The light meter I use is a Sekonic Studio Delux. It can measure light as reflected or as incident. Reflected light bounces off the subject, while incident light falls onto a subject. I had been using reflected light for a long time and never really figured out what the ISO of emulsions was. Now I use incident light readings exclusively and get better results. The light source is independent of the subject and is not dependent on what is being photographed.
A white plastic dome is placed over the sensor in order to measure incident light. Readings are taken from the subject with the meter facing the camera. If it is bright outside a screen is placed in front of the sensor to block some of the light. In that case the dial must be read a special way. Usually, as in low light conditions, black numbers are used. There are red numbers which are used when the "high slide", as it is called, is inserted. Those numbers are to the left; the dial is rotated clockwise and they are revealed.
On a bright sunny day the amount of light is usually 320 give or take a block with the high slide in front of the sensor under the white plastic dome. When you aim the light meter at the camera from the subject and depress the center button the red needle will move up to 320 or 320+1 more block; let go and the needle is fixed in place.
The large dial is in two parts. It looks black but there is a white part under it. That moves a red pointer. The red triangular pointer is aligned with the needle. It serves to remind you of the reading so more can be taken. The black dial moves a scale of numbers. The numbers are the same as the needle numbers and are to be aligned with one or the other triangles in the white scale inside the dial. The red scale is used with the high slide and the black scale is used without it. 320 is put over the red H.
A little white square shows the ISO setting. The lowest setting is one click below 6. ISO of Liquid Light is below that. One full stop will raise or lower the number 6 by twice or half. Above 6 is 12, below it and not shown is 3. 320 by the red H is moved over so that 160 is on the red H making ISO to be 3 when the ISO is on 6 (one click up from the maximum). Then move it again so that 80 is aligned with the red H and ISO is 1 1/2. Move it again and you get 40 on H and 3/4.
There are two dots between the 6 and 12. ISO decends from 6, 5, 4,3, 2 1/2, 2, 1 1/2, 1 1/4, 1, 3/4.
That is how I have been thinking. The dial, however, shows different numbers; ISO is indicated above the f16 number as 1, 2, 4, 8. Not 6. I suppose the lowest number on the dial is actually 6, one click lower than the number and not aligned with it, but since it does not align I don't use it that way.
At the bottom of the dial on the light meter a red 1 will align with white 16. ISO is 1. Bright sun is f 16 at 1 second. That is -3 stops or three more stops of light from 6, when foot candles are 320, my way.