Italics from the original have been set bold in my quote because vbulletin automatically italicizes quotes.
Three-Dimensional Incident-Light Meters
"As will be seen, the salient feature of these meters is the hemispherical type of translucent receptor employed, whose object is to effect automatically and with a single (camera direction) reading the necessary correction for most conditions of lighting (the major categories of which are given on page 126).
The claims made for this meter were investigated by practical testing under carefully controlled conditions, and by comparison with the Duplex method using a flat-receptor meter. These comparative tests confirmed that under all lighting conditions except backlighting beyond about 130 degrees from the subject-to-camera line the exposure indications for a given film speed setting agree within one-third of a stop with those given by the flat-receptor Duplex method.
The application of the Norwood-type meter is quite simple, and consists of merely pointing the meter's hemispherical receptor directly towards the camera from the subject position, irrespective of the type of lighting employed or its direction up to a lighting angle of about 130 degrees from the subject to camera line."