Originally Posted by dancqu
the meter linearity can be an issue, and a bad one. It's very easy to see if the measuring cell response is linear with an usual 4X neutral density filter, placing it before the meter window and take readings with it and without it. These filters are usually well calibrated and absorb the light evenly across the whole spectrum, so the colour won't be a problem. Just try it in under a different lighting conditions, and you'll easily see if your meter says something wrong. Don't mix artificial and sky light, the silicon measuring cell is usually much more optimistic about reds, in spite of blue filter placed before it - its high sensitivity to the red end can't be fully compensated without great loss in sensitivity.