Below are a couple of different ways to determine the ISO of a video camera. The first uses an 18% gray card and the second uses a grayscale chip chart.
18% Gray card method:
Place an 18% gray card where it can be lit evenly from both sides. (See illustration below)
(Using some diffusion and keeping the lights at a 45 degree angle helps reduce glare and hot spots.)
With the camera, make sure that:
Fill the screen with gray.
Change from manual iris to auto, and observe the f-stop.
With the light meter:
Start out with an ISO setting of around 400. (It doesn’t really matter where you start, as you will change this setting. The 400 ought to get you in the general ballpark.)
Using a spot meter set at 1/60th of a second. This is the field refresh rate of NTSC video. (If you have a cine meter, set it to 30 fps, as it takes into account a 180-degree shutter angle.)
Dial the ISO up or down until you get an f-stop that matches the reading on the camera. That is your camera’s ISO reading.
Repeat and refine
Make sure you are getting a good reading by repeating the process a few times. Also, try varying the amount of light falling on the card and take another reading with the camera’s auto iris, and the light meter. They should match, or come close to it.
Once you are sure you have a good ISO for your camera and the particular lens you have on it, write down the figure on a card or write it on some white gaff tape and stick it on the camera somewhere. That way the camera assistant or gaffer can refer to it as needed.
It's good to rate the camera with a few different gain settings (–3 and + 3 for instance). Write these down as well and keep them with the camera.
Note: One problem with the gray card method is that it's possible that the auto iris setting on the lens is off a little. Because of this it's good to use the chip chart method, as it doesn't rely on the auto iris function. (Or take this opportunity to tweak the auto iris pot on the lens.)
Grayscale chip chart method
If you have a waveform monitor, you can use a standard chip chart and the following technique:
Light a chip chart as illustrated above and dial the camera’s iris until the crossover point is 55 IRE on the waveform monitor.
Note the f-stop
Take a few spot readings of the chip chart. Adjust the ISO on the light meter until the light meter's f-stop matches the camera’s f-stop. The ISO you end up with should be the camera’s rating.