PR-680/680L User Manual
4
Once the three Tristimulus values have been derived, useful colorimetric values such as CIE 1931 x, y and 1976
u’,
v’
can be calculated by the following formulae:
CIE 1931 x, y
Z
Y
X
X
x
+
+
=
Z
Y
X
Y
y
+
+
=
CIE 1976 u’, v’
(
)
Z
Y
X
X
u
3
15
/
4
'
+
+
=
(
)
Z
Y
X
Y
v
3
15
/
9
'
+
+
=
P
HOTOMETER
As a photometer, light passes through a custom matched photopic (V
) filter to the highly sensitive
photomultiplier (PMT) detector. The photopic trims the response of the PMT to the CIE 1931 photopic (daylight
vision) response curve (see the
Y
curve in Figure 48). This photometer is especially useful when photometric
brightness (e.g. luminance) of low level sources is required in near real time.
PMT
C
ALIBRATION
If you wish to modify the current Photometer Mode correction factor, use the
PMT Calibration
feature to
perform this task. This is a local factor and does not permanently change any factory calibration factors for the
standard lens or any accessories. However, it will modify Photometer readings regardless of the accessory in
use
–
it is a global factor.
The factory default value for this factor is 1.00. To return the PR-680/680L to its factory state, simply change
this factor back to 1.00 if it has been modified.
The Photometer factor can be modified using one of two methods. The user can enter a calibration value, or the
instrument can be instructed to perform a “One Time Measurement” which consists of both a
Spectroradiometer measurement and photometer measurement.
When measuring sources other than Illuminant A (CCT: 2856K) it is recommended that the onetime
PMT calibration option be used to calibrate the Photometer to the source that is being measured.
This will yield more accurate photometric results.
M
ODIFYING THE
F
ACTOR
M
ANUALLY
The use may manually enter a factor to modify the photometer correction factor.
1.
Access the
PMT Calibration Screen
by touching
Menu
then
Setup
then
PMT Calibration.