Near-Infrared (NIR) Heating and Penetration Depth

Near-infrared (NIR) energy was discussed in relation to coffee roasting and equipment such as Stronghold roasters. A community member stated that Stronghold has near infrared and that it is efficient on the surface. Treat this as a community technical note rather than a full specification of Stronghold heater design. source

Reference Spectrum

A separate NIR spectrum of polystyrene was collected as a reference in the cited discussion. source

Reported Penetration Depth by Wavenumber Range

The same source reported that NIR penetration depth varies by wavenumber range:

• Approximately 1–2 mm in the 4000–5100 cm⁻¹ range. source
• Approximately 3 mm in the 5100–7000 cm⁻¹ range. source
• Approximately 5 mm in the 7000–9000 cm⁻¹ range. source

These values suggest that NIR interaction is relatively shallow and wavelength-dependent, which is consistent with the practical note that NIR heating can be especially surface-effective. source