Statistical interpretation of ISO TC42 dynamic range: risky business

Recently, two ISO electronic imaging standards aimed at di gital capture device dynamic range metrology have been issued. Both ISO 15739 (digital still camera noise) and ISO 21550 (film scanner dynamic range) adopt a signal-to-noise ratio (SNR) criterion for specifying dynamic range. To resiliently compare systems with differing mean-signal transfer, or Electro-Optical Conversion Functions (OECF), an incremental SNR (SNRi) is used. The exposure levels that correspond to threshold-SNR values are used as endpoints to determine measured dynamic range. While these thresholds were developed through committee consensus with generic device applications in mind, the methodology of these standards is flexible enough to accommodate different application requirements. This can be done by setting the SNR thresholds according to particular signal-detection requirements. We will show how dynamic range metrology, as defined in the above standards, can be interpreted in terms of statistical hypothesis testing and confidence interval methods for mean signal values. We provide an interpretation of dynamic range that can be related to particular applications based on contributing influences of variance, co nfidence intervals, and sample size variables. In particular, we introduce the role of the spatial-correlation statistics for both signal and noise sources, not covered in previous discussions of these ISO standards. This can be interprete d in terms of a signal’s spatial frequency spectrum and noise power spectrum (NPS) respectively. It is this frequency as pect to dynamic range evaluation that may well influence future standards. We maintain that this is important when comparing systems with different sampling settings, since the above noise statistics are currently computed on a per-pixel basis.