Posted 20 May 2007 - 07:19 PM
In the early days of film, the stock was only sensitive to blue. So eventually they increased the sensitivity to green as well and called it "ortho" (Greek origin, for "correct", "straight", "right").
But that wasn't "correct" enough for people, especially those using the early two-color processes who needed some red sensitivity (red wavelengths have less energy than blue ones.) Most silent era movies were shot on ortho stock, hence the white-ish skies (blue tended to overexpose and a red filter wouldn't work) and the need to paint faces with white-ish make-up with brown lipstick to get a good tonal range on the face. With a red-insensitive stock, reds in fleshtones could go very dark (underexposed) and blue eyes could go white-ish.
So "panchromatic" b&w stock was invented, sensitive to "all" ("pan") colors, though still a little more sensitive to blues than reds.
You'll notice in lab stocks that there are still some blue-sensitive only and orthochromatic stocks being made.