Alternate Readout Mode for Photometrics PVCAM cameras, specifically the CoolSNAP HQ.

Technical Note: 243
Reads: 7352
Creation Date: 14/09/2001
Modification Date: 12/02/2004

Alternate Readout Mode is a feature of the PVCAM drivers used by Openlab to interface with the Photometrics CoolSNAP range of cameras supplied by Roper Scientific. This mode is controlled by a check box available in the Video Manager dialog or the Video Controls palette.

When the check box is checked Alternate Readout Mode is on, when the check box is unchecked Alternate Readout Mode is off.

For the majority of cameras supported by Openlab this mode is redundant and checking or unchecking the box will have no affect on camera performance or use. It is however always available from the driver and therefore cannot be disabled in Openlab.

In Normal readout mode, when the Alternate Readout Mode checkbox unchecked, antiblooming is enabled. This means that the camera prevents saturated pixels from overflowing into their neighbors which will change the perceived size of oversaturated objects. This reduces slightly the sensitivity and quantum efficiency of the camera compared to Alternate Readout Mode.

In Alternate Readout Mode, when the checkbox is checked, antiblooming is disabled and therefore the camera is at its most sensitive. This mode is designed for low light level applications.

For further information including applications which might require the different modes please see the technical note produced by Roper Scientific entitled "CoolSNAP Interline Cameras" available from the Roper scientific web site:

Note: Autoexposure

Using Autoexposure with the Coolsnap HQ with Alternate Readout mode turned off very often does not work. When antiblooming is switched on, saturation is never actually reached and Openlab cannot tell that the exposure time is high enough. On this particular camera it is therefore impossible for Openlab to accurately detect the saturation point and therefore determine an appropriate value for the exposure. Autoexposure cannot give the same type of results as it does when the camera is in Alternate Readout mode.