19
At Sanken, luminance data are converted by ratio and
chromaticity by relativity. To compare the measured values by
the backlight manufacturer with data of the lamp manufacturer
using the correction factors, we use the equations shown in the
table of Results of Conversion to Measured Values of Lamp
Manufacturer.
Periodical checking of instrumental error and evaluation of
both parties’ measured values after correction are essential in
the optical measurement of CCFL.
Measured by
backlight manufacturer
Measured by
lamp manufacturer
Correction factor
for evaluation
Luminance
(cd/m
2
)
36000
40000
0.260
0.250
0.272
0.260
ChromaticityxChromaticity
y
Measured by
backlight manufacturer
Converted to
measured values
of lamp manufacturer
Luminance
(cd/m
2
)
L1
L1/0.9
x1
x1 –0.01
y1
y1 –0.012
ChromaticityxChromaticity
y
Accuracy of chromaticity measurement published by the
manufacturer is ±0.002 for both SR3 and BM7 as shown in the
table above (Accuracy of Chromaticity *1). The user tends to
believe that the instrumental error (accuracy) of the newly
purchased or manufacturer-calibrated instrument is a maximum
±0.002 for chromaticity measurement.
Actually, however, the published accuracy is subject to the
condition of using the Standard A Light Source (see Note *1).
The declared accuracy is not achievable with CCFLs that have
different color temperatures.
Then, what level of the accuracy (instrumental error after
calibration) can we expect for the CCFL?
According to the equipment manufacturer, the accuracy of
the SR3 spectroradiometer is ±0.005 for the CCFL. The
accuracy of the BM7 luminance colorimeter is ±0.03 of the
maximum value. This is because each tristimulus value is
calculated by measuring the intensity of radiations passing
through three RGB glass filters, and dispersion of the spectral
transmittance of color glass filters installed in the instrument
thus becomes the most important factor.
For supplementary information:
● Refer to Section 4, “Terminology,” for the reference light.
● Refer to Section 5, page 22, for instrumental error of the
Topcon luminance colorimeter (BM-series).
(2) Instrumental Error due to Aging (long-term use)
Measurement becomes unstable, when the instrument is used
over a long period of time. Generally, the measured
chromaticity tends to shift to the higher side, while the measured
luminance tends to shift to the lower side. Major reasons of
these shifts include variation in the transmittance of the optical
system due to stains, variation in the sensitivity of receiving
elements and change in the resistance of the board due to
humidity. To deal with the inevitable instrumental error and
shift from the absolute value caused by aging, the user must
control the variation within a certain allowable range by
compensating the measured values and calibrating the
equipment periodically.
(3)
Instrumental Error with Measuring Instrument beyond Control
With respect to these two types of instrumental error, each
manufacturer warrants and controls the measured values by
setting compensation values for each piece of equipment and
implementing periodical calibration using their standard
measuring instruments and standard light source.
However, the instrumental error of uncontrollable measuring
instruments is, in reality, totally unknown. The same
measurement results are never warranted, even when using
measuring instruments of the same type for the reasons stated
above. To compare with the measurements by uncontrollable
instruments, we need to measure the same light source under the
same conditions to evaluate the instrumental error.
(4) Standardization of Measured Values
As discussed in paragraphs (1) through (3) above, the
measurement in absolute values is difficult for the measuring
instrument for luminance and chromaticity. Lamp manufac-
turers and backlight manufacturers must evaluate correlation of
measured values between them by setting correction factors for
the relevant data. For example, the measured values for the
same lamp by the lamp manufacturer and the backlight
manufacturer are compared in the table below respectively after
data conversion using the correction values shown in the
parentheses.
Spectroradiometer
SR3
Luminance colorimeter
BM7
±2%
±4%
Within ±0.002
Within ±0.002
Within ±0.005
Within ±0.03
*1: When measuring Std A Light Source
*2: For radiations equivalent to CCFL (reference only)
Accuracy
of luminance
Accuracy
of chromaticity
Accuracy
of chromaticity
Instrument
*1 *1 *2
One type of instrumental error of optical measuring
instruments originates from manufacturer’s calibration
(accuracy limit) and another type is attributable to long-term
use of the instrument (aging).
(1) Instrumental Error Originating from Manufacturer’s
Calibration (accuracy)
The accuracy of measurement of the luminance and
chromaticity of the SR3 spectroradiometer which is the current
standard equipment in the industry and the BM7 luminance
colorimeter which was used as the standard equipment in the
past, is summarized in the table below (quoted from the
manufacturer’s specifications).
-0.01
(difference)
0.90
(ratio)
-0.012
(difference)