Images taken using the global shutter module are much darker

OpenMV Boards
1

Hi, I’m trying to capture short-exposure images (~1ms) using the global shutter sensor, but they’re much darker than grayscale images taken using the standard sensor.

For example, using the standard sensor with gain=12dB, whitebalance=(0dB,0dB,0dB), and exposure=5ms, a moderately-bright indoor scene looks like this: 01-openmv-standard-sensor.mp4 - Google Drive

Using the same gain and exposure, but swapping in the global shutter module, the video looks like this: 02-openmv-global-shutter-sensor.mp4 - Google Drive

12dB seems to be the maximum amount of analog gain that the global shutter module supports, but the datasheet mentions that the global shutter module can also apply up to 3.75-fold digital gain (page 24). I gave that a shot, setting the registers 0x80 through 0x98 to 15: 03-openmv-global-shutter-sensor-with-digital-gain.mp4 - Google Drive

This makes things brighter, but there’s an odd flickering that’s very pronounced. Taking the exposure time down to my target value of 1ms it’s pretty hard to see what’s going on: 04-openmv-global-shutter-sensor-with-1ms-exposure.mp4 - Google Drive

Is this a normal amount of read noise, relative to the amplitude of the signal I’m capturing, or could there be something wrong with my module?

*It looks like the Google Drive video previews are pretty aggressively compressed, but downloading the videos should work; they’re very small.

2

Hi, yeah, it’s a different sensor with different properties. So, expect completely different behavior than the normal sensor.

As for the read noise… that’s because our boards have pretty simple filtering on the power supply rails. You have to increase the exposure a lot more to get rid of that. When you run the camera at any of the high FPS rates you typically see line noise.

Note that this chip has no ISP. It’s a raw grayscale sensors. Literally the image you see is the direct output per pixel from the camera sensor array.

The normal camera (MT9M114) has an ISP onboard that cleans up the image significantly.

3

Got it, thanks! Is the physical interface between the camera module and the board some kind of standard connection I could find other modules for, or are the modules listed on the OpenMV site pretty much the only options?

4

No it’s a custom interface, but you could design your own sensor module.

5

We have a custom interface as you’ll notice we have I2C, SPI, 8-bit parallel, and even pins reserved for 2-lane mipi on the camera connector.

1 Like
6

Definitely beyond my current talents, but it’s good to know this is possible, thanks!

7

Any plans to produce a more modern global shutter camera module? The Omnivision sensor/module is listed as ‘obsolete’ and newer chips with 8 bit parallel interfaces have much better sensitiviity (especially IR) and noise figures and are still pretty inexpensive.

8

Right now no, it’s a rather huge time investment per sensor and the current one still works well for a lot of people.