  Indoor light is known to be a new energy source to power uW low consumption
wireless sensor networks (WSNs). For wireless electronic devices that consume
tens of mW, it is still challenging to harvest this amount of power in a low
light indoor environment. The challenge comes from the fact that the irradiance
is low, as well the fact that the source is a multi-spectral direct,
reflective, and scattered mix of artificial and natural light, with intensity
and composition varying in time. This article describes a method providing an
evaluation of the potentially harvestable energy in real light environments.
Measurements of indoor light spectral composition in real condition and
optoelectrical characteristics of photovoltaic converters in a controlled
environment are base on the method s calculation model. Real harvester
prototypes based on GaAs commercial photovoltaic cells and power management
integrated circuit, powering a commercial wireless e-ink display, have been
conceived to compare with the model calculations. For two days of
experimentation, model calculations achieve a mean absolute percentage error
(MAPE) of 2 percents and 6 percents for Day 1 and Day 2, respectively. A method
within this accuracy range would be a helpful tool to assist in designing light
energy harvesting systems in such ways that they can be best tailored to the
indoor lighting conditions to which they will be exposed.
