A pulsed IR led I use in low frequency, 400 Hz. Is different a beam in wich you introduce the frequency, than pulsed the led at this frequency. Maybe pulsed led doesn't measure the temp. but reveals the "phenomenon" around long time buried metals. The precission is very high regarding other LRL systems, but here you assume that you must uses strong pulses.
Other system wich reveals the phenomenon is a cheap Chinese laser pointer, but you use here more low voltage, near 4.5 V. With IR leds you can use 9 V. 10-12 ohms resistor is necessary, and at this point the IR led works in the limit.
Regards
Esteban
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