The real world image will be gray and not clear when I grab the UI+real world image using rgbcam capture function

I tried to grab the UI + real world image using RGBCamera Capture function. But I found the real world image will be gray and no clear in the captured image.
What should I do to brighten the real world image up while capturing image?

In addition, the code in NRHMDPoseTracker.cs:
if (leftCamera != null)
leftCamera.enabled = false;
if (rightCamera != null)
rightCamera.enabled = false;
centerCamera.depth = 1;
centerCamera.enabled = true;
if (leftCamera != null)
leftCamera.enabled = true;
if (rightCamera != null)
rightCamera.enabled = true;
centerCamera.enabled = false;

RGBCamera and Gray Camera can’t be activated at the same time?

Hope for kindly reply.Thanks a lot.