D.1 Emissive Display Systems

D.1.1 Measuring the system Characteristic Curve

Before the characteristic Luminance response of the emissive Display System is measured, it is allowed to warm up as recommended by the manufacturer and is adjusted such that it conforms to the manufacturer’s performance specifications. In particular, adjustment procedures for setting the black and white levels of the display should be obtained from the Display System manufacturer. The goal is to maximize the dynamic Luminance Range of the display without introducing artifacts, resulting in the highest possible number of Just-Noticeable Differences (JNDs).

Note: A simple test that the system is set up properly can be performed by viewing the 5% and 95% squares in the SMPTE pattern. The perceived contrast between the 5% square and its 0% surrounding should be equal to the perceived contrast between the 95% square and a white square.

Measurement of the Characteristic Curve of the Display System may be accomplished using a test pattern (Fig. D.1-1) consisting of:

a square measurement field comprising 10% of the total number of pixels displayed by the system positioned in the center of the display;

a full-screen uniform background of 20% of maximum Luminance surrounding the target.

Note: With a measurement field of 10% of the total number of displayed pixels and a surrounding set to 20% of maximum Luminance, internal light scatter in the monitor causes the Luminance Range to be typically comparable to that found in radiographs, such as a thorax radiograph, when displayed on the CRT monitor.

[pic]

Figure D.1-1. The test pattern will be a variable intensity square in the center of a low Luminance background area.

Notes: 1. For example, on a 5-megapixel Display System with a matrix of 2048 by 2560 pixels, the target would be a square with 724 pixels on each side.

2. Ideally, the test pattern should fill the entire screen. Under certain windowed operating environments, it may be difficult to eliminate certain user-interface objects from the display, in particular, menu bars at the top of the screen. In this case, the background should fill as much of the screen as possible.

The Characteristic Curve of the Display System may be determined by

- turning off all ambient lighting (necessary only when a suction cup photometer is used or when a handheld photometer casts a shadow on the display screen);

- displaying the above test pattern;

- setting the DDL for the measurement field to a sequence of different values, starting with 0 and increasing at each step until the maximum DDL is reached;

- using a photometer to measure and record the Luminance of the measurement field at each command value.

As discussed in Annex C, the number and distribution of DDLs at which measurements are taken must be sufficient to accurately model the Characteristic Curve of the Display System over the entire Luminance Range.

Notes: 1. If a handheld photometer is used, it should be placed at a distance from the display screen so that Luminance is measured in the center of the measurement field, without overlapping the surrounding background. This distance can be calculated using the acceptance angle specification provided by the photometer manufacturer.

2. The exact number and distribution of DDLs should be based both on the characteristics of the Display System and on the mathematical technique used to interpolate the Characteristic Curve of the system. It is recommended that at least 64 different command values be used in the procedure.

3. Successive Luminance measurements should be spaced in time such that the Display System always reaches a steady state. It may be particularly important to allow the system to settle before taking the initial measurement at DDL 0.

As stated in the normative section, the effect of ambient light on the apparent Characteristic Curve must always be included when configuring a Display System to conform with the Grayscale Standard Display Function.

If a handheld photometer that does not cast a shadow on the display screen is used to measure the Characteristic Curve, then the Luminance produced by the display plus the effect of ambient light may be measured simultaneously.

When a suction cup photometer is used to take the Luminance measurements or when a handheld photometer casts a shadow on the display screen, all ambient lighting should be turned off while measuring the Characteristic Curve. The effect of ambient light is determined separately: The Display System is turned off, the ambient light is turned on, and the Luminance produced by scattering of ambient light at the display screen is measured by placing the photometer at a distance from the display screen so that its acceptance angle includes a major portion of the screen and that the measurement is not affected by direct illumination from areas outside the display screen. The Luminance related to ambient light is added to the previously measured Luminance levels produced by the Display System to determine the effective Characteristic Curve of the system.

Note: Changes in ambient lighting conditions may require recalibration of the display subsystem in order to maintain conformance to this standard.

In the following, an example for measurements and transformation of a Display Function is presented. The Display System for this example is a CRT monitor with display controller. It is assumed that the display controller allows a transformation of the DDLs with 8-bit input precision and 10-bit output precision.

The Luminance is measured with a photometer with a narrow (1o) acceptance angle. The ambient light level was adjusted as low as possible. No localized highlights were visible.

1) The maximum Luminance was measured when setting the DDL for the measurement field to the value that yielded the highest Luminance and the DDL of the surrounding to the middle DDL range. From this measurement, the Luminance - 20% of the maximum Luminance - for the surrounding of the measurement field was calculated.

2) The ambient light was turned off. With the photometer centered on the measurement field of the test pattern of Fig. D.1-1, the Luminance was measured when varying the input level Dm in increments of 1 from 0 to 255. The transformation operator of the hypothetical display controller linearly mapped 8 bits on the input to 10 bits on the output. The measured data represent the Characteristic Curve L = F(Dm) for the given operating conditions and this test pattern.

3) Next, the CRT was turned off and the ambient light turned on. The photometer was placed on the center axis of the CRT sufficiently far away so that it did not cast a shadow on the CRT face and its aperture intercepted light scattered from a major portion of the CRT face. The measured Luminance of 0.3 cd/m2 produced by the ambient light on the CRT face was added to the measured Luminance values of the Characteristic Curve without ambient light. The result is listed in Table D.1-1 and plotted in Fig. D.1-2.

[pic]

Figure D.1-2. Measured Characteristic Curve with Ambient Light of an emissive Display System

Table D.1-1 Measured Characteristic Curve plus Ambient Light

DDL Luminance DDL Luminance DDL Luminance DDL Luminance
0 0.305 1 0.305 2 0.305 3 0.305
4 0.305 5 0.305 6 0.305 7 0.305
8 0.305 9 0.305 10 0.305 11 0.307
12 0.307 13 0.307 14 0.307 15 0.307
16 0.307 17 0.307 18 0.307 19 0.307
20 0.307 21 0.307 22 0.310 23 0.310
24 0.310 25 0.310 26 0.310 27 0.320
28 0.320 29 0.320 30 0.330 31 0.330
32 0.340 33 0.350 34 0.360 35 0.370
36 0.380 37 0.392 38 0.410 39 0.424
40 0.442 41 0.464 42 0.486 43 0.512
44 0.534 45 0.562 46 0.594 47 0.626
48 0.674 49 0.710 50 0.750 51 0.796
52 0.842 53 0.888 54 0.938 55 0.994
56 1.048 57 1.108 58 1.168 59 1.232
60 1.294 61 1.366 62 1.438 63 1.512
64 1.620 65 1.702 66 1.788 67 1.876
68 1.960 69 2.056 70 2.154 71 2.248
72 2.350 73 2.456 74 2.564 75 2.670
76 2.790 77 2.908 78 3.022 79 3.146
80 3.328 81 3.460 82 3.584 83 3.732
84 3.870 85 4.006 86 4.156 87 4.310
88 4.456 89 4.608 90 4.766 91 4.944
92 5.104 93 5.268 94 5.444 95 5.630
96 5.864 97 6.050 98 6.238 99 6.438
100 6.610 101 6.820 102 7.024 103 7.224
104 7.428 105 7.644 106 7.872 107 8.066
108 8.298 109 8.528 110 8.752 111 8.982
112 9.330 113 9.574 114 9.796 115 10.060
116 10.314 117 10.560 118 10.820 119 11.080
120 11.340 121 11.620 122 11.880 123 12.180
124 12.460 125 12.700 126 13.020 127 13.300
128 13.720 129 14.020 130 14.360 131 14.640
132 14.940 133 15.300 134 15.600 135 15.900
136 16.240 137 16.560 138 16.920 139 17.220
140 17.600 141 17.940 142 18.240 143 18.640
144 19.120 145 19.460 146 19.800 147 20.260
148 20.560 149 20.920 150 21.360 151 21.760
152 22.060 153 22.520 154 22.960 155 23.300
156 23.700 157 24.080 158 24.600 159 24.980
160 25.520 161 26.040 162 26.480 163 26.700
164 27.380 165 27.620 166 28.040 167 28.580
168 28.980 169 29.400 170 29.840 171 30.540
172 30.800 173 31.380 174 31.880 175 32.400
176 33.060 177 33.400 178 34.040 179 34.400
180 34.840 181 35.360 182 35.900 183 36.400
184 37.060 185 37.400 186 38.300 187 38.420
188 39.160 189 39.760 190 39.980 191 40.840
192 41.540 193 41.900 194 42.800 195 43.060
196 43.620 197 44.520 198 44.620 199 45.500
200 46.100 201 46.380 202 47.400 203 47.600
204 48.320 205 49.060 206 49.380 207 50.320
208 50.920 209 51.600 210 52.420 211 52.680
212 53.520 213 54.220 214 54.620 215 55.420
216 56.100 217 56.600 218 57.400 219 57.820
220 58.660 221 59.320 222 59.800 223 60.720
224 61.520 225 62.240 226 63.040 227 63.480
228 64.460 229 65.020 230 65.500 231 66.500
232 66.960 233 67.840 234 68.600 235 68.980
236 70.040 237 70.520 238 71.420 239 72.180
240 72.900 241 73.980 242 74.580 243 75.320
244 76.200 245 76.540 246 77.720 247 78.220
248 79.200 249 79.880 250 80.420 251 81.560
252 81.960 253 83.140 254 83.720 255 84.340

D.1.2 Application of the Standard Formula

The section of the Grayscale Standard Display Function for the Luminance Range of the CRT monitor Display System is shown in Figure D.1-3. Minimum and maximum Luminance levels correspond to JND indices of JNDmin = 32.54 and JNDmax = 453.85, respectively. Thus, there are theoretically about 420 just-noticeable Luminance differences for the Standard Target (see Normative Section 6). Obviously, with 8-bit input digitization resolution, at best 256 noticeable Luminance increments can be realized.

D.1.3 Implementation of the Standard

The measured Characteristic Curve is interpolated for the available output levels Doutput , in this case, yielding 1024 Luminance levels LI,m . The Grayscale Standard Display Function is also interpolated between JNDmin and JNDmax ( (JND = [ JNDmax - JNDmin ]/1023 = [453.85 - 32.54]/1023) yielding 1024 Standard Luminance levels LI,STD . Interpolations can be performed by a variety of techniques. Here, a cubic spline technique was employed.

For every LI ,STD , the closest L J,m is determined. The data pair I , J defines the transformation between D input and Do utput (Table D.1-2) by which the Luminance response of the Display System is made to approximate the Grayscale Standard Display Function.

Table D.1-2Look-Up Table for Calibrating Display System

Input Output Input Output Input Output Input Output
0 0 1 118 2 131 3 140
4 148 5 153 6 160 7 164
8 169 9 173 10 178 11 182
12 185 13 189 14 191 15 194
16 198 17 201 18 204 19 207
20 210 21 214 22 217 23 219
24 222 25 225 26 228 27 231
28 234 29 237 30 240 31 243
32 245 33 248 34 251 35 253
36 255 37 257 38 260 39 263
40 265 41 268 42 271 43 274
44 276 45 279 46 282 47 284
48 287 49 290 50 292 51 295
52 298 53 301 54 303 55 306
56 308 57 311 58 314 59 317
60 319 61 320 62 323 63 326
64 329 65 331 66 334 67 336
68 339 69 342 70 345 71 347
72 350 73 353 74 356 75 359
76 361 77 364 78 367 79 370
80 372 81 375 82 378 83 381
84 383 85 385 86 388 87 391
88 393 89 396 90 399 91 402
92 405 93 407 94 410 95 413
96 416 97 419 98 422 99 425
100 428 101 431 102 434 103 437
104 440 105 443 106 445 107 448
108 450 109 452 110 456 111 459
112 462 113 465 114 468 115 471
116 474 117 477 118 480 119 483
120 486 121 490 122 492 123 495
124 499 125 502 126 505 127 509
128 511 129 513 130 516 131 519
132 522 133 526 134 529 135 532
136 535 137 539 138 542 139 545
140 549 141 552 142 555 143 559
144 562 145 565 146 569 147 572
148 575 149 578 150 581 151 585
152 588 153 591 154 595 155 599
156 602 157 605 158 609 159 613
160 616 161 619 162 623 163 627
164 631 165 633 166 637 167 640
168 643 169 646 170 650 171 655
172 657 173 663 174 666 175 669
176 674 177 678 178 682 179 684
180 688 181 693 182 696 183 700
184 703 185 706 186 711 187 714
188 719 189 723 190 727 191 731
192 735 193 738 194 743 195 745
196 752 197 754 198 758 199 764
200 766 201 769 202 775 203 777
204 783 205 787 206 789 207 796
208 799 209 805 210 808 211 811
212 818 213 821 214 827 215 830
216 834 217 838 218 841 219 848
220 851 221 856 222 861 223 864
224 870 225 874 226 880 227 883
228 889 229 893 230 897 231 901
232 905 233 911 234 915 235 922
236 925 237 931 238 935 239 941
240 945 241 951 242 955 243 960
244 964 245 969 246 975 247 979
248 985 249 991 250 995 251 1002
252 1006 253 1012 254 1016 255 1023

D.1.4 Measures of Conformance

The FIT and the LUM metrics proposed in Annex C are applied to determine the macroscopic and microscopic approximation of the L J,m to the L I,STD . Figure D.1-3 shows the perceptually linearized Display Function superimposed on the Grayscale Standard Display Function and Figure D.1-4 summarizes the results of the two metrics. A good global fit was achieved as demonstrated by the nearly horizontal-line fit as best fit obtained with the FIT metric. The RMSE is acceptable. All 255 P-Value intervals lead to JNDs on the transformed Display Function for the Standard Target.

[pic]

Figure D.1-3. Measured and interpolated Characteristic Curve, Grayscale Standard Display Function and transformed Display Function of an emissive Display System. The transformed Display Function for this Display System matches the Grayscale Standard Display Function and the two curves are superimposed and indistinguishable.

[pic]

Figure D.1-4. LUM and FIT measures of conformance for a the transformed Display Function of an emissive Display System