Weston Photronic Exposure Meter Model 617 Version 1, USA 1932:

The very first light meter made by Weston specifically for determining camera exposure. It is relatively rare since the twin cell model was only made for two years before being replaced by the much more compact single cell version 2.

Housed in black bakelite, as were all of Weston's meters of this time, it is a truly beautiful piece of engineering. It is designed for reflectance use only, where you measure the light being reflected off the subject rather than the incident light falling onto it. The central top groove aids in the sighting of the meter towards the subject. When the meter was used in its more sensitive twin cell mode the light values on the meter scale were divided by 10.

The disc to the left of the meter gives basic instructions for use. That to the right is the calculator dial of a basic design common to most of the Weston meters leading up to the very last models manufactured. 

The plate / film speeds listed on the dial go up to only 48 - the maximum possible at the time of manufacture. Once this is set, another concentric dial brings the pointer to the measured luminance value. Aperture and shutter speed combinations can then be read off. Besides the normal pointer on the dial, there are others for different types of scene. They allow for exposure compensation for subjects of abnormally high and low contrast and brightness levels.

The original Model 617 Weston used a metal baffle mounted on the photovoltaic cell to make its response more directional. Weston exposure meters from the model 650 onwards used a glass lenticular system to provide that directionality. The lenticular arrangement also delivered more light to the cell, rendering the meter more sensitive.

The Earliest Version of the Model 617 has a different instruction legend to the slightly later one. It is recess moulded into the Bakelite and on many examples, including this one, the original ink had flaked off with age. It can easily be restored with white water-based paint, poster colour or a chinagraph. That this is earlier, is deduced from the smaller list of patents on the meter face. The later variant has an embossed riveted on circular instruction plate which seems to be more durable with age.