Opacity
by Dr. Brad Paulson, Thor Engineering, ICMA Standards Representative
The ISO test method document, 10373-1, Identification Cards – Test methods – Part 1: General Characteristics has been under periodic review, and is currently in final public review stage in preparation for publication. ISO/IEC 10373-1 contains test methods to measure the parameters defined in ISO/IEC 7810, Identification cards – Physical Characteristics. In the next version of 10373-1, a note will be included as part of the opacity test method, that two tests are presented, as a result of planned changes in the base standard, ISO/IEC 7810. As such, this seems to be an appropriate time to describe the planned changes in the 7810 opacity requirement and changes in the 10373-1 opacity test methods.
As Joe Naujokas pointed out in the March/April 2001 issue of Card Manufacturing, opacity was not included in the ISO card standard and test method documents until 1993, then measured with broadband transmission spectrodensitometry. Recognizing the use of infrared (IR) light emitting diode (LED) emitters and sensors in card handling systems, the test method was refined in 1998 to use the spectrophotometer, which could measure IR wavelengths, but limits for measured optical density were not included until the 2003 edition of ISO/IEC 7810. During the last periodic review of ISO/IEC 7810, discussions pointed out that visible light sensors were virtually unknown in current ATM designs, since the advancement of IR sensors. However, the specification limits (an optical transmission density greater than 1.3 for wavelengths from 450 through 950nm and greater than 1.1 from 950 though 1000nm) remain unchanged in the 2003 edition of ISO/IEC 7810. A note was added to give notice that there will be no opacity requirements for wavelengths from 450 through 850nm in the next revision of ISO/IEC 7810.
As noted earlier, the method of measuring opacity for the 2003 edition of ISO/IEC 7810 has been to measure the transmission of light through an object with a spectrophotometer, with an integrating sphere light-diffusing chamber æ clearly, a specialized piece of laboratory equipment. Attenuation of light by the object is calculated by comparing the light transmitted through the object to the sensor with light transmitted between the light source and sensor without the object between them. Optical density is calculated as the negative logarithm of the fractional transmission, so an optical density of 1.3 corresponds to 5% transmission and an optical density of 1.1 corresponds to 8% transmission of the incident light through the object.
During discussions of the opacity test method, it was further observed that no card equipment uses spectro-photometers to sense the cards, but, rather, LED emitters and sensors. Therefore, to more accurately reflect the technology used in equipment to sense cards, a test method was developed with IR emitters and sensors to measure opacity. Currently, the most commonly used IR emitters are GaAlAs (Gallium Aluminum Arsenide) and GaAs (Gallium Arsenide) LEDs, with nominal peak emission wavelengths in the infrared, at 860 and 950nm, respectively. The new test method compares the electric current caused by infrared light passing through the card between the emitter and the sensor to the current created by at the sensor when there is the reference material between the emitter and the sensor, at each of the wavelengths, 860 and 950nm. This test method is intended to support the next revision of ISO/IEC 7810.
In conclusion, while the new test method uses techniques, the standards groups actively worked to develop metrics that are expected to more accurately reflect technology currently used, with a less complicated, and far less expensive, piece of equipment. Further, the reference material was chosen to maintain the past transmission opacity limit in the IR range.