Comment on “A critical assessment of the Hapke photometric model” by Y. Shkuratov et al.

Criticism of theories and models is an important part of science and can lead to increased understanding and improvement of the models. Unfortunately, the paper by Shkuratov et al. [1], which severely criticizes a model for the reflectance of a particulate medium, does little to advance this goal. The paper contains a large number of misrepresentations and statements that are completely false. Important references that address the supposed errors are ignored. Many of the criticisms appear to arise from a profound misunderstanding of the model. The paper correctly criticizes the model for being approximate. However, the assumptions and approximations have been clearly identified and justified in the relevant papers, and it should be noted that all extant models for the reflectance of a close-packed particulate medium are approximate. One of the problems that the model shares with all physically-based models is that some of the parameters have similar effects on the reflectance, which sometimes makes it difficult to retrieve unique values. However, this arises from the properties of the physical processes involved in reflectance. Thus, it is a property of nature and does not mean that the model is wrong. This problem can often be overcome by judicious choice of parameter retrieval methods.

► This paper answers criticisms by Shkuratov et al. of the Hapke reflectance model. ► Nine of the 15 supposed shortcomings of the model listed are completely false. ► One of the “shortcomings” is actually a new discovery enabled by the model. ► The model is admittedly approximate, but so are all extent models. ► When using any physically-based model, parameter retrieval must be done with care.