Experimental Report

Visual Characteristics of Liquid Crystal Displays

Kanchit Rongchai

Cambridge Research Systems Ltd.
Summary This report sets out the investigation into visual characteristics of Liquid Crystal Displays (LCD) in the form of six experimental reports including warm-up behaviour, screen spatial non-uniformity, colour constancy and spectral analysis, the gamma functions and the primary independence, spatial independence and temporal response. The aim of this project is to measure the characteristics of the LCD that are important for generating different types of visual stimuli and to compare such characteristics with Cathode Ray Tube displays (CRT). It was found that the LCD exhibits similar warm-up characteristics to that of the CRT but with slow convergence. The screen of the LCD is not spatial uniform. Spectra of the same primary colour measured from the LCD and the CRT are different in shape. The chromaticity coordinates and spectra of the LCD are not constant when varying its luminance. Temporal response given by the LCD is much better than the CRT to be used as electrophysiological visual stimuli. There are several factors that affect the quality of temporal response of the LCD including the backlight, level of the driving input signal and the white point of the monitor.

Introduction

For over 20 years the mainstay of the visual science laboratory has been the Cathode Ray Tube display. This ubiquitous device offers a simple solution to the need to create spatially variant images for use as visual stimuli. The characteristics of the display are well understood and, for some of them, are well matched to the primate visual system. However low cost flat panel displays, Liquid Crystal Displays (LCDs) for instance, have gradual displaced CRTs in every day applications to the extent that manufacture of low cost CRTs has ceased. There are a vast number of flat panel alternatives which have been designed to replace CRTs in many different applications. Although designed to replace CRTs these displays have different temporal, spatial, and spectral characteristics to CRTs which are not well characterised for applications in the Vision Science laboratory. The aim of this project is to identify the characteristics of LCDs that are important for generating different types of visual stimuli, place acceptable limits upon them and to measure these characteristics. Typical applications for displays as visual stimulators and factors that would make a display suitable for displaying those stimuli are taken into consideration. Such characteristics include, warm-up behaviour, screen spatial non-uniformity, colour constancy and spectral analysis, the gamma functions and the primary independence, spatial independence and temporal response. A series of experiments were designed and performed on real different LCDs. Electronic devices for example a photocell with an amplifier was created. Special pieces of software were written in Matlab to display different stimuli on the LCDs. The CRS VSG System was used to control the LCDs.

Contents

Experiment 1 Warm-up Characteristics Experiment 2 Screen Spatial Non-uniformity Experiment 3 Colour constancy and Spectral Analysis Experiment 4 Gamma functions and Primaries Independence Experiment 5 Temporal Response Experiment 6 Spatial Independence Appendix A Appendix B Appendix C Appendix D Reference Page 51 54

3 Experiment 1

Warm-up Characteristic in CRTs and investigation into the presence of similar effect in LCDs
When a CRT displays an image, luminance and colour change with time. But after a sufficiently long period of time, the luminance and colour of the image then stay relatively constant giving a desirable condition for visual stimuli. This is believed to be as a result of differential characteristics in the thermal-equilibrium of the highvoltage cathodes and control grid accelerators of each channel in CRT (Metha, Vingrys and Badcock, 1993). This experiment studies this behaviour in 3 different LCDs.

Objectives:

1. To characterise the warm up behaviour of a CRT. 2. To investigate the similar effect in the LCDs. 3. To examine the hot, warm and cold warm up behaviour of the LCDs.

Hypotheses:

1. There is a warm up period in the LCDs. 2. The warm up characteristic is constant from day to day. 3. Switching off the LCD for a period of time after it has been through its warm up period has an effect on its consequent warm up behaviour.

Apparatus:

1. 2. 3. 4. 5. 6. CRT SONY Trinitron LCD NEC MultiSync LCD 1510+ LCD DELL E196FP LCD Philips 170BCS A CRS ColorCal A Minolta CS-200 chromameter

Methods:

Using the ColorCal with the CRT: 1. Execute a Matlab program to display a full gray screen on the CRT which is still switched off. 2. The intensity used is half the maximum output range represented by the colormap index of 128. The reason for using such intensity is that the half maximum intensity (128) would allow the greatest dynamic range of modulation. 3. Place the ColorCal on the centre of the screen and record the luminance then switch on the CRT. Carry on measuring until the measurements become constant. Using the Minolta chromameter with the LCDs: 1. Set up the apparatus with followings conditions: - Viewing distance 100 cm, - In a dark room with minimal surrounding illumination, - Temperature of the room is to be observed during the measurements.
4 Measurements are to be taken automatically with measuring interval of 10 seconds until the measurements become constant. 2. Execute the program to display plain grey screen with Matlab colormap index of 128. 3. Start the Minolta chromameter and switch on the LCD. Stop measuring when the measurements become constant.

Results and Analysis:

1.1 Cold Warm up Characteristic of the CRT The warm up characteristic of the CRT is illustrated in terms of luminance and chromaticity coordinates in figure 1.1 and 1.2 respectively. For the cold conditions, the CRT was left switched off over one night before the experiment. The luminance reaches a peak value in about 5 minutes after the CRT has been on, it then drops constantly and approaches an asymptote. The luminance becomes relatively constant after about 40 minutes of the monitors warm up period. In contrast, the chromaticity coordinates are almost instantly constant.
Figure 1.1 The variation of luminance of the CRT over time
Figure 1.2 The variation of chromaticity coordinates of the CRT over time

5 1.2 Cold Warm-up characteristic of the LCDs The luminance variations of the Philips LCD and the Dell LCD are shown in figure 1.3 and 1.5 respectively. In comparison with the CRT, the luminance of the LCDs varies with a similar pattern to that of the CRT but it converges more slowly. The luminance for Philips LCD becomes constant after about 100 minutes of warm up whereas for Dell LCD it takes as long as about 120 minutes.
Figure 1.3 The variation of luminance of Philips LCD over time
Figure 1.4 The variation of Chromaticity coordinates of Philips LCD over time
6 1.3 The hot, warm and cold warm up characteristic of the LCD The Dell LCD was used to investigate whether its warm up behaviour would change with the duration it was temporarily switched off. For cold warm up, the LCD was off over one night, for warm warm up, it had been on for 2 hours and was then switched off for 40 minutes and for hot warm up, the LCD had been on for 2 hours before it was turned off for 30 seconds. Figure 1.5 shows that even thought the LCD has been through its warm up period, switching it off for 40 minutes has a dramatic effect on the variation of luminance when it is switched on again. The monitor in the warm conditions follows almost the same path as the cold conditions. For the hot conditions, the luminance is almost instantly constant, but its initial value is higher than the final value before it was switched off. It takes about 1 hour for the luminance to approach the same value again. However, the difference is negligible which is only about 2 cd/m2.
Figure 1.5 The variation of luminance over time of Dell LCD in 3 cases; hot, warm and cold warm up conditions.
7 1.4 Day-to-day effect on the warm-up characteristic of the LCD The Dell LCD was used in this experiment and 2 sets of measurements were carried out 2 days apart. There was a difference in luminance of about 15 cd/m2 between the two sets of measurements because the viewing angle used was slightly different. In order to compare the pattern of the two variations, the two sets of luminances were normalised by their own maximum value and they were then scaled up as shown in figure 1.6. It can be seen that the two warm-up curves have almost the same shape. Therefore, the variation in luminance over time of the LCD is constant from day to day.

Figure 1.6 Day-today variation of warm-up characteristics of Dell LCD
8 Conclusions LCDs as well as CRTs exhibit similar warm-up characteristics. The variation is mostly due to change in luminance instead of chromaticity coordinates. It takes longer for luminance to become constant in the LCDs than the CRT. Switching off the LCD for 30 seconds after its luminance has been constant does not reset the monitor to warm up again. However, when the monitor is switched off for a longer period e.g. 40 minutes, it undergoes the warm up period as if it was turned off for one night. The warm-up characteristic of the LCD is constant from day to day.
Note: More information on the conditions of the experiment such as temperature variation during the measurement can be found in the Appendix A.

9 Experiment 2

Screen Spatial Non-uniformity
Objective: to investigate the extent to which luminance and colour are constant
across the screen of 3 different LCDs.

Hypothesis:

The luminance and chromaticity coordinates are not constant across the screen.
7. LCD Philips 8. LCD Dell 9. LCD - Samsung 10. A CRS ColorCAL

Method:

1. Ensure that the display has been on for about 1 hour before taking measurements to allow the warm-up characteristic to be negligible. 2. Measurements should be done within a short period of time for the warm-up characteristic to be negligible. 3. Display a full-screen white image at 50% driving level (Matlab colormap of 128) on the LCDs and using the ColorCal measure the luminance and chromaticity coordinates at different positions on the screen.
Results and observations:
The light emitted by the 3 LCDs is most intense at the centre of the screen and drops off quite significantly at more peripheral locations as can be seen from figure 2.1 to 2.3. The maximum drops of luminance relative to the centre are 27%, 16% and 15% for Philips, Dell and Samsung LCD respectively. The chromaticity coordinates are relatively constant across the screen.
Figure 2.1 Spatial non-uniformity across the screen of Philips LCD
Figure 2.2 Spatial non-uniformity across the screen of Dell LCD
Figure 2.3 Spatial non-uniformity across the screen of Samsung LCD
Conclusion If the LCDs are to be used a visual stimulus, the effect of spatial non-uniformity could be significant if extended images are displayed on the screen. For best results, it is recommended that images should be restricted to small and symmetrical regions around the centre of the screen.

Figure 3.1 Chromaticity coordinates variation with digital driving input of the Dell LCD displaying pure red green and blue images
Figure 3.2 Chromaticity coordinates variation with luminance of the Dell LCD displaying pure red green and blue images
Figure 3.3 Chromaticity coordinates variation with digital driving input of the Dell LCD displaying red green and blue images simultaneously
Figure 3.4 Chromaticity coordinates variation with luminance of the Dell LCD displaying red green and blue images simultaneously
There was an issue raised as to whether the measurement of chromaticity from the Minolta chromameter would be reliable at low luminances. According to the specification, for measuring angle of 1, slow measuring interval, room temperature of 23c and relative humidity of 65% max, the accuracy of (x, y) coordinates at low luminance ranges from cd/m2 is 0.004. However, the conditions of the experiment were 29 c room temperature, measuring angle of 0.2 and AUTO measuring interval, therefore were not exactly the same as the manufacturer specifications but it is believed that the accuracy of (x, y) coordinates should be similar to 0.004.

3.2 Spectral analysis

The same experiment was repeated with the CRS SpectroCal spectroradiometer. The spectra of each primary were obtained as shown in figure 3.5 to 3.7. The units of radiance are watt per steradian per square metre.
Figure 3.5 Spectra of Red primary displayed on Dell LCD
Figure 3.6 Spectra of Green primary displayed on Dell LCD
Figure 3.7 Spectra of Blue primary displayed on Dell LCD
19 Discussion on how to analyse the spectra There was a question as to how the colour consistency can be seen from a number of different spectra. If the colour is constant over the range of digital driving input or luminance, the shape of the spectra should be the same for all the input level. However, it is not appropriate to compare the similarity of the shapes of the spectra directly obtained from the measurements because the radiance of the same primary increases with increasing luminance and at low luminances, the spectra are too small to compare with that of high luminances. It was suggested that comparison could be done with normalised spectra; each spectrum is normalised by its corresponding primary peak value for instance, a red spectrum is normalised by the value of the red peak located in the long wavelength region. However, this could result in exaggeration of noises and magnification of unwanted data.

Figure 3.8 Normalised Spectra of Green primary displayed on Dell LCD
In figure 3.8, the unwanted magnification of noise and exaggeration of minor peaks can be clearly seen. The red and blue peaks at low luminances are dramatically smaller than the green peak at high luminances whereas they become similar at low luminances, by normalising similar values the red and blue peaks are excessively magnified at low luminances but are even more attenuated at high luminances. As a result, comparison can be very difficult. However, normalising can be useful to compare spectra at similar luminances. This method was used in section 3.3. An alternative way of comparing the spectra is to use vertical logarithmic axes. The spectra at high luminance and low luminance become closer to each other. Thus the shapes of the spectra can be compared more easily without having to be modified.
20 Analysis of the spectra It can be seen from the logarithmic graphs that the shape the spectra for all 3 primaries become variant at the digital driving level of 64 and below relative to the spectrum at maximum level (256). This suggests that the colour displayed by the Dell LCD is not constant over the range of luminance. However, the analysis of spectra cannot provide the limit of digital driving level in which the LCD displays a colour difference that cannot be perceived by the human eye. The spectral analysis, however, enables us to conclude that at low luminances the colour displayed by the LCD does change. A further analysis on the spectra could be carried out to reinforce the conclusion and define the boundaries in which the LCD should operate to allow highest colour constancy. This method is to convert the spectra into equivalent (x, y) chromaticity coordinates so that further analysis can be performed.
3.3 Spectra variation across 5 different LCDs
The 5 different LCDs mentioned in the Apparatus section were used to display fullscreen red, green and blue images. The maximum digital driving level (256) was sent to the VSG system before the LCDs displayed the images. The spectra were then measured and a number of graphs are plotted to see the variation across the LCDs. Normalisation was used to manipulate the spectra so that comparison can be done more easily. It is justified to normalise the values because all the LCDs were operating at the maximum digital driving level of 256 so they had similar luminance.
Figure 3.9 Normalised Spectra of red, green and blue primaries displayed on 5 different LCDs
It can be seen from figure 3.9 that the spectra produced by all the 5 different LCDs are very similar. This leads to a conclusion that spectra are constant from one LCD to another.
22 Comparison with the CRT The same experiment was carried out with the CRT mentioned in the apparatus section and the spectra were obtained and are shown in figure 3.10.

Figure 3.10 Normalised Spectra of red, green and blue primaries displayed on 5 different LCDs
The CRT gives a different set of spectra because its operating mechanism is different from the LCDs.

Conclusions

1. The chromaticity coordinates of the LCD stay quite constant at high and medium luminances but vary as the luminance approaches zero. For gray, red, green and blue image displayed by the LCD, the chromaticy coordinates move towards the same point as the luminance decreases to zero. 2. The shape of the spectra of a colour displayed by the LCD remains quite constant at high and medium luminances and varies as the luminance decreases. 3. The spectra obtained from the 5 different LCDs are of similar shape.
Note: More information on the effect of the SpectroCal on the measurement of spectra can be found in the Appendix C.

23 Experiment 4

Gamma functions and Primaries Independence
Objectives: 1. To obtain the function of luminance and digital driving input of the LCD. 2. To investigate whether the gamma functions would hold from day to day. 3. To investigate whether the 3 primaries (red, green and blue) are independent of each other. Hypotheses: 1. The luminance has a non-linear relationship with the digital driving input. 2. The gamma functions remain unchanged from day to day. 3. The primaries are not independent of each other. Therefore, 1. The calculated gamma function of the white image, based on measured gamma functions of red, green and blue primaries, is not the same as the measured gamma function of the white image when all primaries are operating simultaneously. 2. The gamma function of one primary is changed when operating with the other two primaries. Variables declaration: Independent variable: Digital driving input Dependent variable: Luminance (gamma function and primaries independence) Control variable: Viewing distance = 100 cm, constant viewing angle Apparatus: 1. LCD Dell 2. LCD Philips 3. A Minolta CS-200 Chromameter Methods: 1. Write a Matlab program to display a full-screen pure red, pure green, pure blue and white (all 3 primaries together) of which luminance and be varied by the user. The digital driving input ranges from 0 256. 2. Allow the LCD to warm up before taking measurements. 3. Measure the luminance as a function of the digital driving input of each primary separately and all primaries operating together. 4. With 2 primaries fixed at maximum and half maximum operating levels, vary the digital driving input of the other one and measure the luminance.

Results and Observations:
4.1 Gamma functions of the LCD and its day-to-day variation The gamma functions are illustrated in figure 4.1. The functions are non-linear with some degree of distortions on the curves. It can be obviously noticed that there are some saturations at upper level of the digital driving input from 248 to 256. The same experiment was done on a different LCD (Philips) and it was found that the saturations are also present at upper driving levels. The identical experiment was carried out on the next day and it was found that the gamma functions are almost exactly constant as can been seen from figure 4.1 that the two sets of curves more or less exactly coincide.
Figure 4.sets of Gamma functions of Dell LCD measured on 2 separate days
25 4.2 Primaries Independence The gamma functions of each separate primary were measured, so the gamma function of a grey image, when all primaries are working simultaneously, can be predicted by adding the 3 gamma functions together. The reason of doing this is to compare if the calculated function will be the same as the measured one. The luminance of the grey image was then measured as a function of digital driving level. It was found that the experimental values are higher than the predicted ones as illustrated in figure 4.2. This suggests that when 3 primaries work simultaneously, more light is emitted than when each one is operating separately. As a result, the primaries are not independent of each other.
Figure 4.2 Comparison of experimental and calculated luminance as a function of Digital Driving level
To reinforce the conclusion, the gamma function of each primary was measured in 3 different conditions. For instance, for the gamma function of the red primary, the levels of green and blue were fixed at 3 different values including 0%, 50% and 100%. The level of red was then varied by varying the digital driving level and the luminance was then recorded. In order to obtain the corresponding gamma function of the red primary in the 50% and 100% green and blue conditions, the luminances were subtracted by the initial luminance the luminance when red level is 0%. The results are shown in figure 4.3. It can be seen that the gamma function of red primary increases with the operating levels of green and blue primaries and the same concept is applied to green and blue primaries. Therefore, the primaries are not independent of each other.

Figure 4.3 Gamma functions of red, green and blue primaries in the 3 different conditions.
Conclusions The luminance has a non-linear relationship with the digital driving input. The gamma functions remain unchanged from day to day. The primaries are not independent of each other. Therefore, the gamma function of a primary is changed by the operating level of the other two primaries.

27 Experiment 5

Temporal response of LCDs Objectives:
4. To measure and analyse the response of LCDs to the driving signals. 5. To examine the effect of backlight to the response. 6. To investigate the effect of driving level and the level of Red Green and Blue on the monitor on the response. 7. To measure the decay of each primary. 8. To compare the response of different LCDs in terms of the rise time, the decay time and the shape of the response.
16. LCD-Samsung 17. LCD NEC MultiSync LCD 1510+ 18. LCD DELL E196FP 19. A Photocell with an amplifier 20. An oscilloscope Control conditions: the LCDs are allowed to warm-up for about 1 hour before starting the measurements.
Measuring the response of the LCDs The LCD is controlled by a program written using the CRS Matlab toolbox with the VSG system to display a cycling sequence of full-screen maximum black and maximum white images. The duration of the images is to be determined by the user. The photocell with an amplifier is connected to the oscilloscope which is controlled by the computer to measure the light from the LCDs. Effects of the backlight to the waveforms of the LCDs The aim of this measurement is to prove whether the fluctuations on the response are caused by the backlight. The oscilloscope is used to measure the light variation of the LCD displaying a stationary full-screen maximum white image. Primary decay The cycling page program is used to produce a series of full-screen red or green or blue image alternating with a maximum black image. The length of the coloured image is 0.5s whereas that of black screen is 1s. The reason for using these durations is to allow the LCD enough time (0.5s) to fully response to the driving signal before the light decays after the driving signal is set to 0 and it is easier to detect the decay part of the response when the off state is long (1s).

31 5.2 Backlight effects on the shape of response Apart from The Dell LCD, it is possible to vary the backlight level by adjusting the brightness on the display. The results from all LCDs are consistent; as the brightness decreases the modulation of the signal becomes more obvious and significant. The result from the Samsung LCD as an example is shown in figure 5.7.
Figure 5.7 The graph from the oscilloscope measuring a stationary white image on the Samsung LCD at 100% backlight level
At 100% backlight level, the luminance appears to be relatively constant regardless the presence of the sinusoidal modulation at the frequency of 6.3 kHz which is due to the operating frequency of the backlight. CCFLs, used to backlight LCD displays, typically operate at 30 kHz to 70 kHz with measurable harmonic content into the low MHz region.[1] The brightness is then lowered to 50 percent and the result is shown in figure 5.8. It can be seen that the mean value is lower and the modulation becomes very significant which results in flickers at a high frequency of about 370 Hz which is not perceived by the human eye. However, this modulation due to the backlight is not synchronous with the frame rate e.g. 60Hz. It is then unlikely that the number of peaks will be perfectly constant from one frame to another which results in variation of luminance across the frames.
Figure 5.8 The graph from the oscilloscope measuring a stationary white image on the Samsung LCD at 50% brightness level
Reference [1] www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,C1028,P1219,D4300 - 21 Aug 2006
Figure 5.10 The response to a driving signal at the frame rate frequency (60 Hz) seen on the Samsung LCD at 100% (up) and 50% (down) brightness level
From figure 5.10, it shows that the response seen on the LCD is continuous at the maximum backlight level but the degradation of the response due to 50% level manifests itself in a series of high frequency flickers (370 Hz) which is fortunately not resolved by the human eye.

33 5.3 The effects of Red/Green/Blue level (white point) on the monitor and the level of driving signal on the response As a result of the presence of the frame rate steps on the response of the Dell LCD, it was suggested that the steps could be removed if the driving signal is lower than maximum level, say 50%. An experiment shows that even though the driving level is halved or lower, the steps are still present on the response. The same experiment was carried out on the Samsung LCD which initially gave a very smooth response to the driving signal at maximum level. In figure 5.1 and 5.11, it can be seen that the response degrades as the driving level decreases.
Figure 5.11 The response of the Samsung LCD to the driving signal at 50% (left) and 25% (right)
The presence of steps due to the frame rate on the response of the Dell LCD can be eradicated by fully increasing the level of Red/Green/Blue setting on the monitor. From figure 5.11 and 5.12, it can be seen that the resultant response improves significantly; the rise time becomes dramatically shorter whereas the decay time is constant.
Figure 5.12 The response of the Dell LCD with RGB setting at factory default Rise time ~ 60 ms Decay time ~ 20 ms
Figure 5.13 The improved response of the Dell LCD with RGB setting at 100% Rise time ~ 22 ms Decay time ~ 20 ms
Measurements were performed to examine the effect of driving signal level on the response when Red/Green/Blue level is 100%. It was found that the steps become more noticeable as the driving signal level decreases as shown in figure 5.14.
Figure 5.14 The response of the Dell LCD to the driving signal at 50% level with the RGB setting at 100%
It can be concluded that the LCDs give the best response to the driving signal when the Red/Green/Blue setting is set to the maximum level and when the driving signal is high or maximum.
35 Comparison with the CRT The response to the step driving level of the CRT is shown in figure 5.15. Due to the raster scan of the CRT, an image is refreshed at the frame rate in this case 60 Hz. The luminance peaks at the beginning of the frame time and decays down until the next frame. Therefore, the response is not continuous like the response of the LCD.
Figure 5.15 The response of the CRT to the step driving signal of length 0.1s

36 5.4 Primary Decay

Figure 5.16 Primary decay of the Dell LCD
There are 3 primary colours in a pixel of an LCD including red, green and blue. They are produced by different light filters. The 3 curves in figure 5.16 have been normalised by their initial luminance level in order to compare their decay characteristic. It can be seen that all 3 graphs have more or less the same shape. This suggests that the decay characteristic is independent of the type of primary but it is determined by the active matrix circuits and the switching characteristic of the liquid crystals.

Amongst the four LCDs, the Samsung LCD gives the best response to the driving signal because it has the shortest rise time and the shortest decay time. The time delay for which the pixel response to the driving signal depends on the spatial location on the screen; the response is more or less synchronous with the driving signal at the top left corner but the longest delay takes place at the bottom right corner. In the factory default conditions, the Dell LCD is the only one that presents the artefact on the response due to the frame rate. The consequent steps on the response give a discontinuous rise in luminance and slow down the rise time significantly. Despite its rapid decay time, it is undesirable to be used as visual stimuli in electrophysiology unless its Red/Green/Blue setting is set to maximum level and it is to be operating at the maximum driving level. The backlight has an impact on the response. The desirable backlight level is 100%, at this level there is hardly any modulation that would result in flickering. The dramatic modulation represented on the response at 50% backlight will not also be perceived by the human eye. However, in electrophysiology, the modulation is not desirable. It is concluded that different colour filters have no effects on the decay characteristic of the Dell LCD. The decay purely reflects how fast the active matrix circuits of the LCD operate and how fast liquid crystals can switch themselves. Note: More information and graphs are in the Appendix D

37 Experiment 6

Spatial Independence
In CRTs, the luminance at one point of the image is often affected by changing the luminance elsewhere on the image. Does this effect happen with the LCD? Objective: to examine the change in luminance of a small area at the centre of the screen of LCDs etc. when the background luminance surrounding it changes. Hypothesis: There is spatial independence in the LCD; the luminance and chromaticity of the image are not affected by changing luminance and chromaticity of the area surrounding it. Apparatus: 1. LCD Dell 2. a chromameter 3. a ColorCal light measuring device Method: 1. Write a program to display a small maximum white square at the centre of the screen and another program to display a full maximum white screen. 2. The size of the square should fill the view circle of the chromameter. However, it should not be too small or else flare from other parts of the screen would affect it. In this experiment the square is pixel. 3. Set the viewing distance to 30cm so that the small square appears to be large enough to fill the viewing circle of the chromameter. 4. Display the small the white square and measure the luminance. 5. Display the full white screen and measure the luminance of the same point. Results and observations: Chromameter: Luminance (cd/sq m) (x,y) chromaticity coordinates Averaged value 92.36 (0.3294, 0.3307) 92.59 (0.3294, 0.3307)