The details of “object as detector” option is mentioned in:

The Setup Tab > Editors Group (Setup Tab) > Non-sequential Component Editor > Non-sequential Detectors > Objects as Detectors

One important thing is the number of Pixels:

 The pixels are numbered in the order the polygons are defined, and there is no general way to describe the pixel ordering. For these objects, some experimentation may be required to determine the exact pixel ordering method.

For most of the objects, the pixel size can be changed in the Object Property > Draw > Drawing Resolution.

It is mentioned in:

The Setup Tab > Editors Group (Setup Tab) > Non-sequential Component Editor > Non-sequential Detectors > Detector Surface Object

OpticStudio uses a collection of triangles to conform to the shape of the curved surface. There are Nr radial zones, and Na angular zones. Together these two parameters define Nr * Na polygonal regions. Each region is generally divided into two triangles, so the total number of triangular pixels is 2 * Nr * Na. However, if the minimum aperture (parameter 4 above) is zero, then the innermost radial zone has only Na triangles rather than 2 * Na. In this case the total number of triangles is 2 * Nr * Na - Na. The algorithm that selects the triangle coordinates attempts to make the area of each triangle approximately the same, however the area from triangle to triangle can not be held exactly constant. The corners of each triangle will lie on the aspheric surface defined by the radius, conic, and aspheric coefficients. The triangles are flat and do not lie exactly on the surface if the surface is not flat.

The triangles are the individual pixels used to record irradiance and flux data. The pixels are numbered in consecutive radial zones. Pixel 1 starts at angle zero in the innermost radial zone. The first 2 * Na pixels (or just Na pixels if the minimum aperture is zero) are numbered going counterclockwise around z to complete the first annular zone. The pixel numbering continues at angle zero in the second radial zone for the next 2 * Na pixels, until all radial zones are complete.

For extruded object detector, the pixel number can be modified like mentioned in this community post:

How to change pixel size in extruded object detector? | Zemax Community

For a STEP/IGES/SAT CAD file, the pixel size is controlled by the parameter Chord Tolerance in Object Properties > CAD. See 'Chord Tolerance' for more information. For a STL file, the pixels are decided by the triangles defined in the file and cannot be changed.

Another important thing is how to read the data on this detector.

It’s mentioned in:

 The Setup Tab > Editors Group (Setup Tab) > Non-sequential Component Editor > Non-sequential Detectors > Objects as Detectors

The intensity detected will be displayed on the Shaded Model display, and on the text listing of the Detector Viewer window. This means that you can’t see the result in the graph tab of the detector viewer.

Please note that: Detector Surface objects use a collection of triangles to approximate a curved surface that can detect flux and irradiance. The display of the detected data requires conversion of the triangles into square pixels. This conversion is done automatically when displaying the detector data as a false color, grey scale, or cross section plot. The number of square pixels selected is adjusted to provide a reasonable display resolution, and will vary depending upon the number of triangles. For text listings of Detector Surface objects, the X, Y, Z, R, and Angle data will be listed along with Flux and Irradiance values for each pixel. The X, Y, and Z values are the coordinates of the center of each detector triangle. The R value is the distance from the center of each detector triangle to the object vertex. The Angle values are the angle in degrees to the center of each detector triangle as projected to the XY plane.