IBIS has several observing modes, for engineering and calibration purposes. However, for scientific use there is only one operating mode, Science Mode.
In Science Mode, ISGRI registers and transmits events on a photon-by-photon basis, i.e., every event is tagged with ( , ) position on the detector plane, event energy (from the pulse height and rise time) and event time.
PICsIT in principle can also operate in photon-by-photon mode. However, with the higher background compared to ISGRI, there would be unacceptable data loss. Therefore, the standard mode for PICsIT is histogram. Images and spectra (full spatial resolution, 256 energy channels) are accumulated for about 30 minutes before transmission to ground. There is no time-tagging internal to the histogram, i.e., spectral imaging has time resolution of 30 minutes.
In addition, coarse spectra, without imaging information, are accumulated by PICsIT and transmitted with far higher time resolution, but without imaging information. Thus their usefulness is limited to observations of very strong sources where the source countrate dominates the background. The time resolution, and the number of energy channels, for this spectral timing data can be commanded from ground. The time resolution can take values between 1 and 500 ms; the current default is 16 ms and eight energy channels.
In Table 2 the properties of all the modes are summarized.
| Detector Image | Timing | Spectral | |
| Observing Mode | Resolution | Resolution | Resolution |
| (pixels) | (channels) | ||
| ISGRI | |||
| photon-by-photon | 128 128 | 61.035 s | 2048 |
| PICsIT | |||
| Photon-by-Photon | 64 64 | 64 s | 1024 |
| Spectral-Imaging | 64 64 | 30 min | 256 |
| Spectral-Timing | None | 1 - 500ms | 2 - 8 |