230201 1L2L0X0A_(I), http://farbe.li.tu-berlin.de/AECI.HTM or
http://color.li.tu-berlin.de/AECI.HTM
For this main page with general information and special images
of the corresponding image page with 10 colour series, see
AECI in English,
AGCI in German.
For the previous main page, see
AEBI in English,
AGBI in German.
For the next main page, see
AEDI in English,
AGDI in German.
For links to the chapter A
Colour Image Technology and Colour Management (2019), see
Content list of chapter A:
AEA_I in English or
AGA_I in German.
Summary of chapter A:
AEA_S in English or
AGA_S in German.
Example image part of 26 parts AEAS to AEZS:
AEAS in English or
AGAS in German.
Chapter A: Colour Image Technology and Colour Management (2019),
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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For the archive information (2000-2009) of the BAM server "www.ps.bam.de"
(2000-2018)
about colour test charts, colorimetric calculations,
standards, and publications, see
indexAE.html in English,
indexAG.html in German.
Back to the main page of this TUB web site (NOT archive), see
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