As a photographer, I use the term first light ( Last Light ) to define a time when
there is some ambient light in the sky but we have lost perception of
colour. This happens because special cells in the retina, called rods register black and white
only - these are more sensitive to light than the cones which read colour. Even though we can see it there is still enough
light to take photographs and register colour. Again how long
this light lasts depends upon your post ion on the earth and the
time of year. In summer it is longer in winter shorter.