. For the exponent it is valid
n = 1 / 2,4.
This formula includes the absolute gamma ga = 2,4.

The relative gamma gP is the ratio of two absolute gamma values. For example it is approximately valid for the standard office according to ISO 9241-306:
gP = 1,8 / 2,4 = 0,75.

The lightness range of L* between black N and white W decreases with the contrast of the scene. If for example the luminance of a daylight projector and the luminance of the room daylight produced on the display are equal, then the contrast of W and N is only 2:1. This requires a new gamma gP = 1,2 / 2,4 = 0,50 or n = 1 / 1,2 for visual equal steps between W and N.

The exponent is then more near the value 1. The new L* is then approximately a linear function of Y. This result corresponds with the mathematical basic result, that any nonlinear function is a linear function in an small range.

By ergonomic reasons the many real visual application on the display require an adjustment according to the application contrast. On this web page this adjustment is done with many achromatic and chromatic ISO-test charts. Similar adjustments are often necessary for print or scan applications.

For further information about output linearization see the Reportership Report CIE R8-09:2015 (download only available for CIE-members) or a paper for free download of the author K. Richter with approximately the same technical content, see
OUTLIN16_01.PDF

2. Photos of the image flower motif on slide and negative material with seven exposures


Figure 2 shows seven exposures between under and over exposure on slide and negative film
For the download of this figure in the VG-PDF format, see AEC1L0N1.PDF.

The left and right side show eight figures based on slide and negative film, respectively. On both sides the left top figure is the normal exposure. Then a series of seven figures follows between under and over exposure on both sides. It can be clearly seen, that the series is taken from under to over exposure.

In Fig. 2 the cut points of the black grid define the measure point of the digital rgb data. The data were taken directly from files which are in the Adobe eps-image format. The 8bit data in the range 0 to 255 are used in Fig. 1, 5, 6, and 7. The 16 (=2x8) positions in the later Fig. 5 correspond to the 16 positions in Fig. 2.


Figure 3 shows the linearized images based on slide and negative film on both sides with a the grid.
For the download of this figure in the VG-PDF format, see AEC3L0N1.PDF.


Figure 4 shows the linearized images based on slide and negative film on both sides without a the grid.
For the download of this figure in the VG-PDF format, see AEC4L0N1.PDF.

In Fig. 3 and 4 all linearized images appear now very similar. In addition the result of both the slide and negative film is similar. The simple input-output relation of three rgb data sets as function of the lightness L* leads to this result. It shows the success of the output linearization method for the production of rgb image data which are approximately independent of both the exposure and the film material.


3. Properties of the ISO/IEC-image "flour motif" in the PG-PDF format in four resolutions.


The ISO-test charts AE18 and AE28 in figure 3 and 4 include the ISO/IEC image "flower motif" in two versions based on slide film (sf, AE18) and negative film (nf, AE28). The analog reflective original "flower motif" (about 70cm x 100cm) includes a black and white Siemens-star and test patches.

An equally spaced 16 step grey scale with CIE lightness L* in the range L*=15, 20, 25, ..., 90 was used. The 14 CIE-test colours for colour rendering according to CIE 13.3 include a hue circle and the four elementary colours Red Re, Yellow Ye, Green Ge, and Blue Be (no. 9 to 12) with a high chroma. The taking illuminant was standard illuminant D65. As measuring and taking (viewing) geometry the CIE standard 45/0 degree was used.

Seven different exposures between under to over exposure were used. For the slide film the exposures -1,5, -1,0, -0,5, +0,0, +0,5, +1,0, and +1,5 stops were used. For the negative film the exposures -2,0, -1,0, +0,0, +1,0, +2,0, +3,0, and +4,0 stops were used. The standard Kodak Photo CD process creates the rgb-colour data as function of exposure and for the two film materials.


Figure 5 shows the input - output relationship for slide film sf (left) and negative film nf (right).
For the download of this figure in the VG-PDF format, see AEY1L0N1.PDF.


Figure 6 shows the input - output relationship for slide film sf with the exposure +0,5.
For the download of this figure in the VG-PDF format, see AEY10-6N.JPG.


Figure 7 shows the input - output relationship for negative film nf with the exposure +1,0.
For the download of this figure in the VG-PDF format, see AEY11-5N.PDF.

By output linearization of the standard rgb values of the Photo CD the intended ergonomic relationship was reached, see the dashed black line. The three relations for the rgb data are nearly perfect, and are therefore located behind or near the dashed line.

Fore more information about output linearization see the Reportership Report CIE R8-09 (download only available for CIE members) or a paper for free download of the CIE author K. Richter with approximately the same technical content, see
http://farbe.li.tu-berlin.de/OUTLIN16_01.PDF


Figure 8 shows the flower motif for slide film with the exposure +0,5.
For the download of this figure in the VG-PDF format, see AEC61-3N.PDF.


Figure 9 shows the flower motif for negative film with the exposure +1,0.
For the download of this figure in the VG-PDF format, see AEC71-3N.PDF.

Visual inspection leads to te result that it is hard to see differences between the film material slide film (sf) and negative film (nf). This is similar for exposure variations of slide film in the range between -0,5 and +1,5 stops, and of negative film in the range between -1,0 and +3,0 stops.

The exposure range is about four stops for negative film and two stops (half) for slide film. The output linearization method produces in this exposure ranges images which appear approximately identical.

For other resolutions and in addition images with cmyk instead of rgb data go to the image part AEES.

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