Auto Exposure in Openlab

Technical Note: 162
Reads: 6594
Creation Date: 29/01/2001
Modification Date: 12/02/2004

Auto exposure is used to quickly estimate an appropriate exposure setting for a particular illumination situation. It may well require further manipulation to achieve the exact image required.

Auto exposure will operate on a region of interest (ROI) drawn on the image, if an ROI is not present it will operate on the current live document size, otherwise it will use the whole chip for the calclulations.

The sample must be illuminated and the camera set up so that the best quality image is formed. If you wish to perform auto exposure on an ROI make sure the region is selected before starting the auto exposure process. Click on the Auto Expose (AE) button and the software will vary the exposure, a progress bar may come up from which you can stop the process should you wish to. While that progress bar is on the screen the auto exposure protocol can be described thus:

Auto exposure samples an image, it then checks the maximum pixel value of the image, on the basis of this it estimates a new exposure and captures another image. It continues to perform the operation until one of the following:

1. The exposure time is too short (shortest exposure time for camera has been reached). The image is too bright. Openlab will present a message to that effect.

2. The exposure time is too long (either 1 minute or the longest exposure time for the camera, which ever is shorter). The image is too dark. Openlab will present a message to that effect.

3. The brightest pixel of the image is within 80% of the max pixel value for the camera. Approximate optimum exposure reached.

If you believe the message that the image is to dark is incorrect, ie the image you view down the eyepieces should be detected by the camera, check the microscope video port to make sure light is going to the camera at the time when you click the AE button. Check for the presence of filters or barriers which may prevent light reaching the camera. Also check other camera controls such as offset and digital gain which can make the image appear darker than it actually is.

The length of time the Auto Exposure process takes is mainly camera and exposure time dependent. The weaker the signal and the slower the camera, the longer Auto Exposure will take. It is therefore inappropriate in some situations.

Auto Exposure cannot function on a moving target! The more change occurring in the field of view, the less likely Auto Exposure is to settle on a value. This also applies to samples which photobleach rapidly. In these situations Auto Exposure is not an appropriate tool to use.

Auto exposure is not relevant for video cameras as the video standards set fixed frame rates and therefore exposure times.