You may adjust the exposure/lighting of your webcam via the following methods:
1) If you are NOT using AR FX:
Open PBU and browse to your Camera tab. Under 'Video Device', choose your webcam (HD Pro Webcam C920) and then click 'Configure'. A Properties window will open where you can adjust exposure levels. Use the live-view preview on the right side of the screen to see the results until you are satisfied with the changes. Click on 'Apply' when finished and your settings will be saved.
2) If you ARE using AR FX:
If you plan on using AR FX (Augmented Reality) then you will need to change your exposure settings with both PBU and SnapCam not running. To Verify they are not running, make sure PBU is closed and that SnapCam is not showing in the Windows Quick Access Menu. If you see the Snap Camera icon (which looks like a yellow circle), right-click on it and choose 'quit'. This is shown below:
You will now type 'Logitech' into the Windows Search bar and click on 'Logitech Capture' as shown below:
If you do not have Logitech Capture installed on your photo booth, you may download and install it here.
With the Logitech Capture app opened, you may now make changes to your webcam exposure. Change the Image Adjustments options on left side as shown below, using the live view on the right side to view the changes until you are satisfied.
Once you are satisfied with the changes, simply close the Logitech Capture app to save the settings. Launch PBU and your new exposure settings will be ready to use.