Providing confidence in indoor light harvesting measurements

The challenge

Lightricity’s innovation is a highly efficient ‘indoor solar panel’, which can permanently power a wide range of Internet of Things (IoT) and other low-power devices. The stamp-sized device eliminates the need for batteries, which come with a large environmental footprint on disposal. This has attracted interest from warehouses and manufacturing facilities with large suites of connected sensors, often in hard-to-reach areas.

Lightricity needed a way to reliably measure how effectively its panels convert different types of light into energy, and, unlike outdoor solar cells, which have well-established standards and off-the-shelf solar simulators, there was no universally accepted standard or measurement technique for indoor lighting.

As a result, Lightricity developed its own indoor light simulator, the ‘LightBox’, which can create the full range of indoor lighting intensities in a highly controlled way. The test product is placed in the box beneath a panel of LEDs and its response is measured and analysed under varied lighting conditions.

Measuring and analysing light responses is a complicated process, since indoor light conditions and sources vary enormously, from a few lux (a very dimly lit room) to several thousand lux (a brightly lit workshop), and solar materials perform differently under different light levels. Lightricity needed to measure how light was converted to energy at indoor light levels and to show how performance would be affected in a room where lighting levels regularly changed. This created a multitude of measurement uncertainties, many of which were beyond Lightricity’s capabilities.

“We quickly realised that we needed to ensure all measurement uncertainties had been properly considered and accounted for, and that measurements could be compared to an independent standard” says Matthias Kauer, Managing Director of Lightricity., “so no one could think that we designed it to favour our own product”. This led them to approach NPL through A4I.

The solution

Over three A4I projects, NPL helped Lightricity to understand the sources of measurement uncertainty in the Lightbox and provided recommendations for enhancements.

“Indoor light harvesting really is a whole new area, albeit one that is growing rapidly,” says James Blakesley, Principal Scientist at NPL who led the A4I project. “So we didn’t have a template to work from and didn’t know what the sources of uncertainty were going to be. We had to bring all of our team’s expertise to really get our heads around what needed improving and how to do it”.

Amongst the key improvements that arose from the project were adjustments to the uniformity of the LED light source, and the angular light distribution, i.e. how the light spread throughout the lightbox. These changes created a more consistent and precise light source, which significantly improved the accuracy and repeatability of measurements.

The project also improved accuracy by comparing performance against calibrated reference devices, which are matched to the light harvesting materials under test. This minimises angular and spectral errors over the typical approach of using photometers which "see" a different spectrum and angular distribution of light to the devices under test.

The impact

Although the Lightbox was initially designed to support Lightricity’s R&D, it has become a product in its own right, used by university researchers and commercial labs to assess a growing number of indoor solar products. “Since the project we’ve seen a more than doubling of sales of the Lightbox, and most of that can be attributed to the A4I improvements” says Kauer.

For its own indoor solar products, the Lightbox serves Lightricity with highly accurate performance data for its light harvesting panels in a range of indoor lighting conditions. The research allows them to prove long-stated claims that their technologies are up to six times more efficient than other light harvesting technologies.

The advancements in indoor photovoltaics (PV) characterisation developed through this project have directly contributed to a new international standard for indoor PV measurement, which NPL was involved in developing. “This will really boost the nascent indoor light harvesting industry” says James Blakesley. “There are a wide range of exciting new ‘indoor solar’ technologies that are just now emerging. Each uses different materials with different properties, so having a consistent way to compare them will spur innovation and support sales by providing clear comparable product data.”