Gray sphere or reference card logs light quality, direction, and color temperature on set — used later to reconstruct lighting for VFX or grading.
You unpack the gray sphere and place it in the scene — that's your insurance for later. The lighting reference documents the exact state of the lighting at the moment you are shooting. Without it, you have to guess how the lighting situation actually was during grading or VFX work. With it, you have an objective representation: color temperature, intensity, direction, quality (hard or soft), ratio between key light and fill light.
In practice, you document the reference at least once per setup — ideally with every lighting configuration. Place the gray sphere (or reflector card) exactly where your talent will be later. Exposure metering according to the Zone System or direct comparison with the later subject is crucial. Some DoPs also photograph the reference from different angles: frontally to the camera to capture the reflection, then from the side and from the back to understand the entire lighting architecture. The image with the reference often runs for a few frames through the footage — directly at the beginning or end of a take — or you can shoot it separately. The VFX supervisor and later the compositor need this information to correctly integrate 3D objects, green screen elements, or subtitles into the existing light. Incorrectly calculated light in CGI elements is immediately noticeable; with good reference, you avoid expensive rework.
During digital grading, lighting references are less critical, but still helpful: they show you and the colorist what the original lighting atmosphere was before color adjustments are made. Especially in scenes with extreme or artificial lighting (neon light, fire, special practical lights), the reference helps to ensure authentic reconstruction. You document not only color values but also the consistency across multiple takes or even shooting days — an underestimated advantage.
Technically: Gray spheres reflect neutrally and evenly. Reflector cards (white, gray, silver) give you a quicker impression of light intensity and direction. Modern sets also use 360° HDRI (High Dynamic Range Imaging) recordings as an extended lighting reference — these capture the entire lighting environment for precise VFX integration. The old rule remains: one minute of documentation saves you hours of post-production.