# IR thermometer

> An IR (infrared) thermometer reads surface temperature from emitted infrared radiation; growers use it to measure leaf temperature for leaf VPD.

- Canonical: https://genetixbay.com/knowledge/glossary/ir-thermometer.html
- Also written: infrared thermometer, IR gun, temp gun, laser thermometer, thermal camera
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

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 leaves. 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](https://genetixbay.com/knowledge/glossary/hps.md) right after moving them. Use the [leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md) with room [RH](https://genetixbay.com/knowledge/glossary/relative-humidity.md) and air temperature to calculate leaf [VPD](https://genetixbay.com/knowledge/glossary/vpd.md).

## 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.

## Related terms

[Leaf temperature](https://genetixbay.com/knowledge/glossary/leaf-temperature.md), [Leaf VPD](https://genetixbay.com/knowledge/glossary/leaf-vpd.md), [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), [Temperature](https://genetixbay.com/knowledge/glossary/temperature.md)

## Read more

- [VPD explained: vapour pressure deficit for cannabis growers](https://genetixbay.com/knowledge/environment/vpd-explained.md)

## Sources

1. Nelson JA, Bugbee B, 2015. Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS ONE 10(10): e0138930. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598144/