Experiment 07 with IR indicators:

Detection of the IR part of sunlight

This experiment shows in a very impressive way the efficient conversion of infrared sunlight into visible green light when using the IR indicator IRI 4400.

Required equipment

IR source: Sunlight without clouds
Dark sunglasses
Magnifying glass (Fresnel lense in credit card format or lense with max. 5 cm diameter)
Infrared filter for separation of visible light (foil)
Black paper
IR indicator: IRI 4400

Safety measures

Do not direct focussed sunbeam onto black surface of indicator card (heat damage/danger of fire).
Do not expose the passive card material to temperatures above approx. 50 °C (heat damage). The phosphor chip withstands locally and for a short time 200 °C. This is important to consider if the magnifying glass has a diameter above approx. 5 cm, or if operated without IR filter which would cause a too high energy concentration in the focal spot.

Preparation: Positioning

Wear dark sunglasses.
Hold the magnifying glass into the sun without IR filter; direct the sunbeam onto black paper to determine the focal point (smoke development).
Try the same with an IR filter until the paper smokes. Appropriate IR filters: many black plastic films (try out!) or developed unexposed photo slides.

Realization: IR detection

Change the paper against the IR indicator card. Take care that only the frontside of the phosphor chip is hit. It will emit a bright green light at the focal point of the infrared part of the sunlight. The phosphor chip of the IRI 4400 does not absorb all energy like the black paper, but passes a major part or reflects it. Another part is changed into green light which is emitted again. Therefore, if the safety measures mentioned above are observed in detail, there will be no heat damages.