The first model appeared in 1935. The model II which differed considerably appeared about a year later.

The first model was slightly larger than the later model and differed in operation. In use the film speed is set on the calculator. A knob is turned until the required shutter speed is indicated by the needle. The f number is then pointed to by one of three arrows, these corresponded to three shutter speed scales over which the needle moves. The scales and arrows are coloured red, black and green. There is a further scale (A - G) for cine use, this is for cameras having a shutter setting of 1/32, the needle is aligned to one of these letters, the same letter on the calculator will then show the aperture to use.

With the Avo II, the film speed is set, a pointer is lined up with the meter needle, the shutter speed and aperture pairs can then be read. The price was 55/-. Also available with continental f number values.

Early versions of the model II have a silver inner dial and are marked 'prov. patent', later the dial was black and the face plate carried the full patent number.

When using a conventional photo-electric exposure meter the reading was taken facing the subject and therefore measured the light reflected by the subject. In taking the reading it was common to expose for the shadows, these would then be located near the foot of the characteristic curve. In black and white still photography there was considerable scope at the printing stage to adjust the balance between shadow and high-light areas and achieve an optimum print. This was not the case with colour reversal films or in cinematography where the processing was automated.

Smethurst's interest was in exposing cine film where it was most important that the actors face remained at a consistent brightness between scenes i.e. a high-light region would be fixed on the characteristic curve. Rather than measure the light reflected from the scene he measured the light reflected from a standard subject (an artificial high-light), the next step was to include the standard subject in the exposure meter and, since the standard subject reflected a known percentage of light, measure the transmitted light falling on the face of the meter. This was incorporated in the Avo Smethurst High-Light Meter making it the first incident light meter.

In use the meter is pointed to the light source or sometimes the camera. The film speed is set. A dial moves a pointer which is lined up with the meter needle. The shutter speed and aperture pairs can then be read.