Method for characterizing an LCD projection display

Method for characterising an LCD projection display Youngshin Kwak∗, Lindsay W. MacDonald** Colour & Imaging Institute, University of Derby, Derby, UK ABSTRACT In this study, the characterisation method for a typical desktop LCD colour projector is reviewed. Measurements were made with a spectroradiometer to establish the additivity of the primaries, inter-channel dependence, colour gamut, tone scale, contrast, spatial non-uniformity, temporal stability and viewing angle variation. In the case of tone characterisation, LCD projectors show S-shaped curve between input digital values and output luminance unlike the conventional CRT monitor represented by a power function. Mathematical models to predict the S-shaped electro-optical transfer function have been empirically derived. Four mathematical models including PLCC, GOG, S-Curve Model I and II were compared for their accuracy in predicting the colours generated by the display for arbitrary signal inputs. It is proven that the newly derived SCurve Model I and II work successfully for an LCD projector. Keywords: LCD display, projection display, characterisation, electro-optical transfer function, mathematical modelling.

1. INTRODUCTION For colour management in multimedia systems, understanding the colour performance of imaging devices is very important. To get an objective and consistent result, standardised methods of measurement and characterisation are required. However there is no standard characterisation method for projection displays. Although a proposal was made by the International Electrotechnical Commission (IEC) Project Team 61966, little progress has been made since first working draft1 published in 1998, supposed to cover either front or rear projection system and also CRT, LCD and DMD based projectors. Even though the first working draft by IEC needs more research and revision, it is still worthwhile to follow its method as a guideline for characterising the projection displays. In this study both calibration and characterisation for an LCD projector were performed. Some of the assessment methods proposed in this study were based on the IEC draft, although various details were changed. Especially for tone characterisation, new empirically derived mathematical characterisation methods were introduced, which define the relationship between the input digital values and output colours displayed on the screen. Performance of these new models was compared with traditional CRT characterisation techniques – GOG model and LUT model.

2. MEASUREMENT PROCEDURE The characterisation techniques for the LCD projector proposed by IEC project team 61966 as first working draft were reviewed and revised. The working examples were tested with experimental data using LCD projector. The LCD projector used in this experiment was Sanyo PLC-5605B. 2.1 Conditions 2.1.1 Environment conditions In this study every measurement was done in a dark room and one hour warm up time preceded any measurement as recommended. 2.1.2 Conditions of measurement a) Contrast, Brightness and other additional adjustments are recommended to use pre-set positions. However in this study optimum setting of Contrast and Brightness, which shows maximum dynamic range without clipping and has most linear relation between input digital value and output luminance, was found by comparing the several combinations of

∗

[email protected]; ∗∗ [email protected]; phone 44 1332-622217; fax 44 1332-622218; http://colour.derby.ac.uk; Colour & Imaging Institute, University of Derby, Kingway House East, Kingsway, Derby DE22 3HL United Kingdom

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Projection Displays VII, Ming H. Wu, Editor, Proceedings of SPIE Vol. 4294 (2001) © 2001 SPIE · 0277-786X/01/$15.00

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Contrast and Brightness settings. In this case, the optimum setting was the pre-set setting of the LCD projector. To adjust the output luminance and screen size to be typical viewing condition, zoom control was set to the middle position. b) For measurement of projection colours, non-contact measurement equipment, like spectroradiometer PhotoResearch PR-650 used in this experiment, is required. Experimental settings, especially distance between the projector and screen, is very critical for projection systems affecting the output luminance, display uniformity and other characteristics including display size. In the IEC draft, typical viewing position as defined by the manufacturer of the display equipment under test is recommended. However it is unknown in many cases by customers. In this study the distance between screen and the LCD projector was about 260 cm, making 88cm picture height. The spectroradiometer was put at a distance of about 3 picture heights from the screen, followimg the ANSI/SMPTE 196M-1986 2 recommendation. 120 cm Image

Spectroradiometer

~300 cm

~ 88 cm ~16.4° ~260 cm ~2.5°

~17.6 cm

LCD projector Screen

Figure 1: Experimental Geometry

c)

A colour patch of the size of h/5xh/5 (~17.6x17.6 cm, h: image height) was displayed in the centre of the screen and background was black. Test images were made using Microsoft PowerPoint. d) The screen is not a part of the projector itself but plays an important role as a part of projection system, affecting output colour performances of the projector. However no standard screen or method to describe the state of the screen is recommended by IEC draft. In this experiment a wooden panel painted with Dulux White paint was used to make matt screen which has minimal angular dependency (see 2.4.5). 2.2 General characterisation measurements 2.2.1 Spectral and basic colorimetric characteristics Measurements of spectral radiance distributions and corresponding absolute and relative tristimulus values for peak three primary colours, red, green, blue and white colours are recommended by IEC draft. Also the correlated colour temperature has to be reported. In this study only absolute tristimulus values were measured. DR DG DB Black

0

0

0

YL (cd/m2) 0.38 0.47 XL

White 255 255 255 114.6 Red

ZL

x

y

0.55 0.2706 0.3372

137.5

134.1 0.2967 0.3560

255

0

0

33.45

18.10

0.66 0.6407 0.3467

Green

0

255

0

57.47

112.0

5.47 0.3285 0.6402

Blue

0

0

255 23.99

8.15

130.1 0.1479 0.0502

Correlated Colour Temperature for White : 7073 K

0.008

Green

0.006 0.004

Blue

0.002

Red

0.000 380

480

580

680

780

Wavelength (nm)

Table 1: Chromaticities of the primary and white colours

2.2.2 Inter-channel dependency Inter-channel relationship between input digital value and output tristimulus values is defined as 3x8 matrix T by measuring 32 colours. First column of T matrix represents 0th order term, possibly the noise term of each channel, and next three columns represent 1st order terms. 5,6 and 7th columns are 2nd order terms coming from the combination of two channels, and last column is 3rd order term from the combination of all three channels.

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 1     R   G     R  X '      B  = ⋅ = ⋅ Y S G S T '      RG  ,  B  Z'        GB     BR   RGB   

 0.241 0.417 0.172    S =  0.129 0.814 0.056   0.001 0.036 0.945     − 0.0023 1.0033 − 0.0011 0.0032 0.0004 − 0.0019 − 0.0043 0.0030    T =  − 0.0008 0.0008 1.0011 0.0005 − 0.0012 − 0.0007 − 0.0006 0.0009   0.0000 0.0002 0.0002 1.0000 − 0.0021 − 0.0002 − 0.0002 0.0023   

In the above equation, R, G, B means normalised monitor luminance levels computed using the spectral radiance of the red, green, and blue channels at maximum excitation as primaries. Another easy and simple way of testing inter-channel dependency is to check the additivity of three channels, which is not mentioned by the IEC draft. Y (cd/m2) 137.50 138.25 0.54%

X 114.60 114.91 0.27%

White Red+Green+Blue Difference (%)

Z 134.10 136.23 1.59%

Table 2: Additivity test result

2.2.3 Colour gamut Describing colour gamut is not recommended by the IEC draft. However reproducible colour range of the specific projector is important information for image processing application especially when reproducing colours outside the display colour gamut. Figure 2 shows the colour gamut of the Sanyo LCD projector in CIELAB space. 150

100

100

80

Yellow

Green

Green Red

50 Cyan -150

60

0

-50

50

-50

40

150

Red

-100

Blue

-150

Blue

20

Magenta

0 0

a*

50

100

150

200

Chroma (C*)

Figure 2: Colour gamut boundary in CIELAB space

2.3

LCD-specific measurements

2.3.1 Tone characteristics There is no standard mathematical method to represent the tone characteristics of an LCD projector like GOG model for CRT monitor. In the IEC draft, it is recommended to measure 32 steps per channel to make one-dimensional LUT of each channel and report as logarithmic plot. However using logarithmic scale has no meaning for the tone characteristic of the LCD projector because it does not necessarily follow a power function. Therefore a normal scale plot was used instead of logarithmic scale in this study. Figure 3 shows the measured transfer functions for the three channels of the Sanyo projector. Normalised Luminance

1 0.8 0.6

Red Green

0.4

Blue

0.2 0 0

50

100

150

200

250

Digital Count

Figure 3: Electro-optical transfer functions of three channels

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2.3.2 Colour tracking characteristics Colour tracking characteristic means the locus of chromaticity changes of primary colours and achromatic colours corresponding to the input digital values of each channel. Chromaticities of 8 steps per channel were measured and reported on the CIE 1976 UCS diagram, (u’,v’). Left graph in Figure 4 shows chromaticity changes of each channel, approaching that of black as the input level approached zero because the chromaticity of black arises from the leaked light through LC cells and this leaked light is always added to any colour. Therefore the chromaticities could be corrected by subtracting the black values. (Figure 4, right). Even after black corrections, chromaticities of blue and green channels were significantly changed compared to red channel. 0.7

0.7

0.6

0.6

0.5

0.5

Grey 0.4

Red Green

0.3

0.2

Blue

0.2

Blue

0.1

0.0

Red

Grey

0.4

0.3

0.1

Green

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.0

0.1

0.2

u'

0.3

0.4

0.5

u'

Figure 4: Changes of chromaticities of the primaries before (left) and after (right) black correction

The reason for chromaticity changes of primary colours is not clear without information about transmittance characteristics of each optical element inside LCD projector. However it is considered to be an intrinsic characteristic of LC cell5, which shows different spectral transmission curve according to switching level. This characteristic was checked indirectly by comparing tone characteristic against wavelength. Figure 5 shows that relative luminance changes for input grey level changes were not the same for every wavelength inducing chromaticity changes. Also Figure 5 shows the performance of short wavelength for blue channel are quite different from others explaining large chromaticity change of blue channel. 1.0

420 nm

0.8

460 nm

0.6

500 nm

0.4

540 nm 580 nm

0.2

620 nm 0.0 0

100 200 Input Grey Level

Figure 5: Tone characteristic changes by wavelength

2.4 Assessment measurements 2.4.1 Contrast Measurement of contrast was tried in this study even though it is not recommended by the IEC draft. The checkerboard method recommended by ANSI 3 was used. With this method, a 4x4 checkerboard pattern was generated consisting of black and white rectangles that cover the entire image area, as illustrated in Figure 6. The luminance at the centre of each rectangle was measured and the eight white values were averaged together, as were the eight black values. Black to white ratio was 1:91.61. 1

2

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5

6

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11

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25

h

h/5 w/5

w

Figure 6: ANSI checkerboard pattern for contrast measurement (left) and Measurement points for spatial non-uniformity (right)



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2.4.2 Spatial non-uniformity To check the spatial non-uniformity of lightness and chromaticity co-ordinates over the entire projected image, white colour was displayed and 25 equally spaced points were measured. (Figure 6, right) From this test, it was found that luminances and chromaticities of 25 points were not symmetrically distributed from the central position implying an internal defect of the LCD projector used in this experiment, as shown in Figure 7. CIELAB values were calculated using the values of point 13 as a reference white. Region 2 1

12

2

3

6

7

8

11

12

13

Region 1

16 21

17 22

4

5

8

Region 3 9

10

4

14

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0 -6

18 23

19 24

-4

-2

Region 1 Region 2 0

20

-4

25

-8

2

4

6

Region 3

-12

a*

Figure 7: Variation in Image Chromaticity

2.4.3 Dependency on background/internal flare (spatial independence) Dependency of background is defined as the degree of colorimetric change at the centre of the screen, depending on brightness of the background in the IEC draft. For this test, calculating colour difference of white colours for white and back background is recommended. However chromaticity of central colour patch is affected by not only brightness but also colourfulness of the background. Also white is the colour least affected by background colours. Therefore the method proposed by Fairchild and Wyble was employed.4 A set of colour stimuli was defined, including black (0,0,0), grey (128,128,128), white (255,255,255), red1 (128,0,0), red2 (255,0,0), green1 (0,128,0), green2 (0,255,0) and blue1 (0,0,128), blue2 (0,0,255). Each of the nine colour stimuli was measured on nine different backgrounds made up of the same set of colours. Overall the average CIELAB colour difference was 6.10 indicating that there is a large spatial dependence for the LCD projector. Colour differences were larger when the background colour had high chroma (Green2, Blue2) and high lightness. 2.4.4 Temporal stability Short-term and mid-term stabilities are recommended for test. For short-term stability, a white screen has to be measured every 20 minutes for duration of two hours, which might be a print error in the draft, and every 10 minutes for duration of 24 hours for mid-term stability. However only short-term stability was tested and the method of Fairchild and Wyble4 was used. A full white (255,255,255) and a medium grey (128,128,128) were alternately displayed and measured every two minutes. These measurements began from a cold start (initial power-up of the LCD projector) and continued for about 40 minutes. It was found that tristimulus values for white and grey colours showed good stability from the beginning, with steady state reached after about 10 minutes. 2.4.5 Viewing angle characteristics The dependence of luminance on horizontal viewing angle was evaluated for 11 test colour patches – 7 grey levels, peak white, peak red, peak green and peak blue colour. The luminance of each colour was measured successively over a specified range of horizontal viewing angles (in 10 degree increments) from the normal viewing direction to ±40 degrees. The result showed that the painted white matt screen used in this experiment has minimal angular dependence except for a slightly higher reflectance at the normal direction and no colour dependent angular characteristics. The luminance of white at 40 degrees from normal direction was reduced by about 8% relative to the normal direction. 2.5 Suggested improvements to the proposed standard Based on our experience, the following suggestions for improvement to the proposed standard can be made: a) The relative geometry of the projector and screen (distance between projector and screen, screen size, setting of zoom control, etc.) is critical. To get a meaningful and repeatable characterisation result, a clearer recommendation is needed. b) Viewing angle characteristics are dependent on both projector and screen. The characteristics of the screen have to be specified separately from those of the projector.

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c)

It is not necessary to report the measurement results as tristimulus values normalised to the luminance of peak white, a technique adopted from reflectance colorimetry. The absolute values reported as Yxy would be acceptable, where Y is the luminance in cd/m2. d) Additivity should be checked, by comparing the measured luminance of white with the sum of the luminances of the individual red, green and blue channels. e) Determination of contrast and colour gamut should be included. f) To report the tone characteristics of an LCD-based projector, the standard recommends that three one-dimensional tables should be constructed and reported as logarithmic plots. However the LCD transfer function is not a power function but an S-shaped curve, and so logarithmic plots do not properly represent the tone characteristics, especially at the high-signal end where the curve is very compressed. g) Flare is a significant factor affecting the performance of a projection display. Stray light from both internal scattering in the projection optics and the surrounding environment affects the dark colours and limits the dynamic range of the projected image. In particular, the luminance and chromaticity of a test colour patch at the centre of the screen is affected by both luminance and chromaticity of the surrounding region. More attention should be paid to the influence of surround.

3. MATHEMATICAL MODEL FOR TONE CHARACTERISATION OF THE LCD PROJECTOR. The mathematical models for tone characterisation of the LCD projector were empirically derived from this experiment result and their performances were compared with PLCC and GOG models. 3.1 S-Curve Model I To characterise the LCD projector more effectively, a new mathematical model to predict s-shaped tone characteristics was proposed, which will be called S-Curve model I in this report. The S-Curve model has same two-stage structure as the GOG model6,7 but uses a different function for the non-linear relationship between DAC signal values and monitor RGB luminance levels, i.e. the electro-optic transfer function. The proposed hyperbolic function is a mathematical construction, suggested by analogy with Hunt’s use of a similar function for retinal cone responses8, except that a second exponent has been included to allow for different curvature at the black and white ends. < Non-linear Relationship between DAC Values and Monitor Luminance Values >

R = Ar

dr

dr

βr

αr

+ Cr

,

G = Ag

dg dg

βg

αg

+ Cg

,

B = Ab

db

db

βb

αb

+ Cb

- dr, dg, db : normalised input digital values for red, green and blue channels - R, G, B : normalised monitor luminance levels computed using the spectral radiance of the red, green, and blue channels at maximum excitation as primaries. - A, α, β, C : constants determined by measurement data < Linear Transformation Matrix >

 X pixel   X   X r , max X         + Y  +  Yr ,max  Y pixel  =  Y   Z pixel   Z  ambient  Z  inter −reflection  Z r ,max    flare flare

X g ,max Yg ,max Z g ,max

X b, max   R   Yb ,max  G  Z b,max   B 

Table 3 shows the model parameters calculated for the LCD projector used in this experiment.

A α β C

Using 32x3 colours Red Green Blue 3.54 2.37 2.09 3.29 3.20 3.15 11.77 6.94 7.85 2.55 1.39 1.12

Using 8x3 colours Red Green Blue 3.39 2.55 2.20 3.31 3.16 3.12 10.78 7.17 7.96 2.39 1.55 1.20

Using 8 grey colours Red Green Blue 3.85 2.62 2.15 3.30 3.16 3.09 10.37 7.49 7.87 2.77 1.63 1.17

Table 3: Coefficients for S-Curve Model I



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3.2 S-Curve Model II In the S-Curve Model I, it was assumed that the normalised monitor luminance levels are independent each other. Therefore R, G and B were functions only of dr, dg and db respectively. However the measurement data showed that the R, G and B are not perfectly independent each other. This means, for example, that the input signal to the blue channel affects not only B values but also R and G luminance values. This effect is represented in Figure 8, which shows negligible change for the red channel but large changes for the green and blue channels, and therefore the relatively poor performance of S-Curve Model I for blue and green channels. 0.03 0.02 R by Dg

0.01

R by Db 0.00 -0.01

G by Dr 0

100

200

G by Db B by Dr

-0.02

B by Dg

-0.03 -0.04

Digital Count

Figure 8: Normalised monitor luminance level generated by input signal from the other two channels. R by Dg means R value generated by green channel input signal.

The normalised monitor luminance level driven by another channel is small when it is considered that the vertical axis of Figure 8 has a scale from 0 to 1. However to predict the colour tracking characteristics accurately and to improve the tone characterisation performances for green and blue channels, this component must be included in the characterisation model. It is observed from Figure 8 that all curves have a similar form, which appears to follow the gradient of the S-Curve function. The function for the non-linear relationship between DAC values and monitor luminance levels was therefore extended by including in each term a component based on the first derivative of the other two channels. < Non-linear Relationship between DAC Values and Monitor Luminance Values >

R = Arr ⋅ f R (d r ) + Arg ⋅ f G ' (d g ) + Arb ⋅ f B ' (d b ) G = Agr ⋅ f R ' ( d r ) + Agg ⋅ f G (d g ) + Agb ⋅ f B ' ( d b ) B = Abr ⋅ f R ' ( d r ) + Abg ⋅ f G ' ( d g ) + Abb ⋅ f B ( d b ) xα (α − β ) x α + β −1 + α ⋅ C ⋅ x α −1 = , f ' ( x ) xβ + C (x β + C)2 f ' ( x) : first - order derivative of f ( x ) f ( x) =

- dr, dg, db : normalised input digital values for red, green and blue channels. - R, G, B : normalised monitor luminance levels computed using the spectral radiance of the red, green, and blue channels at maximum excitation as primaries. Note that Agr and Abr were set to 0 because of their negligible contribution.

Anr Ang Anb αn βn Cn

Red (n=R) 3.539 0 0 3.292 11.770 2.552

Using 32x3 colours Green(n=G) Blue(n=B) -0.033 0.016 -0.007 2.365 0.002 2.092 3.201 3.147 6.939 7.851 1.389 1.119

Red (n=R) 3.394 0 0 3.308 10.783 2.394

Using 8x3 colours Green(n=G) -0.030 2.550 0.002 3.157 7.166 1.551

Blue(n=B) 0.016 -0.007 2.203 3.118 7.956 1.204

Table 4: Coefficients for S-Curve Model II

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Using this S-Curve Model II, the colour tracking chromaticities were predicted. The result in Figure 9 shows a good match between predicted and measured values. 0.6 0.5

Red-model Green-model

0.4

Blue-model Red-measure

0.3

Green-measure 0.2

Blue-measure

0.1 0.0

0.1

0.2

0.3

0.4

0.5

u'

Figure 9: Comparison of the Colour Tracking Characteristics between measured and predicted by S-Curve Model II

3.3 Comparison of the model performances The accuracies of these four methods – PLCC, GOG model and S-Curve Model I, II – were compared using 94 test colours. The measured data were compared with the values predicted by each model in terms of CIELAB values. Table 5 summarises the results.

AVG STDEV Max

PLCC (32x3) 1.29 1.01 5.70

GOG (32x3) 9.11 6.99 23.78

GOG (8x3) 10.84 9.80 33.92

∆E*ab S-Curve I S-Curve I (32x3) (8x3) 2.18 2.21 1.64 1.60 7.32 7.53

S-Curve I (Grey 8) 1.90 1.49 7.02

S-Curve II S-Curve II (32x3) (8x3) 1.55 1.41 0.95 0.89 4.39 4.65

Table 5 Model Performance Comparison using 94 Test Colours

Simple one-dimensional look-up-table showed the best result followed by S-Curve Model II and S-Curve Model I using 8 grey colours and GOG model showed worst result. By using the S-Curve Model I, a reasonably good characterisation result was achieved by measuring the CIE tristimulus values of only eight grey levels.

4. CONCLUSION As described in the introduction, device characterisation is very important in cross-media reproduction. Many mathematical models and techniques for characterisation of colour-imaging devices have been under development. However characterisation of LCD projectors is not well understood. One of the difficulties for characterising LCD projectors is the problem of the screen, which is not a part of the LCD projector but an essential part of the whole projection system. Therefore, the colour appearing on the screen is determined not only by projector but also by the screen. However separating the effects of LCD projector and screen is not an easy task. In this report, the characterisation included both the LCD projector and the screen. To characterise the LCD projector itself, excluding the effect of the screen, a different technique would be needed. The calibration and characterisation for an LCD projector, Sanyo PLC-5605B, were performed. The LCD projector showed imperfect constancy of channel chromaticity, causing slight chromaticity changes in the blue and green channels by input signal levels. The ANSI contrast ratio was 1:91.61. The spatial uniformity test showed that not only luminance but also hue and chroma changed according to the position in the image. The darkest part of a white screen had only 78.1% luminance of the brightest point. Also in the case of projected image, the colour in the centre was seriously affected by the background colour. However this spatial dependence could be minimised when achromatic and low luminance colours were used as background. The warm-up characteristic of the projector was stable without abrupt changes over time. The traditional tone characterisation methods for a CRT monitor – PLCC and GOG models – were applied to characterise the LCD projector. Also two variants of a new method, S-Curve Model I and II, were tried. The results showed that the PLCC and S-Curve Model II performed better than other methods and the GOG model performed worst.



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S-Curve Model I and II were empirically derived from the measurement data. However the function used in the S-Curve model (below) has a very generalised form. When β is equal to 0, f(x) becomes a power function, which could be used to characterise a CRT based monitor.

xα f ( x) = A β x +C Even though the performance of the two S-Curve models was slightly worse than PLCC, S-Curve model I needed the data of only 8 grey colours instead of the 32x3 colours that have to be measured for the PLCC model. Further investigation will be necessary to establish the theoretical basis of the S-curve function and also the first derivative terms in S-Curve Model II. The derivative terms in S-Curve Model II mean that it could be controversial to recommend this as a standard method for characterising a conventional LCD projector, even though the model performed very well in this study. Further studies with other LCD projectors are in progress to reveal how effective the model is in general.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

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IEC 61966-6, “Colour measurement and management in multimedia systems and equipment – Part 6: Equipment used for digital image projection”, Committee Draft, 1998. ANSI/SMPTE 196M-1986, “Screen Luminance and Viewing Conditions – Indoor Theater Projection” ANSI IT7.215-1992, “Data Projection Equipment and Large Screen Displays – Test Methods and Performance Characteristics”. Fairchild, M. D., Wyble D. R., “Colorimetric characterization of the Apple Studio Display (Flat Panel LCD)”, Munsell Color Science Laboratory Technical Report, 1998. Silverstein, L. D., Fiske, T. G. “Colorimetric and Photometric Modelling of Liquid Crystal Displays”, IS&T and SID’s Color Imaging Conference: Transforms & Transportability of Color, pp.149-156, 1993. Berns, R.S, Motta, R J, Gorzynski, M E. “CRT colorimetry, part I: theory and practice” Col. Res. Appl., 18, pp.299-314, 1993. Berns, R.S, “Methods for characterizing CRT displays”, Displays, 16, pp.173-182,1995 Hunt, R. W. G., Measuring Colour, 3rd Edition, p.212, Fountain Press, 1998.

