Fig. 2.

The response waveform comparison (RWC) method. a, b, c The interpolation of the calibration responses. d, e, f The principle of estimation of effective intensities of the responses. As an example, we estimated the effective intensities of the responses from the same calibration run to perform self-validation of the method. a Measured calibration responses to light pulses with intensity increasing in 0.25 log unit steps over a range of 4 log units (from −4 to 0). The response at logI = −2 is shown in red, and the responses at logI = −4, −3, −1 and 0 are given in blue. The black line marks the shutter opening. b Waterfall plot of the interpolated response array. The measured responses are shown interlaced at their nominal intensities. c Comparison between the measured and interpolated responses. Interpolated responses (spaced 0.05 log unit apart) are shown in black. The measured responses are shown as in a. The inset below the responses shows the difference between the measured and interpolated responses. d Waterfall plot showing the absolute sample-wise difference between the measured response to a light pulse at intensity logI = −2 (red trace in c) and the interpolated response array (shown in b). As expected, the sample-wise difference is close to zero (black colour) in the region around logI = −2. See also the red RMSD profile in e. e RMSD profiles (root mean square sample-wise difference between the measured response and each of the interpolated responses) for some of the calibration stimuli. The positions of the minima of the RMSD profiles were used as the estimators of the effective intensities of the applied stimuli. The red curve presents the RMSD profile for the response to a light pulse at logI = −2 (compare with d). The RMSD minimum for the measured response (0.47 mV) is reached at logI = −2.04 (red circle in e, f). Blue curves, showing the RMSD profiles for the measured responses at logI = −4, −3, −1 and 0, and the black thin curves (responses to intensities at 0.5 log units in between) have their minima close to the applied intensities. f Waterfall plot of the RMSD profiles of all the measured responses from the calibration run. The white crosses, representing the minima of RMSD curves for each stimulus (repeated as solid blue diamonds in Fig. 3), show a close correlation between the applied and estimated intensity (colour figure online)