As with many things, the history of this is obscured in the mists of time. I have been casually researching this, but as I am not an expert it takes me about eighteen times longer to ferret out information than it would someone competent. This is what I've found so far.
A Quick Bit of History
The modern era of measuring light (for our purposes) begins in 1932 with the introduction of the photoelectric meter by Weston Electrical Instrument of New Jersey. The original Weston Model 617 (as meters would for the next 30 years) used a selenium cell which converted light into a tiny electric current; this current could measured by an ammeter: so the more light, the more current and the farther the needle would swing. Weston was able to make a cell that responded to light much the way film did, calibrated the scale, and came up with a little device that allowed photographers to accurately measure light and make smart decisions on how to expose film.
All light meters do the same thing: they measure the amount of light that falls on the photocell. It's up to the photographer to interpret it into something meaningful.
But there was another way of measuring light, and that was looking at the light falling on the subject itself, instead of the reflectance of light from the subject. It had to be interpreted differently, but there were many applications where this was more useful than reading reflectance. So a diffuser, perhaps of a piece of frosted glass, for instance, was placed over the photocell. The cell was placed next to the subject and the cell was aimed back at the camera lens as if the camera were photographing the cell. This meter was known as an "incident" meter. And this bring us to
Donald W. Norwood
who was interested in meters. Incident meters with flat diffusion panels were useful for photographing flat objects, like paper in copying, but less so for three-dimensional objects. Norwood came up with the idea of a diffuser that was hemispherical and would cover the photocell — it would provide a much better reading. For this, Norwood got US Patent #2,214,283 in 1940.
Now about this time, things start to happen. . . .