Clear Cards and Infra-red Blocking Technology

By Dr. Brad Paulson, Thor Engineering, ICMA Standards Representative

We’ve all seen them, those neat looking clear cards. In fact, clear cards have done well in the voting for the Elán awards over the past few years. Although the base card standard, ISO/IEC 7810, avoids defining the materials of construction, it is possible that no other design has created so much materials development in order to meet the materials requirements. Clearly, few innovations in card design have generated so much controversy and discussion.

Generally speaking, ISO/IEC 7810, 7811, and 7816 specify the minimum requirements for international data interchange. Fundamentally, the requirements are dimensional, that is, the size, shape, and thickness of the card and the location and placement of the machine-readable components, such as magnetic stripes or integrated circuit (IC) chip contacts. Typically, these requirements are sufficient for cards to be used successfully in swipe and insertion readers, where the readers are designed to read data as the card is manually presented to the reading head.

However, equipment with card transport mechanisms, such as some personalization equipment and automated teller machines (ATMs), require that the card be “seen”. Initially, these switches were mechanical, but, as they have become more readily available, they are now almost entirely the more reliable light emitting diodes (LEDs) and sensors. Furthermore, to avoid spurious signals caused by ambient light, equipment manufacturers have tended to use LED emitters with peak emission wavelengths in the infrared region of light, commonly nominal peaks at 860 and 950 nm. This allows for design of cards that are opaque to infrared light, meeting the equipment requirements, but transparent to visible, white light—the clear card.

One method of making a clear card visible to the card sensing equipment is to reflect the wavelengths of the LEDs. To this end, inks have been developed that will reflect infrared radiation, yet allow visible light to pass. When the card, which appears translucent to the eye, passes under an infrared LED, the light is reflected, blocking the light and making the card visible to the sensor.

A second method elaborates on the reflectance principle, using multiple layers with different indices of refraction to set up destructive interference of light with predetermined wavelengths. An example of destructive interference is to place an object in a pool of water; waves reflect from the object and at particular locations, no disturbance is evident as the reflected waves cancel the arriving waves. By selecting clear layers of different indices of refraction and thickness, selected wavelengths of light can be eliminated from passing through the clear card. This technology is evident as a shifting of the apparent color of the card when viewed at different angles.

A third method is to use materials that absorb, or block, infrared light. As pigments tend to be opaque, blocking visible light as well as infrared light, these components are typically dyes dissolved in a solvent or ink. However, it is possible to incorporate them in the clear plastic as well. Typically, dyes and pigments are selected by the wavelengths of light they reflect, which determines the their visible color. In the case of infrared blocking dyes, dyes are selected by the wavelengths of light that they block. As blocking bands at 860 and 950nm is important for ATM use, dye manufacturers are working to provide blocking at those wavelengths, while minimizing the reflection of light in the visible wavelengths.

Since light is a continuous blend of wavelengths, blocking infrared usually impacts visible light as well. Selecting the appropriate technology requires balancing the desired activity to infrared light, while limiting the activity in visible light. So in conclusion, the quest for true clarity in materials development groups will be maintained, as long as card designers and manufacturers continue to push the creative limits of transparency.

Brad Paulson, Ph.D., is the ICMA Official Standards Representative and concurrently serves as principal and founder of Thor Engineering, a company he started in 2001 to test and evaluate media and materials to determine failure modes and recommend material and process improvements. Views expressed here are his own. Questions? Contact Brad at Tpaulson@rconnect.com.