Experiment 13 with IR indicators:

IR permeability of materials

With an infrared indicator and an IR source the permeability of foils and others for infrared light may be determined in a simple way.

Required equipment

Foil/filter/material to be examined
IR source: any IR LED of suitable wavelength or remote control
IR indicator: IRI 1100, IRI 1400 or IRI 4400
Daylight source or UV-A fluorescent lamp (e.g., a bill detector; only IRI 1400)

Safety measures

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

Preparation: Charging

Expose IRI 1100 or 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 by UV-A radiation. In this case it shows a visible green fluorescence. Incandescent bulbs are inapproriate because of their high IR level. It would discharge the card simultaneously during charging. The IRI 4400 requires no charging.

Realization and analysis

Darken the room as much as possible.
Hold the backside of the active area of the IR indicator card near the light-emitting diode/remote control. The frontside will emit a well perceptible luminescence. (Too close a distance will lead to an accelerated discharge for IRI 1100 and IRI 1400; therefore it should be avoided.)
Hold the material to be examined into the beam between IR source and IR indicator in such a way that it will cover only a part of the phosphor chip. The difference of emission intensity between the covered and the uncovered part provides information about the absorption of the material to be examined. Consider that the IRI 4400 has a contrast enforcing effect because its green emission has a square law dependence from the stimulating IR intensity.

This experiment shows that many black plastic foils are IR permeable. Therefore they are suitable cheap IR filters. This method allows to read informations hidden between two IR filter foils.