PR-655/670 User Manual
55
FIGURE 49 - CIE 1931 TRISTIMULUS FUNCTIONS
CIE XYZ Tristimulus and Photometric Values
( ) ( ) ( )
=
780
380
683
X
S
X
( ) ( ) ( )
=
780
380
683
Y
S
Y
( ) ( ) ( )
=
780
380
683
Z
S
Z
Where X, Y, and Z are the three CIE Tristimulus values. X
represents the red component, Y is the green component and Z is
the blue component.
Y also serves as the photometric value
–
when using the standard
MS-75 lens, Y gives candelas / meter² - the SI unit for luminance.
To find footlamberts (English unit of luminance), multiply cd/m²
by 0.2919.
683 is a constant used to convert lumens to watts. There are 683
lumens per watt at 555 nm for photopic (daylight) vision.
S(
) = the corrected spectral data,
( ) ( ) ( )
Z
Y
X
,
,
are the three
CIE Tristimulus functions (curves) and
(
) is the data increment
–
for the PR-655 the increment is 4(nm) and 2 (nm) for the
PR-670.
