FIGURE 4.

Fits (lines) of the experimental dual-color PCH (diamonds) to the ideal model (A) and to a model including afterpulsing (B) and their normalized residuals. The sample is a dilute aqueous solution of Alexa 488 () so deadtime effects are negligible. The ideal model fails to describe the experimental dual-color PCH as evidenced by the large reduced χ² (χ² = 24.0) and residuals. Including afterpulses into the model improves the fit significantly, yielding a reduced χ² of 1.5. The afterpulse probabilities returned by the fit are q_A = 0.0070 ± 0.0006 and q_B = 0.0049 ± 0.0006, in excellent agreement with our independent measurements (Table 1, Instrument 2). A neutral density filter was used in the emission path to reduce the brightness of the sample.