As Brian Shaw mentions, the Dunn and Wakefield book is an excellent guide. Here is an excerpt on incident metering and the use of flat disc vs hemispherical (Norwood style) diffusers with incident meters. (The Exposure Manual, 4th edition, page 131, column 1)
What's implied here is that the Duplex method is mainly for flat-receptor incident meters, and that domed incident meters compensate in the same way as the flat-receptor Duplex method until the light source is more than about 40 degrees behind the plane of the subject.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 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.