DICOM PS3.14 2018c - Grayscale Standard Display Function

D Illustrations for Achieving Conformance with the Grayscale Standard Display Function (Informative)

The following sections illustrate how conformance with the Grayscale Standard Display Function may be achieved for emissive (soft-copy) Display Systems as well as systems producing image presentations (hard-copies) on transmissive and reflective media. Each section contains four sub-sections on 1) a procedure for measuring the system Characteristic Curve, 2) the application of the Grayscale Standard Display Function to the Luminance Range of the Display System, 3) the implementation of the Grayscale Standard Display Function, and 4) the application of the conformance metrics as proposed in Annex C.

It is emphasized that there are different ways to configure a Display System or to change its performance so that it conforms to the Grayscale Standard Display Function. In fact, conceivably, a Display System may calibrate itself automatically to maintain conformance with the Standard. Hence, the following three illustrations are truly only examples.

Luminance of any Display System, hard-copy or softcopy, may be measured with a photometer. The photometer should have the following characteristics:

Note

The photometer may be of the type that attaches directly to the display face (with a suction cup) or of the type that is held away from the display face. If of the latter type, the photometer should be well baffled to exclude extraneous light sources, including light from the background area of the test pattern.

For a film Display System the photometer may be appropriately used to measure the background Illuminance and the Luminance of the light-box on which the film will be displayed. The Luminance characteristics of the film Display System may be measured directly with the photometer or indirectly using measured optical density of the film and the values for the measured background Illuminance and the light-box Luminance.

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 (Figure 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.

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

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


Note

  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.

Note

  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 (1°) 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 Figure 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/m2produced 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 Figure D.1-2.

Measured Characteristic Curve with Ambient Light of an emissive Display System

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 LJ,m is determined. The data pair I,J defines the transformation between Dinput and Doutput (Table D.1-2) by which the Luminance response of the Display System is made to approximate the Grayscale Standard Display Function.

Table D.1-2. Look-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,mto 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.

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.

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.


LUM and FIT measures of conformance for a the transformed Display Function of an emissive Display System

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


DICOM PS3.14 2018c - Grayscale Standard Display Function