Main electrical panel beside a subpanel with its cover removed
Generic photo: Gvolk · CC BY-SA 3.0

Using an infrared thermometer on a service call: emissivity, spot size and what it cannot read

An IR thermometer reads the infrared energy coming off a surface, not the air or what is behind glass. Shiny metal emits little infrared and reads low; a strip of electrical tape or flat paint gives a surface it can read. The meter measures a circle that grows with distance (the distance-to-spot ratio), and the laser dot only marks its center. Glass blocks the infrared it uses, steam and dust scatter it, and a meter brought in from a cold truck needs time to settle. Use it to compare and find hot spots; use a contact probe when the number matters.

Cross-checked ~10 min easyBrands: All

Before you start

  • Never point the laser at eyes
  • Measuring live connections means working near exposed voltage: keep distance and use the meter's spot ratio instead of reaching in

Diagnostic steps

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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Using an infrared thermometer on a service call: emissivity, spot size and what it cannot read · OnCall Tech