Experiment 08 with IR indicators:

Indirect IR detection by local discharge

This experiment uses the limited storage capacity of the chargeable IR indicators IRI 1100 and IRI 1400 for detecting even very small or very high IR intensities. This method is also good for detecting IR radiation at visually inaccessible places.

Required equipment

IR source: any IR LED
IR indicator: IRI 1100 or IRI 1400
Daylight source or UV-A fluorescent lamp (e.g. bill detector; only IRI 1400)
IR remote control as second IR source for observation of inverse image

Safety measures

Shortwave UV radiation decreases the life span of the indicator card and may harm eyes! Use UV absorbing sunglasses!

Preparation: Charging

Expose the IRI 1100 or IRI 1400 to daylight or fluorescent light prior to use (charge). The charging time depends on the intensity of the exciting light. Normally, it takes less than a minute. Important is the blue part of the charging light. The IRI 1400 may also be charged with UV-A radiation. In this case it shows a visible green fluorescence. Incandescent bulbs are inappropriate because of their high IR level. It would discharge the card simultaneously during the charging.

Realization: IR detection

Hold the (partly covered) phosphor chip of the IR indicator card near the light-emitting diode (with ambient light shielded or in darkness) for a while and fix it there. If there is IR radiation, the non-covered part of the phosphor chip will emit visible light (IRI 1100: orange; IRI 1400: green).
As soon as the IR stimulated emission decreases, remove IR source or switch it off.
Expose the phosphor chip in darkness to weak IR light extensively with e.g. remote control. The parts exposed to the IR radiation in the first session will glow weeker than the unexposed ones.

Advantage: IR radiation is detectable at visually inaccessible places and can be analysed afterwards. This procedure works also for detection of higher IR intensities. But also very small IR intensities can be detected: by using the integrating effect of the discharging and by increasing the time of exposure.