The displays included in this study are NEC 1990Sxi (NEC Display Solutions of America, Itasca IL) used with the color diagnostic workstations in our MRI practice. The displays are calibrated after an initial “burn-in” period of 2 days using NEC Spectraview II software with a GretagMacbeth Eye-One Display V1 colorimeter/photometer. The Spectraview II system makes color measurements and adjustments during calibration. The luminance of the display is calibrated to the DICOM part 14 grayscale standard display function (maximum
within the ACR accreditation guidelines for MRI). Displays are deployed as pairs and are initially color calibrated to have matching white points within 0.004 mean radial distance (see Eq. 6
) in the CIE 1931 (x
) color space.
The color measurements included in our analysis were accumulated quarterly over the course of many years from five locations on each display (see Fig. ). The color measurements were made on a white screen which had been adjusted (using the ons-screen display brightness control) to read between 90 and 110 cd/m2
. Measurements were made using a Sencore ColorPro IV colorimeter (Sencore Inc, Sioux Falls, SD) with an x
accuracy of ±0.004 (meaning absolute agreement with RGB color standard in the CIE 1931 color space). Repeated measurements have a precision of ±0.002 in x
. For much of the data discussed in this paper, the color point measurements were converted to the 1976 CIELUV (u′, v′) color space which shows better correlation with the human visual system's interpretation of color [5
]. The conversion is given below:
Shows the measurement locations on paired displays and calculation for the MRD between displays
The uncertainty in CIELUV color space for a single measurement is given by
is the measurement uncertainty which equals ±0.004. A typical measured color point for a display in this study is (x
(0.316, 0.331, 100 cd/m2) which yields Δu′=
The same colorimeter was used over the period of 3 years without recalibration. The displays had a staggered deployment such that during a single measurement time period, the ages of the tested displays could range from 0 to 24,000 operating hours; however, the displays measured at later dates were on average older than those measured at earlier dates. Thus, it's anticipated that monotonic drifts in the colorimeter measurements would be dampened, but might not be totally eliminated.
The radial color “distance” between two points 1, 2 in CIELUV color space is defined by:
The uncertainty in any radial distance measurement would be Δradial distance
0.003. Similarly, the mean radial distance (MRD) between two displays (A
) is calculated from radial distance between the average u′ and v′ values (CIELUV 1976) on each display.
For quality control, if a visual color difference appeared between displays or the MRD measured greater than 0.006 in CIELUV 1976 color space, the displays were recalibrated to reduce the MRD. In addition, if the maximum luminance of the display fell below 90 cd/m2 or artifacts appeared, the display was removed from service.
We retrospectively analyzed QC data taken over the last 3 years (the average display lifetime) for 38 displays. Typical data from an individual pair of displays is shown in the Results
section (Fig. ). For each time point and for each display, the average color point and the intra-display mean radial distance (between each combination of the five measurement points) was calculated. For each pair of displays at each time point, the mean radial distance was calculated between the average color points of each display and also for the central color points of each display. Individual display and display pair measurements were compared with quality control limits and averages to attain the rate of failure and measurement spread.
Fig. 2 Shows typical color point measurements (CIELUV 1976 color space) from a single display (with five measurement positions) over time. Each series represents a different position on the display and each point represents a different time. The points in this (more ...)
To ascertain the evolution of the average display color as well as the spread of display properties and trends, the data points from displays of the same age were averaged as follows. The average time between quality control tests was approximately 2,000 h. Data were binned with a granularity of 2,000 operating hours and include only one data point per display per bin. If two display data points happen to fall into one bin, only the most central time point was retained. For each time bin, the color points, radial distances, or net change in color point from t
0 (initial quality control test), were averaged. The uncertainty (standard error) in the average value was taken from the standard deviation of the mean over the time bin.