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Color measurement methods for medical displays

Anindita SahaHongye LiangAldo Badano

Abstract — The effect of different measurement methods on the characterization of display color,maximum color difference, and luminance uniformity of medical liquid-crystal displays are reported.We use a telescopic colorimeter and a custom-designed collimated probe with an internal lensattached to a spectrometer. The maximum color-difference variations were found to be between0.0047 to 0.0073, in the same range as variations among methods, displays, and screen locations.

Keywords — Medical display, color measurement, color uniformity, luminance uniformity.

1 IntroductionColor is becoming an important aspect in the charac-terization of medical-imaging display devices due to the factthat gray-scale images are more and more commonly mixedwith color images in a multi-modality mode. Display devicesthat had been targeted for gray-scale applications such ascomputed tomography, projection radiography, and mammo-graphy, which require a fine control of the gray-scale pres-entation states, benefit from the addition of color for thesuperimposition of color-coded images from other modali-ties (i.e., from nuclear medicine, PET, or SPECT studies)and for the effective addition of computer aids based ondetection and classification algorithms.

Several standards and recommendations for displaycolor measurements are available. The VESA Flat PanelDisplay Measurement Standard Version 2.0 guidelines1 arewidely accepted testing procedures for display devices.Though first created specifically for consumer electronics,the guidelines are generally used across many applications.2

The standard defines how to measure various properties offlat-panel displays using specific techniques and room con-ditions.

The ISO 13405-2 “Ergonomic Requirements for Workwith Visual Displays Based on Flat Panels” is more commonlyused for ergonomics and safety rather than for metrology.2

The ISO standard, gaining in recognition due to EuropeanUnion requirement guidelines, deals with the classificationof devices based on calculations, but no specific measure-ment method is given.

The Standard Panels Working Group (SPWG) dealswith manufacturing the most popular types of displays inregards to the electrical and mechanical components.3 Themetrology is similar to that used in VESA. On the otherhand, the TCO standard4 uses a combination of the VESAand ISO recommendations. Little information is availableon actual methodology for optical methods, but the standardencompasses mandates in ergonomics, electromagneticemissions, electrical safety, and energy efficiency.

The American Association of Physicists in Medicine(AAPM) TG18 document “Assessment of Display Perform-ance for Medical Imaging Systems” is the most relevant formedical displays. TG18 comprehensively deals with the the-ory behind the measurements as well as with how to meas-ure and compute the results.5

In this paper, the effect that different instruments anddifferent methodologies have on color metrics of interestwere studied. In particular, telescopic measurements ofcolor and uniformity using perpendicular and rotated orien-tations between the meter and the display surface normalwere compared. A new approach to measure color based onthe adaptation of previous work on a luminance probe designis also reported.6,7

2 Measurement methods

2.1 Metrics

By using the measured luminance, the uniformity of the dis-play can be assessed by determining the minimum andmaximum luminance on a particular display screen. Thenonuniformity metric1 is calculated according to

Under the EUREF guidelines for digital mammogra-phy, the maximum non-uniformity should be less than 10%to be considered acceptable.8 The accepted calculation fortesting color uniformity is present in all the different stand-ards. The calculation is based on the CIE 1976 uniformchromaticity-scale diagram using chromaticity coordinates(u′, v′). Equations to convert the earlier CIE (x, y) coordi-nates to (u′, v′) are given in all of the standards. The colornonuniformity is computed using the distance formula betweenthe measurements of (u′, v′) for the various locations for thedisplay screen1:

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The authors are with CDRH/NIBIB Laboratory for the Assessment of Medical Imaging Systems, Division of Imaging and Applied Mathematics,Office of Science and Engineering Laboratories, Center for Devices and Radiological Health, U.S. Food and Drug Administration, 12720 TwinbrookParkway, Rockville, MD 20952; telephone 301/443-5020, fax -9101, e-mail: [email protected].

The mention of commercial products herein is not to be construed as either an actual or implied endorsement of such products by the Departmentof Health and human Services. This is a contribution of the U.S. Food and drug Administration and is not subject to copyright.

Journal of the SID 14/11, 2006 979

and

The maximum change in (u′, v′) is the reported colornon-uniformity parameter, which should be less than 0.01for an acceptable clinical workstation.5

2.2 InstrumentationFor our telescopic measurements, we used a spot colorime-ter (CS100, MINOLTA, Mahwah, NJ) in two approaches:perpendicular and rotated. The colorimeter is placed on astand approximately 1 m from the display. The perpendicu-lar method of measurements is done by moving the color-imeter from center of the display in a vertical thenhorizontal direction in order to reach the corner points ofmeasurement. The rotated method was performed by mov-ing in a diagonal direction from center to an outside point ofinterest. A clear plastic overlay with a mark for the fivemeasurement points is used to focus the colorimeter to ensurethe measurements are taken at the same point on the screenby taping the overlay to the edge of the display. Once thecolorimeter is focused on the measurement point, the over-lay is flipped behind the display so as to not distort the meas-urement. A third method involving a color probe isdescribed for the first time in this paper.

2.3 Characterization of the color probeIn previous work, we used an automatic goniometric setupthat consists of a five-axis motorized stage, a conic colli-mated luminance probe,9 a high-gain Si photodiode sensorwith an active area of about 5.7 × 5.7 mm, a photopic filter,and a research radiometer (SHD 033 sensor, IL 1700 radi-ometer, International Light Inc., Newburyport, MA). In thisapproach, the probe has to be positioned at a constant dis-tance and at a specific angular viewing direction from theliquid-crystal display (LCD), and the required pattern hasto be displayed before luminance measurements are made.At each gray level, after a warming time of 10 sec, the aver-age of 10 consecutive measurements of luminance, 0.5 secapart, is recorded by the software application. The standarddeviation of these 10 measurements is typically on the orderof 0.01 cd/m2.

Although the probe is highly collimated [see Fig. 1(a)],the contamination of luminance measurements by straylight is typically on the order of 10–5 9 and depends on thedistance between the probe and the emitting surface. Tominimize this effect, automatic corrections based on theangle with respect to the display normal are made in order

to ensure that at each angle the distance between the probeand the LCD screen remains constant. The corrections arecalculated within the LabView application that controls thepositioning of the probe, the acquisition of luminance data,and the display of test patterns in the screen. More detailson the experimental setup can be found in Ref. 10.

We have modified the luminance probe design to al-low for a fiber-optic adapter that connects the light path tothe spectrometer (MAS40, Instrument Systems, Ottawa,Canada) rather than a high-gain photodiode for use in spec-tral measurements. We use a lens that has a 19-mm focallength and a working distance of 12 mm connected to the fi-ber-optic adapater to the MAS40 to focus the beam (Lens77646, Oriel Light, Stratford, CT). The first probe designwas based on collimation. This probe design allowed too lit-tle light to pass to the spectrometer causing erroneousmeasurements. Thus, a new probe was designed by widen-ing the tip of the probe to approximately 2 cm to increasethe light input. The modification can be seen schematicallyin Fig. 1(b). All measurements were performed with the tipof the probe being 1 mm from the display screen.

The characterization of the new probe design is basedon a similar methodology described in Ref. 6 though the useof a different light source. The light source used was a stand-ard flat-panel-display set up with a white line in the centerof the screen and a black background on the remainder ofthe screen. The slit plate was positioned over the displaysurface with the slit placed over the white line on the screen.The slit plate was made out of black vinyl plastic with athickness of about 2 mm. The actual slit was constructedfrom two razor blades tinted black.11

Figure 2 shows the measurements taken along the boxand across the slit for the luminance probe from Ref. 10 [seeFig. 1(a)] and for the redesigned color probe [see Fig. 1(b)].The plots show that the maximum height was in the middleof the slit at each distance. The tails of the scans indicate the

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FIGURE 1 — Design modification to use a luminance probe for spectralmeasurements. The color probe has a larger aperture and no internalbaffles to allow more light into the fiber.

980 Saha et al. / Color measurement methods for medical displays

luminance probe has an ultimate contrast sensitivity of 10–4,while the color probe has a sensitivity of less than 10–2. Thechange as the distance increases can be seen in Fig. 3, wherethe widths of the peaks were calculated at the different dis-tances for both the luminance and color probe. The widthsof the peaks increased dramatically at the full-width-at-thousandth-maximum for the luminance probe, while thecolor probe had no values. The lower widths occur when theprobe is placed closer to the slit.

Four different displays were tested with the colorprobe: a one-million-pixel (1 Mpixel) color display, a 3-Mpixelcolor display, a 5-Mpixel monochrome display, and a 1-Mpixel TV display. The 1-Mpixel color display was testedusing both the probe and colorimeter to compare the meth-ods. The results compare the use of the colorimeter usingthe rotated and perpendicular methods versus the colorprobe attached to the spectrometer. The 1-Mpixel TV dis-play was tested using different measurement points. Twotests were performed by taking five or nine points that werein a 2.5-cm square from the screen border while two testswere performed using five or nine points for a 5.1-cmsquare.

3 ResultsFigure 4(a) shows the luminance non-uniformity in terms ofpercentage for each color on the screen for each of the threemeasurement methods for the 1-Mpixel display. Errorpropagation techniques using root-sum-of-squares of thedeviations from the average values were used to computethe error in non-uniformity. The lowest non-uniformity occurs

FIGURE 2 — Luminance scans of a 0.1-mm slit for six distances fromprobe to slit for the luminance and color probes. The tails of the colorprobe are at 10–2 compared to 10–4 for the luminance probe.

FIGURE 3 — Full-width-at-half-maximum, full-width-at-tenth-maxi-mum, and full-width-at-thousandth-maximum comparison forluminance and color probes. The FWHM and FWTM are higher for thecolor probe while FWTHM increases for the luminance probe as afunction of distance.

FIGURE 4 — Uniformity as a function of the different color backgroundsfor 1-Mpixel display. The results for the telescopic methods exhibit lessnonuniformity than those obtained with the spectrometer method.


for the red screen using the telescopic meter, while theother colors had a higher non-uniformity. Figure 4(b) showsthe color uniformity on the 1-Mpixel display for each of thethree measurement methods with the telescopic measureshaving consistently smaller color differences than the spec-trometer method. The color uniformity was determined bycalculating the maximum color difference using the aboveequations for converting the 1931 CIE (x, y) coordinates to1976 (u′, v′) coordinates. Each color screen has a maximumcolor difference of less than the maximum accepted value of0.01. Error propagation techniques using root-sum-of-squares of the deviations from the average values were usedfor the repeatability of maximum color differences.

Figure 5(a) illustrates the luminance non-uniformityfor each of the four displays with the corresponding error.The higher resolution displays had lower non-uniformitythan the lower-resolution ones. The four different displaysare compared for their color-uniformity characteristics inFig. 5(b) where the color difference was lower in the 3- and5-Mpixel displays.

The results for the 1-Mpixel TV in Fig. 6(a) indicatethat non-uniformity was lower for less points of measure-ment on the screen. Figure 6(b) compares the color uni-formity of the 1-Mpixel TV display using the five or ninepoints of measurement and squares of different size.

The color coordinates of (u′, v′) were averaged foreach color and measurement method and presented in Fig. 7.Figure 8 reflects the difference in color coordinates for each

FIGURE 5 — Uniformity as a function of the four different displays forthe different color backgrounds. Higher pixel-count displays exhibitgreater uniformity.

FIGURE 6 — Uniformity of 1-Mpixel TV using variable-pointmeasurements for the different color backgrounds. Different screenlocations have little effect on the uniformity results.

TABLE 1 — Color coordinates in all tests shown in Fig. 8. TABLE 2 — Maximum color differences in all tests showed in Figs. 7 and8.


display and compares that to the variability of the colorcoordinates for each point on the 3-Mpixel display.

Table 2 is a compilation of the color-difference data.For each set of results, the maximum color difference foreach color and each experiment set were determined. Table 1quantifies the color coordinates (u′, v′) for each of the fourdisplays.

4 DiscussionOur results show that the difference in color-measurementmethods for medical displays is significant and comparableto the value of the quantity to be measured.

The scans of the probe in Fig. 2 show that the redes-igned color probe has a larger width but smaller range ofluminance values than the luminance probe. The increasedwidth is further delineated in Fig. 3 where the full-widths-at-half-maximum and at-tenth-maximum are significantlyhigher than for the luminance probe. The color probe hadno full-width-at-thousandth-maximum values further indi-

cating that it is less sensitive than the luminance probe. Themain cause in the differences between the two probes is dueto the design of the lens of the color probe which is 2 cmwide and captures much more light as the probe is movedacross the slit. Another factor in the difference in luminancevalues between the luminance and color probe could befrom the different light sources used for measurement. Theluminance probe was tested using a uniform light sourcewith four halogen lamps inside which were supplied with aconstant 12-V source, while the color probe was tested usinga 1-Mpixel flat-panel display with a white background.

The first testing method done was to compare the effectsof measurement techniques on the luminance and color-uniformity metrics using the 1-Mpixel display. In Fig. 4, themeasurements using the telescopic meter have a smallernon-uniformity on the order of a 2–5% difference, depend-ing on the color over the probe. Other than the red back-ground, using the rotated and perpendicular methods, theluminance non-uniformity was above 10%. The color-differ-ence metric shows similar results in Fig. 4. The proberesults yield a constantly higher color difference than thecolorimeter. A cause of the discrepancy in the results couldbe due to the precision of the instruments as well as thecalibrations of the two instruments. The spectrometer was

FIGURE 8 — Color coordinates u′ and v′ comparison. Different displaysvary in (u′, v′) while the measurement for one display across the displaysurface shows a small color coordinate change.

FIGURE 7 — Color coordinates u′ and v′ comparison. There is asignificant difference among measurement methods, but very littledifference among measurements with five or nine data points anddifferent screen locations.


more precise in measurement with at least six significantfigures for the color coordinates and luminance measure-ments, whereas the telescopic meter only had three signifi-cant figures.

A test was done to check the color calibrations using ared and green laser with both the colorimeter and probe todetermine the accuracy of the instruments. The spectrumgiven by the probe software was compared to the knownwavelength of the red and green lasers and shown to becorrect. The color coordinates were measured and the colordifference was determined between the two instruments.For the green laser, the color difference was found to be0.006 between the colorimeter and probe while the red laserhad a much larger color difference of 0.05. The results indi-cate further work needs to be done to verify the accuracy ofthe color measurements with both instruments and to deter-mine how to account for the variability between the instru-ments.

The second set of testing was performed using theprobe as the only instrument of measurement for the fourdifferent displays. Figure 5 shows that the luminancenonuniformity was below 10% only in the 5-Mpixel mono-chrome display. The greatest non-uniformity was in the1-Mpixel display, while for the most part the 1-Mpixel TVand the 3-Mpixel display had similar non-uniformity rangesfrom about approximately 10–15%. The color differencemetric from Fig. 5 also shows the same pattern as the lumi-nance non-uniformity. The 5-Mpixel monochrome had thelowest color difference with nearly no difference for thegray background. The low gray color difference in themonochrome display versus the color displays might be dueto the color filters in those displays.

The third round of testing was performed by changingthe points measured from five to nine and changing the dis-tance from 2.5 to 5.1 cm from the corner of the display. Thenon-uniformity was found to be lower when measuring fivepoints on the display screen instead of nine as seen in Fig. 6.The non-uniformity of all the points did not deviate far fromthe range of 10–15%.

The CIE color coordinates for the various testingmethods shown in Figs. 7 and 8 correlate with the uniform-ity results. The color coordinates drawing shows wherealong the color spectrum the (u′, v′) coordinates wereobtained and the differences between the methods.

Table 2 summarizes the results from the color differ-ence and CIE graphs to numerically quantify the maximumcolor difference. Maximum-color-difference variations are,in general, in the same range as variations among methods,displays, and screen locations and are between 0.0047 and0.0073. The results suggest that color-performance meas-urements on the order of 0.01 could be confounded by vari-ability from the measurement method and instrumentation.Factors that affect these color-measurement methodsinclude angular dependence of the spectral emissions and,perhaps, lens flare and stray light contamination. Ourresults suggest that caution should be used when comparing

display devices using measured parameters that are not con-sistent across methodologies and device technologies. Aunified manner in which to conduct these measurements isneeded for the meaningful reporting of the color charac-teristics of medical displays.

AcknowledgmentsThe authors acknowledge equipment loans used in thisstudy from Barco, Chi Mei Optoelectronics, Eizo/Nanao,National Display Systems; useful technical discussions withKen Richardson (Instrument Systems) regarding the designand calibration of the color probe; and design and help fromEdward F. Kelley (NIST).

References1 Video Electronics Standards Association (VESA) Flat Panel Display

Measurements Standard Working Group. Flat panel display measure-ments standard, version 2.0. Technical report, VESA, May 2003.

2 P A Downen, “A review of popular FPD measurement standards,” ProcADEAC, 5–8 (2004).

3 Standard Panel Working Group (SPWG). SPWG Notebook PanelSpecification, Version 3.5, Technical report, SPWG (March 2005).

4 TCO Development. TCO ‘05 Desktop Computers. Technical Report,TCO (June 2005).

5 E Samei, A Badano, D Chakraborty, K Compton, C Cornelius, KCorrigan, M J Flynn, B Hemminger, N Hangiandreou, J Johnson, D MMoxley-Stevens, W Pavlicek, H Roehrig, L Rutz, J Shepard, R AUzenoff, J Wang, and C E Willis, “Assessment of display performancefor medical imaging systems: Executive summary of AAPM TG18report,” Med Phys 32(4), 1205–1225 (2005).

6 A Badano, S Pappada, E F Kelley, M J Flynn, S Martin, and J Kanicki,“Luminance probes for contrast measurements in medical displays,”SID Symposium Digest Tech Papers 34, 928–931 (2003).

7 E F Kelley and A Badano, “Characterization of luminance probe foraccurate contrast measurements in medical displays,” Technical ReportNISTIR 6974, NIST (2003).

8 R van Engen, K Young, H Bosmans, and M Thijssen, “Addendum onDigital Mammography,” European Reference Organisation for QualityAssured Breast Screening and Diagnostic Services, 3rd ed. (November2003).

9 A Badano and M J Flynn, “Method for measuring veiling glare in highperformance display Devices,” Appl Opt 39(13), 2059–2066 (May2000).

10 A Badano and D H Fifadara, “Comparison of Fourier-optics, tele-scopic, and goniometric methods for measuring angular emissions frommedical liquid-crystal displays,” Appl Opt 43(26), 4999–5005 (2004).

11 The slit plate was provided by Edward F. Kelley (NIST).

Anindita Saha received her B.S. degree in bioen-gineering from the University of Pittsburgh in2005. She is currently working as an engineer atthe Division of Imaging and Applied Mathemat-ics, Office of Science and Engineering Laborato-ries, Center for Devices and Radiological Health,U.S. Food and Drug Administration, Rockville,MD, where she works in the display charac-terization lab. She is a member of the BiomedicalEngineering Society and Tau Beta Pi.


Hongye Liang received his Ph.D. in electricalengineering from the University of Maryland,College Park, MD. He is currently working as apostdoctoral fellow in the Division of Imagingand Applied Mathematics, Office of Science andEngineering Laboratories, Center for Devices andRadiological Health, U.S. Food and Drug Admini-stration. His current research interests includeLCD temporal performance measurements andsimulation on the visual effect of LCD temporalresponse.

Aldo Badano received his Ph.D. degree in nuclearengineering from the University of Michigan in1999. He is currently a scientist with the Divisionof Imaging and Applied Mathematics, Office ofScience and Engineering Laboratories, Center forDevices and Radiological Health, U.S. Food andDrug Administration, where he developed a pro-gram on the characterization and modeling ofmedical image acquisition and image display devicesusing advanced experimental and computational

methods. He is a referee for several scientific journals and a reviewer oftechnical grants for DoD and NIH. He has authored or co-authoredmore than 70 publications and is a member of SID, AAPM, and SPIE.
