A modified cheap handheld car gadget could offer a low-cost way for farmers and fire services to monitor wildfire risk and spot crop drought stress.
Researchers at King's College London have converted an off-the-shelf product, usually used to measure if car window tint is legal, into a scientific instrument that can measure leaf water content in seconds.
Comparable field instruments can cost many tens of thousands of pounds, while the standard laboratory approach requires leaves to be picked, oven-dried and weighed over many hours.
However, the modified device, which costs about £140 in total to build from readily available components, neared the performance of laboratory methods across six plant species, explaining at least 81 per cent of the variation in leaf water content.
Leaf water content is a key indicator of how easily vegetation could ignite, and the researchers note the meter could support fuel moisture monitoring as well as irrigation decisions in agriculture, which accounts for about 70 per cent of global freshwater use.
The study was led by Dr Luke Brown, Senior Lecturer in Earth Observation in the Department of Geography at King's College London, working with Professor Mark Danson of the University of Salford
Water content tells you whether vegetation is healthy or ready to burn but previously to do this you had to destroy the foliage you are measuring," says Dr Brown. "We wanted to show that a sensor cheap enough for any farm, fire service or classroom can do the job in seconds - and without damaging the leaf."
Dr Luke Brown, Senior Lecturer in Earth Observation at King's College London
The instrument began life as a £70 meter sold to check whether car window tinting is legal. It measures how much light passes through whatever is slotted into it. The researchers replaced its light sources with two precisely chosen LEDs, one at a wavelength that water strongly absorbs and one that passes through water almost untouched.
Comparing the two readings isolates the water inside the leaf from its structure.
The team first tested the design in theory, simulating 1,000 virtual leaves using an established physics-based model of how light travels through plant tissue. They then ran dehydration experiments between April and June 2026 at Rothamsted Experimental Farm in Hertfordshire, tracking leaves from wheat, rapeseed, beech, birch, hazel and sycamore as they dried out over several days.
The meter's readings closely tracked two of the standard measures of leaf water status used in agriculture and fire science.
The findings arrive amid the UK's worst wildfire year on record. Fire chiefs confirmed in August that the number of wildfires in 2026 had already passed the previous record of 1,017, set across the whole of 2025.
The authors caution that the meter requires calibration for each plant species, and that its readings become less reliable once leaves are severely desiccated.
"Environmental monitoring has long been rationed by the price of instruments," says Dr Brown. "Bringing down that cost changes who is able to make these measurements, and that has potential to enable better management."
The research, Rapid non-destructive assessment of leaf water status with low-cost transmittance meters, is published open access in Methods in Ecology and Evolution.
