What are Grey Balls used for?

Question

I know that a Grey Card is used for the white balance of the camera, but whate are the Grey Balls used for?

On big Productions, there is usually a guy (what is his job?) that holds up a grey ball and a "mirror"-colored/reflective ball. What is this used for? Is this for digital effects added in post and to see the light and the direction of light? What is the purpose of them?

Also, sometimes, there is the use of a reference card (a paper with many little colored squares on there). What is that used for?

Is this to adjust the color of the digitally added material?

This can't be added to adjust the images in post, because they should be the same (because of white balance), is it? Does dynamic range matter with these kind of things?

What are these balls called and what for are they used? What Role/Job does the person holding them have? Does he has other duties or is he/her just holding them?

Answer 1

The grey balls are actually used like the grey cards but with added lighting data cause they are spherical and refer to the surrounding light sources better, they usually reflect 18% of the light and are used to match the temperature of the CG Element to the footage and also exposure.

We create the CGI versions of the grey and chrome ball since they are very easy to make and mostly mathematically very close to the real balls. So if your CGI balls don't match the pictures of the chrome/grey balls then you need to adjust:

1. intensity of the CGI Lighting,
2. exposure of the lights (individual or mostly overall) and finally
3. the white balance or the temperature of the CGI Lighting to make it fit.

Sometimes they use the Mirror or Chrome Balls to take HDRIs to roughly recreate the surrounding lighting using a DSLR and multiple bracketed shots but nowadays VFX or On-set supervisors, the guy, you were asking about, used a compact 360 cameras like the Thetha Z1 or Insta 360 One X2 or even a 180 degree fisheye lens on a camera and quickly snapped the images without worrying about capturing what's behind the chromball or the other tedious job of actually paining out the camera and/or cameraman.

The small colour squares you see are called colour checkers and they come in many types. They are used to unify the CG Camera and the footage in terms of colour profile, dynamic range and white/black points; usually there is a separate bigger card for white, black and grey points but all in all it's to mimic and dumb down CG Cameras to the characteristics of the real world cameras used to capture the image.

Answer 2

Yes, they are used for adding visual effects in post. The mirrored ball is for creating an environment map or reflection map. These maps provide the same lighting from the real scene to the virtual scene where graphics are created to composite into the existing scene.

I believe the grey ball is used for recording more diffuse lighting from all angles, though I'm not positive.

The cards with either black and white or colored square patterns on them are for motion tracking, image stabilization, and orientation. They're often used to replace computer or mobile phone screens, for example.