One simple technique is to white balance on a card that is not white. Using a card that is slightly blue under the main source of light, the camera is white balanced. The white balance circuit assumes that the card is white and equalizes the output of the red, green and blue channel outputs to produce the correct ratio for white. This means increasing the yellow/orange gain to counteract the bluish tint of the ‘white’ card. When a normal scene is exposed, the camera will produce straw coloured pictures. Alternatively, if the card used for white balance has a yellowish tint, the balance circuit will produce pictures that are bluer or ‘cooler’ than normal.