In firebrand deposition scenarios, there are a few possible outcomes: no ignition, smoldering, and flaming ignition. The criteria for determining smoldering or flaming ignition is generally qualitative in nature. Experiments were conducted to determine a quantitative ignition criterion. Firebrands were deposited on cellulose insulation fuel beds under an impinging air jet at two different velocities while thermocouple measurements were taken in the fuel bed and cameras recorded the tests in the visible and IR spectrum. The firebrand temperature and the temperatures within the fuel bed were insufficient to predict the ignition of the fuel bed. However, using the IR camera to monitor the growth of the reacting area in the fuel bed, a quantitative definition of ignition was found. Over the course of a test, different growth rates, representing different phases in the ignition process, were apparent in the non-dimensional reacting area. The initial phase, right after deposition, increased as 0.003 1/s, and the final phase average growth rate was 0.18 1/s. The time to the final growth rate matched well with the flaming ignition times recorded in the visible videos. The average error between the observed and predicted time to flaming ignition was 12%. This non-dimensional reacting area analysis provides a framework for determining ignition in a quantitative way.