For most Department of Defense (DoD) installations, the prime method of determining compliance with air regulations (and usually the primary backup for in-stack instruments) relies on trained observers to visually estimate the opacity of a plume every 15 seconds for a specified period, then average those estimates to determine a single reading that is compared to permitted levels. This is known as Environmental Protection Agency (EPA) reference Method #9. The sampling period may be as short as 6 minutes or as long as 1 hour. To qualify as a visual observer, an individual must attend a “smoke school” every 6 months. A smoke school consists of both classroom and field training, with a series of visual observations of a calibrated smoke source constituting the qualification examination. That examination requires that the observer estimate the opacity of 25 white and 25 black smoke plumes within ±15% accuracy for an individual observation and be within 7.5% overall. Sample opacity ranges from 0 to 100% in 5% increments.

 While Method #9 has an extensive history of use, it is both subjective and expensive in terms of dedicated work force. In addition to the cost of the school (approximately $350 per student and per diem/travel as needed), the five days of missed work add up significantly when multiplied by the number of students (e.g., 35 at Hill Air Force Base [AFB]) and the twice-yearly training requirement.


The objective of this demonstration was to field a digital opacity compliance system (DOCS) to be usable as an alternative to certified EPA Method #9 observers. To accomplish this, a digital recording and analysis system for opacity compliance measurements was demonstrated and validated as an alternative to the current opacity measurement procedure. The system uses a commercial off-the-shelf digital programmable camera to capture images of a smoke plume. These images are then downloaded to a standard personal computer where they are analyzed to determine opacity. DOCS is designed to produce opacity readings, which are accurate, and to provide a permanent digital record of the image for future reference. Preliminary testing showed the system to be highly effective in quantifying smoke plume opacity and in determining the opacity of fugitive dust. For example, DOCS had been used to take preliminary, limited measurements at a visible emission-training program; this preliminary investigation showed that DOCS easily passed the requirements of precision and accuracy as established by Method #9.

Demonstration Results

Demonstrations of the DOCS image acquisition and processing system were conducted at three commercial smoke schools—Ogden, Utah; Augusta, Georgia; and Columbus, Ohio—to demonstrate the system’s ability to qualify as a smoke reader would. Also, demonstrations at commercial and government sites in Ohio and Alaska were conducted to evaluate quantitatively how the system performs compared to Method #9 observers in the field. Site measurements were necessarily confined to the less than 40% opacity that is allowed by existing permits for most measurements with some excursion measurements at start-up. It was a good test of the suitability of the technology for use in the regulatory environment that exists currently rather than a test of its ability to replicate observers’ readings over the complete range of 0 to 100% opacity.

 Results indicated that the digital camera system can replace certified smoke readers in instances where contrast is expected to be good, where weather is most often clear, and where expected opacities are low. When opacities are regulated at 0%, the system is very attractive because of its forensic record of the plume. Technicians at Hill AFB have been using the system since November 1999. They and State of Utah air quality regulators are pleased with the results and are committed to future use of the methodology.

 DOCS is more costly to install than training Method #9 certified observers, but a comparison of the annual operating costs showed an annual savings of $8,090 using DOCS versus certified readers at a large urban facility and an annual savings of $14,890 at a remote facility. In both cases, the purchase cost of a DOCS system, which is continually trending downwards, would easily be paid back within a year.

Implementation Issues

The use of digital opacity measurements will assist primary end users such as Hill AFB, Utah; Wright Patterson AFB, Ohio; Fort Stewart, Georgia; and Eielson AFB, Arkansas, in demonstrating compliance with emission standards. Each base/facility will benefit to an extent, depending on the level of compliance mandated by the size and mission of the facility. Regulators’ air pollution control programs will benefit because the records of visible emissions will be better documented. This technology has far-reaching potential and could be used by many agencies currently attempting to comply with opacity requirements via EPA Method #9, particularly if the permitted opacity is very low, as is most often the case.