Instrumentation & Measurement Magazine 26-3 - 52

Calculation of Irradiance from
Illuminance for Artificial Light
Photovoltaics Applications
Peter R. Michael, Danvers E. Johnston, and Wilfrido A. Moreno
M
any indoor sensors are powered by artificiallight-harvesting
photovoltaic (PV) cells. The
performance evaluation and application of PV
devices require irradiance measurement to determine input
power. Standard solar irradiance meters provide measurements
calibrated to sunlight spectra and do not have the
low-light-level capability needed for indoor applications.
Light meters, specifically designed for human visible artificial
light applications, measure illuminance. This article describes
a low-cost method to calculate irradiance from illuminance
measurement of artificial light sources. An application example
is provided.
Analysis and Application of PV Power
and Energy
PV systems input photons and output electrons, as described
in the well-known photoelectric effect, which earned Albert
Einstein the Nobel Prize in 1921. Input irradiance expressed
in the SI system has units of watt per square meter (W/m2
).
With irradiance and the PV system area, the input power in
watts (W) is calculated. The input power compared to output
power determines the conversion efficiency and the
power available to a device. With the collection of power
over time, one can determine the energy available. With the
power and energy measurements, a PV system design can be
implemented.
Outdoor photovoltaic systems are evaluated by measuring
irradiance under Standard Test Conditions (STC) [1].
STC irradiance is 1,000 W/m2
, and the related illuminance is
approximately 120,000 lx [2], 240 times the recommended illumination
level of 500 lx for an office [3]. Standard irradiance
meters are cost-effective but typically have a low-level limit of
100 W/m2
, approximately 12,000 lx, which only provides an
accurate reading for sunlight solar spectra. More expensive
thermopile sensors work with any spectra but require specialized
amplification and conversion circuits to enable low
irradiance measurements. These limitations mean that for
low-cost measurement of artificial light PV systems, an alternative
method is needed.
52
Illuminance
Light meters measure illuminance, the luminous flux of light
per unit area, in the unit of lux (lx). One lux equals one lumen
per square meter (lm/m2
IEEE Instrumentation & Measurement Magazine
1094-6969/23/$25.00©2023IEEE
), calibrated to the human eye's
May 2023
Illuminance meters, which measure light in human visible
range, provide a reading in lux (lx). These meters offer a
low-cost, easy measurement method for lighting design and
applications. The issue is how to use illuminance measurement
to derive irradiance values useful in PV applications.
This article includes theoretical derivation, limitations,
laboratory measurement confirmation, and an application
example of the conversion of illuminance to irradiance. The
theoretical calculations use fundamental values of light and
power. Laboratory confirmation uses a US $165 Digi-Sense
20250-00 lux meter for illuminance measurement. Irradiance
measurement utilizes a US $650 Apogee thermopile sensor,
model SP-510, an AT-100 μCache logger, and an Apogee application
running on an iPhone. An Internet of Things (IoT)
sensor provides an application example.
Irradiance
Irradiance in units of watt per square meter (W/m2
) measures
power density. The photoelectric effect takes the energy of
photons, E = hc/λ and converts it to electrical energy. PV module
measurements utilize STC irradiance input of AM1.5G
sunlight. In the acronym AM1.5G, AM1.5 specifies Air Mass
with a solar zenith angle of 48.2°, G stands for Global, and photon
wavelengths range in nanometers (nm or 10−9
m) from 280
nm to 4,000 nm.
Fig. 1 shows the spectral irradiance power density fsipd
·nm) of AM1.5G sunlight over the wavelength range
in
W/(m2
from 280 nm to 4,000 nm. The National Renewable Energy Lab
provides the data as a spreadsheet [4]. Integration of the fsipd
data over the wavelength range considered in Fig. 1, using (1),
results in an irradiance, I, of 1,000 W/m2
:
4,000 nm
I 

280 nm
f d
sipd

(1)
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