Auto exposure

From edgertronic high speed video camera
Jump to navigation Jump to search

New v2.5.2 feature: Exposure is the amount of light per unit area reaching the surface of the image sensor in the edgertronic camera (in units of lux-seconds), and is determined by the shutter speed, lens aperture, and scene luminance. Since the lens aperture and scene luminance can not be controlled by the edgertronic camera, the edgertronic camera auto exposure algorithm can only modifiy the shutter speed.

Auto Exposure is valuable in variable lighting situations,for example partly cloudy outdoor.

Enabling auto exposure

Auto-exposure-v2.5.2.png

To control the auto exposure setting, click on the wrench to bring up the settings modal, then click on the Options tab.

Auto exposure best practices

On the SC1, auto exposure will adjust the shutter speed between a maximum of ~1/frame_rate and a minimum of 4us. The user will need to set the aperture and ISO so that anticipated changes in lighting doesn't cause the auto exposure to bump into either shutter speed limit. While every application is different, you are more likely to hit the limit when lighting decreases, so setting the aperture, ISO and frame rate for the lowest anticipated lighting is often the best option.

Comparing normal exposure and auto exposure

I used a genaray MonoBright 5200K LED light source set to 10%, 20%, 30%, 40% and 50% of maximum brightness, with auto exposure disable and then again enabled. For each setting, I browsed to http://10.11.12.13/images/image.jpg to capture the live view frame. The results are shown below.

Lamp
Brightness
Auto Exposure On Auto Exposure Off
50% Autoexposure50on.jpg Autoexposure50off.jpg
40% Autoexposure40on.jpg Autoexposure40off.jpg
30% Autoexposure30on.jpg Autoexposure30off.jpg
20% Autoexposure20on.jpg Autoexposure20off.jpg
10% Autoexposure10on.jpg Autoexposure10off.jpg


CAMAPI support for auto exposure

Both configure_camera() and run() CAMAPI methods support a new dictionary key auto_exposure, which is an enum with the following definition (as found in camconstants.py)

AUTO_EXPOSURE_OFF = 0
AUTO_EXPOSURE_SATURATION = 1
AUTO_EXPOSURE_AVERAGE = 2

The version 2.5.2 webUI presents auto exposure as an on/off binary setting. However, the CAMAPI definition is an enumeration to allow for future additional auto exposure algorithms to be supported. None are planned at this time.