Laser Viewing Cards

The probably most common sensor cards for the near infrared wavelength region (used e.g. for working with YAG lasers) contain some kind of phosphor. Before use, the phosphor must be “charged” by illumination with visible light, i.e., sunlight or artificial light. The material then generates a low level of phosphorescence (afterglow), which can hardly be recognized. However, when hit by infrared light, the material releases the energy stored during the charging process much more quickly, and the illuminated spot can be seen (often in orange color) with the naked eye. This visual impression lasts only until the stored energy has been exhausted; one then needs to recharge the card or just move the laser spot to another location which has not yet been used.

Depending on the used phosphor, the wavelength region with good sensitivity can be narrower or broader. The wavelength range specified by the manufacturer often includes regions where the sensitivity is actually very low, while quoted numbers for the sensitivity often apply to wavelengths with maximum sensitivity. Under ideal conditions (optimal wavelength region, short-term use after charging the card with bright visible light), some cards allow one to detect infrared intensities of the order of a few μW/cm², whereas in other cases one requires multiple mW/cm².

There are rare-earth-doped materials which can upconvert infrared radiation to the visible region. Such materials do not need any charging before use. However, the upconversion process requires higher optical intensities, i.e., the sensitivity of such cards is substantially lower. (It can actually be better with pulsed light.)

Laser viewing cards for ultraviolet light can directly produce visible fluorescence light and do not need any charging before use. Because such processes can be very efficient, the achieved sensitivity is usually quite high. In some cases, there is also substantial level of phosphorescence, which may last for several minutes.

Particularly for use in the mid-infrared spectral region, thermochromic detection cards are available. These contain a material (e.g. a liquid-crystal film or a material containing certain pigments) which changes its color when it is heated to an elevated temperature. Depending on the type of material, the color may change e.g. from green to brown or black, or from dark to white.

This operation principle means that the laser intensity must be high enough (e.g. above 100 mW/cm²) to raise the local temperature by some tens of Kelvins. On the other hand, the laser intensity should not be far higher, because otherwise the material could be destroyed. Therefore, the usable dynamic range is fairly small – often with a factor less than 10 between the damage threshold and the minimum detectable intensity.

The visible spot remains until the material has cooled down again. That process may be accelerated by pressing the card against some cool metal part, for example. If the ambient temperature is too high, it may not be possible to use such a card.

An advantage of the thermochromic operation principle is that it works in a very wide wavelength range, because only sufficiently high absorption of the laser light is required to achieve the temperature rise. For shorter wavelength, however, other cards (e.g. based on phosphorescence) are normally preferred due to their much higher sensitivity and larger dynamic range.

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