All Things Color for Film and Digital Cinema
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Posts Tagged ‘CIE’

DCI P3 White Point and color primaries – Measurement Reference – 6300k

Projecting a set of white and color primary  test images from your DCP player or software/Hardware solution should give the following readings when properly projected through a calibrated system and measured with an accurate  color meter.

Measuring and calibrating a projectors DCI white point should not only be performed with the projectors internal test patches. I personally argue to test calibration with both the projectors internal test patterns as well as external test patterns because depending on the image interface to the projector from the DCP player or SDI hardware output, experience tells me some of these interfaces/modules can and do add additional adjustments to the color gamut and levels.

Luminance 14 Ft Lamberts

Gamma: 2.6

White:  x 0.314   y 0.351

Red:      x 0.680   y 0.320

Green:  x 0.265   y 0.690

Blue:     x 0.150  y 0.060

 

 


18 Percent Grey …. “Middle Grey” and Magic Numbers

 

    For a while I have been pondering the function of the 18 percent grey card. Why 18%, what is its historical reference…ect.

    After a lot of reference reading I came up with the following ….

    Traditionally 18 % refers to the statistical average reflectance of a photographed scene. ‘Normally’ exposed skin tones also generate an average incident light reading of about 18%.

    Photographically, if you were to make a set of 11 patches starting with 100% reflectivity and each subsequent patch was halved in reflectance, you would end up with a logarithmic scale where the light intensity is being halved each patch. The seventh step, middle grey, would yield a photographic Status M Neg density value of 0.7

    Using our Density math described in previous posts we know Transmission = 1/10^Density   

    Therefor    T=1/10^0.7= 0.18      AND       Transmission is directly proportionate to Luminance L*. Not boring you with the math the result is 49.496.

   So    0.7 Density = 0.18 Transmission = 50% Luminance(L*).

 

 

Fig.A - Status M against Status A Density

Fig.A - Status M against Status A Density

 

 

  Interestingly If we map our 21 step sensitometry readings of a 21 strip grey scale test wedge over Neg and Print densities we see that 0.7D is the cross over point !   See Fig.A

 

 

Fig.C

Fig.B

 

 

 

 

Looking at the image below, Fig.C, you will notice that this point of around 18 % grey is the mid point at which the cineon Log file is expanded when overlayed with a Print emulation 3DLookUp table. Both Mid Grey LAD Patch’s are almost identical as seen on the corresponding waveform Representations. 

 

 

18percent

Fig.C

 

Looking at the graph in Fig.C one can also see the reference point of 18 % on each of the mapped targets averaging around 50% luminance. Interestingly with this chart I have mapped Cineon Log levels against the 2.2 and 2.5 video gamma transfer functions and CIE Luminace L* values. They are all relatively close to each other in their Logarithmic encoding. I can see im going to have to rewrite this as its going to get messy from about here on in, although quite fascinating. 2.5 gamma looks like its the best match for CIE L* but somehow we got stuck with 2.2. Near enough is good enough I guess. It was decided back in the 8Bit video days that to help save on visual data that video/TV could also have a perceptual gamma encoding, once again mimicking the eyes response to nature. A gamma of 2.2 was decided apon.  

 

 

Adrian


The Macbeth Chart

The Macbeth chart is a color evaluation chart that represents and reflects the colors of nature under any illumination. It is used as a color rendition and reproduction target for both stills and motion picture imaging labs. Using the  given set of digital scientific values for each patch an operator can evaluate a given image exposed with the card and correct for color inconsistencies inherent in Digital imaging devices, film stocks, lens coatings, old lamps, printing stocks, printing paper, monitoring devices,….ect.

Below is the Macbeth Color Chart and its relative CIE and SMPTE-C RGB values.

Macbeth Color Chart

Macbeth Color Chart

 

 

 

 

 

 

 

 

 

No Name CIE_x CIE_y CIE_Y Hue Value Chroma RGB_TRIPLET
1 dark skin 0.400 0.350 10.1 3 YR 3.7 3.2 116 80 67
2 light skin 0.377 0.345 35.8 2.2 YR 6.47 4.1 196 149 129
3 blue sky 0.247 0.251 19.3 4.3 PB 4.95 5.5 91 122 155
4 foliage 0.337 0.422 13.3 6.7 GY 4.2 4.1 88 109 67
5 blue flower 0.265 0.240 24.3 9.7 PB 5.47 6.7 128 127 175
6 bluish green 0.261 0.343 43.1 2.5 BG 7 6 90 190 169
7 orange 0.506 0.407 30.1 5 YR 6 11 221 119 44
8 purplish blue 0.211 0.175 12.0 7.5 PB 4 10.7 71 90 164
9 moderate red 0.453 0.306 19.8 2.5 R 5 10 198 81 98
10 purple 0.285 0.202 6.6 5 P 3 7 93 60 107
11 yellow green 0.380 0.489 44.3 5 GY 7.1 9.1 158 88 64
12 orange yellow 0.473 0.438 43.1 10 YR 7 10.5 234 161 49
13 blue 0.187 0.129 6.1 7.5 PB 2.9 12.7 47 59 151
14 green 0.305 0.478 23.4 0.25 G 5.4 8.65 65 150 71
15 red 0.539 0.313 12.0 5 R 4 12 181 40 59
16 yellow 0.448 0.470 59.1 5 Y 8 11.1 241 200 38
17 magenta 0.364 0.233 19.8 2.5 RP 5 12 190 78 146
18 cyan 0.196 0.252 19.8 5 B 5 8 0 134 164
19 white 0.310 0.316 90.0 N 9.5 0 242 242 236
20 neutral 8 0.310 0.316 59.1 N 8 0 200 200 199
21 neutral 6.5 0.310 0.316 36.2 N 6.5 0 159 160 159
22 neutral 5 0.310 0.316 19.8 N 5 0 122 121 119
23 neutral 3.5 0.310 0.316 9.0 N 3.5 0 84 84 84
24 black 0.310 0.316 3.1 N 2 0 53 53 53