Infrared light made visible:

The IR indicator cards of the IRI series

A new series of infrared indicator cards in credit card form has been launched, the IRI 1100, 1400 and 4400. They distinguish themselves from the first generation both by partly considerably improved qualities and by additional functions. IR indicator cards are used for industrial and scientific examinations for many years yet; their price is accordingly high. By limiting the active surface on a measure sufficient for most applications, their price was now reduced such as they are now affordable even for hobby users.

Why infrared indicators?

Modern technical communication and storage systems are almost not conceivable without infrared light. Infrared remote controls for TV, hifi audio gear, VCR, even workstations: these are just some examples which have almost completely replaced other types of controls such as cable, ultrasonic and radio. The optical fiber connections represent the most modern form of transmission of news. Here IR light is mainly the information transmitter, because the absorption losses of optical fibers are insignificant for certain IR wavelengths. Finally there are CD players which caused the triumphant advance of the IR semiconductor lasers in the entertainment electronics.

This was made possible by the progress in semiconductor technology, which makes suitable economical IR emitters available in a large selection as light emitting diodes (LED) and laser diodes. Unfortunately, IR light is invisible for the human eye. (The normal view is limited to the wavelength coverage of approx. 380 nm (violet) to approximately 780 nm (dark red).) In contrast to ultraviolet light which excites many natural and artificial materials to show visible fluorescence with wavelengths below 380 nm, IR light (wavelengths above 780 nm) is not so easy to detect. Responsible is the Stokes law for fluorescence, which states that the wavelength of the emitted light is always longer than that of the exciting light. For IR light this means: Once IR - always IR.

Nevertheless, today a series of crystalline phosphors are known that are complicated to produce. They allow to visualize IR light without the need of electronic equipment. For the IR indicator cards of the IRI series two functional principals are used: chargeable and converting indicator phosphors.

- Chargeable IR indicators apply a phosphor which would show visible fluorescence while obeying the Stokes law under blue or ultraviolet excitation. After the excitation fluorescence remains for a while. This effect is called phosphorescence. A special additional crystalline doping largely suppresses the spontaneous emission of phosphorescence; the exciting energy remains stored (charged). Only IR light sets the stored energy free, under by-passing the Stokes law. This is called stimulated emission. When the stored energy is used, the card has to be recharged. (Fortunately, comparing to normal rechargeable batteries the number of charging cycles is not limited.) The IRI 1100 and IRI 1400 are part of that group of chargeable IR indicators.

- Converting IR indicators are able to transform IR light continuously to visible light. Converting phosphors circumvent the Stokes law by multi-photon processes. That kind of phosphors has been developed about 1970 for the production of green and blue light emitting diodes. (However, this principle was never used because of its cost and the rapid progress in developing directly emitting LEDs.) Because of the square law dependence of the visible emission on the intensity of the stimulating IR light (assuming two-photon processes) and since there is no need for recharging, this functional principle is especially advantageous for higher radiation power. The IR indicator card IRI 4400 benefits from this mechanism.

What is the structure of the new infrared indicators?

The cards of the IRI series are essentially structured like a telephone card. Thus, the proven format of credit cards of the last generation was maintained (example IR-A2). The telephone module is replaced by a plastic chip which contains the phosphor. The chip is square and has a surface of 1 cm². Thus, the active surface is smaller than the predecessor series but still big enough to cover the major part of applications. Since the surface of the phosphorescence chip determines the price, it became possible to make IR indicators with much more expensive phosphors affordable for the normal user (examples: IRI 1100 and IRI 4400). The embedding plastic has the task to protect the phosphor from environmental influences and to guarantee a long life span. In contrast to the telephone module, the phosphorescence chip is placed in a circular transparent window. Thus, a further important characteristic of the last generation is preserved: the possibility of transmissive operation. Usually it is more practical to hold the IR indicator between IR source and eye than to detect infrared light in an illumination process. (Naturally, the latter is also possible with this card.) Furthermore is has been proven advantageous to place the active surface no more in the center, but in a corner of the card. Thus, even inaccessible places become accessible.

Which indicator for which utilization?

The IRI 1100 emits visible orange light after charging in visible light when illuminated by IR radiation. Because of its extreme short rise time (in the nanosecond range), its high sensitivity in the range of 1000 nm and its high storage capacity, the IRI 1100 is ideal for testing remote controls. IR remote controls usually work with 950 nm pulsed LEDs (high frequency with a visible low frequency component). Darkening the room is only necessary for detecting smallest IR intensities. Useable from approx. 780 nm to approx. 1300 nm.

The IRI 1400 is chargeable like the IRI 1100. However, it emits green light when illuminated by IR radiation. Special characteristic is the high sensitivity for low IR intensities in a wavelength range of approx. 780 nm. However, darkening is necessary for observation. (Of course higher intensities are also detectable, but they require a recharge after a short time.) Above 900 nm, the range for conventional IR remote controls, sensitivity drops and rises again at about 1300 nm. The IRI 1400 is useable from approx. 650 nm (red) to approx.. 1500 nm. Examples are 880 nm IR LEDs or the far-field of 780 nm laser diodes for CD players. Further applications are light emitting and laser diodes for optical fibers. Because of the green emission this card is especially adapted for use with laser safety goggles which filter the red part of the spectrum but are permeable to green. The dark adapted human eye being especially sensitive for green, the IRI 1400 may be used with complete darkening for detection of very low IR intensities.

The converting IR indicator IRI 4400 represents a real novelty that was not available under this form before. This card needs no charging and recharging. It changes IR light continually in visible green light. Thus, the domain of the IRI 4400 are high power emitters under one condition: the radiation should not overheat the card (the phosphor chip supports over 200 °C for a short time, but the card material only approx. 50 °C). Maximum sensitivity near 970 nm. Useable (depending on intensity) from about 830 nm to about 1100 nm. Application examples: 950 nm IR LED LD 242 from Siemens directly at the top edge of the package; focused 830 nm laser light (30 mW); 950 nm LED remote controls (need darkness because of relatively low radiation density). Even focused IR part of sunlight is detectable without problems. Here the green emission can be seen without darkening. Like the IRI 1400, the IRI 4400 is particularly adapted for use with IR laser safety goggles.

Comparing the three types of cards, we notice that the IRI 1100 is best for detecting medium IR intensities (remote controls) in shielded ambient light. For low IR intensities (viewing in the dark) we recommend the IRI 1400. It has the highest sensitivity of all cards with wavelengths below 900 nm. The IRI 1100 is best for the 1000 nm range. In comparison with the IRI 1400, the IRI 1100 is qualified by its particularly low "self discharge" glowing. For observing high power IR LEDs without recharging we need the IRI 4400. As we see, the different characteristics of these cards are complementary. With a complete set you are equipped for most applications.

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