Instrumentation & Measurement Magazine 26-3 - 56

Table 3 - Calculated versus measured irradiance (W/m2
Illuminance (lx)
600
683
824
2089
6500
Power Density Factor
(W/m2
)
0.011
0.013
0.016
0.040
0.124
Calculated Irradiance
(W/m2
1.93
2.19
2.66
6.71
)
20.89
Table 4 - Calculated versus measured irradiance (W/m2
Illuminance (lx)
1142
1925
2390
2770
3600
5820
Power Density Factor
(W/m2
)
0.022
0.036
0.045
0.052
0.068
0.110
Calculated Irradiance
(W/m2
3.20
5.40
6.70
7.77
)
10.09
16.32
) GE 4US7
Measured Irradiance
(W/m2
1.94
2.28
2.63
6.71
)
20.10
) Utilitech E359760
Measured Irradiance
(W/m2
3.06
5.12
6.54
8.05
)
10.09
16.73
Table 5 - Calculated versus measured irradiance (W/m2
Illuminance (lx)
850
2570
3720
3900
9650
Power Density Factor
(W/m2
)
0.018
0.053
0.077
0.081
0.200
Calculated Irradiance
(W/m2
2.55
7.70
)
11.14
11.68
28.91
illuminance and irradiance within their respective wavelength
ranges. This spectral independence is unlike standard
solar irradiance meters that require solar spectra for accurate
measurements. The Digi-Sense meter measures illuminance
+/- 3% of reading and +/- 0.5% of full scale. The Apogee meter
measures irradiance +/- 5% of the reading. The maximum
difference of calculated versus measured is less than the RMS
value of the two meter's error of +/- 6%.
Irradiance Measurement Based on Datasheet RSI
The above calculation requires the light sources RSI, obtained
from a single spectral measurement which requires a relatively
expensive meter such as the US $1,200 Sekonic or a
Hopoocolor alternative for US $700. A no-cost alternative is the
use of datasheet RSI provided by many LED light manufacturers.
For example, Cree provides RSI data for the XM-L2 LED.
Fig. 7 shows measured versus datasheet RSI values of the
Bolder Cree XM-L2 LED light. The calculated irradiance values
(1.599 W/m2
± 1%, measured; 1.522 W/m2
± 1%, datasheet)
include consideration of the standard error of a sum in the calculation.
A systematic error must be present as these irradiance
values calculated from the two methods differ by about 5%.
56
Fig. 7. Cree XM-L2 LED comparison of recorded and datasheet RSI.
IEEE Instrumentation & Measurement Magazine
May 2023
) GE LED 9A19
Measured Irradiance
(W/m2
2.50
7.70
)
11.03
11.72
28.96
% Difference
2.0%
0%
1.0%
-0.3%
-0.2%
Differences include meter accuracy, data digitization errors,
and LED manufacturing tolerance.
Table 6 lists the illuminance measurements from the
Bolder Cree XM-L2 LED light source, Apogee irradiance
measurements, irradiance calculated from the Sekonic RSI
measurements, the percent differences, the datasheet RSI
calculated irradiances, the percent differences, and the
Irradiance %
Difference
4.5%
5.3%
2.4%
-3.5%
0%
-2.5%
Irradiance %
Difference
-0.5%
-4.0%
1.1%
0%
3.8%

Instrumentation & Measurement Magazine 26-3

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