# ISET Documentation

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The main ISET page describes downloading, installation, and programming tips. There is a great deal of general ISET documentation at ImagEval. This ISET Documentation page contains preliminary documentation and conceptual notes. It is a place for thinking out loud and for clarifying some of the complex issues.

#  Units

##  Scene

A general description of radiance units is shown at the right. There are more detailed descriptions of radiance units in various class presentations and the SPIE Tutorials. This note explains why the scene is stored as a set of spectral radiance values rather than a set of spectral radiant intensity values.

Spectral radiance is a measure for an extended surface. The radiance has units that include the normalization for area: [itex] W / sr / nm / m^2 [/itex]. The spectral radiance surface area is normalized by the surface seen from a specific direction.

Images are not extended surfaces; rather they have only localized scattering that seems as if it would be more appropriate as specifying in terms of the point scattering of a spectral radiant intensity. The spectral radiant intensity differs from the spectral radiance in that the radiant intensity [itex] W / sr / nm [/itex] is not normalized for the surface area of the emitting region (which is assumed to be a point).

In ISET we frequently change the scene field of view (and sometimes the viewing distance). This changes the surface area represented by each sample point (shown in the upper right text field of the scene window). If we stored a value that is not normalized for surface area, we would have to scale the spectral radiant intensity by the new surface area every time we changed the field of view (or the viewing distance).

Instead, ISET stores the spectral radiance. This is the value we would measure if we arrayed the data at the sample point to form an extended surface and measured the spectral radiance. This value is normalized per square meter of the extended surface. When we plot the spectral radiance from a region of interest (ROI) that includes multiple points, the plot shows the weighted sum of the spectral radiances of the ROI points.

To calculate the spectral radiant intensity from the spectral radiance, it is only necessary to multiply the spectral radiance by the surface area (in square meters) of the region of interest.