Alexandre Maller asked the following question:
You’ve answered your question. To the meter, the white surface of the incident light receptor is the subject. The hemispheric-shaped Lumisphere approximates the 3-D nature of a face and reads light striking it from 180-degrees averaging what it “sees.” The flat disk allows the metering cell to only “see” light directly in front of the meter. It is used to determine the brightness of lights in a multi-light setup. This difference in brightness is the building blocks for lighting ratios. If the main light is two times brighter (f/8) than the fill light (f/5.6) the ratio is 1:2. The Lumidisc is also used to measure light falling on a flat surface light a painting or a green screen for video shooting.
Please get back to me if you have any other questions about Sekonic meters.