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