An infrared (IR) thermometer measures the thermal infrared radiation a surface emits and converts it to a temperature. It does not touch the surface and reads in under a second. The red laser on many units only shows where you are aiming; it does not measure anything. Thermal cameras do the same job across a whole image.
In practice
Set emissivity to about 0.95–0.98 for leavesbeing reviewed. Check the distance-to-spot ratio: a 12:1 gun held 30 cm (12 in) away reads a circle about 2.5 cm (1 in) wide, so get close enough to fill the spot with one leaf. Aim at leaves on the canopy top that face the light, read several, and average.
Common errors: reading the pot or the floor through gaps in the canopy, reading a shiny surface (low emissivity) and reading leaves in the fixture's direct radiant output under HPS right after moving them. Use the leaf temperature with room RH and air temperature to calculate leaf VPD.
Don't confuse with
- Probe thermometer: measures air or liquid temperature by contact.
- PAR meter: measures light, not heat.
- Thermal imaging of root zones: shows only surface temperature, not inside the medium.
HPS (high-pressure sodium) is a gas-discharge lamp with orange-yellow light peaking near 589 nm; it was the standard flowering light before LEDs.Leaf temperature is the actual surface temperature of a leaf, which often differs from air temperature by 1–3 °C depending on light source and transpiration.Relative humidity is the water vapour in air as a percentage of what it could hold at that temperature, so the same RH dries plants differently at different temperatures.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.Photosynthetically active radiation: light in the 400–700 nm waveband that plants use for photosynthesis, counted in photons rather than brightness.The root zone is the volume of medium or solution where a plant's roots live, including its water, air, temperature, pH, EC and microbes.Leaf VPD is the vapour pressure deficit between the inside of a leaf, at leaf temperature, and the surrounding air; it is what actually drives transpiration.In growing, temperature usually means grow-room air temperature; cannabis grows best around 24–28 °C (75–82 °F) by day and a few degrees cooler at night.
