Suppose we have a portrait with two light sources.
When we use an incident lightmeter to give us a correct average exposure for a tridimensional object which is non-evenly illuminated, we use the dome and we point the instrument to the camera taking it in front of the subject. Part of the dome will receive more light, part less. Inside the dome there will be a great shake of photons and the result will be the reading of the light meter, a form of averaging. We will call this method A.
When we want to determine the illuminance ratio, or how you call it, I mean the difference in lighting on let's say the two sides of the face, we use the flat cover, not the dome, and we point the light meter to the light source, not to the camera. First we measure light source A, then we measure light source B, than we see that they are let's say in 2:1 relation. If you want more contrast you diminish light on the "darker" side etc. So no dome, meter pointed at the light source, and two readings in our case. We will call this method B.
A third way is to use the dome, but on two different sides of the face. You first use the light meter on one side, so that the dome is invested only by one light source. You put it "by the cheek" rather than "by the nose" so to speak, but pointed toward the camera. Then we use it on the other side, so that it only takes the other light source. Again using the dome, and pointed toward the camera. So dome used, meter pointed at the camera, two readings in our case. We will call this method C.
My incident light meter doesn't have the flat cover, only the dome. At the moment I am not doing the kind of work that would require method B, but I might begin soon.
Can't I use method C to investigate the illuminance ratio of the scene? Do I really need the flat cover over the sensor?
Or to be clearer: What is the difference between method B and method C?
Which also begs the question: should I buy another incident lightmeter, that has the option of the flat cover?
PS I specify I don't have the flat cover lest somebody tells me to try and find by myself