okay probably used a lazy answer... l know red filters were used in the old black and white movies ,scenes were shot over various times of day and days.. the red filter gave the consistent blend showing no change of time in light and shadowing..so that when the whole scene was place together the quality of black and white film remained the same.. the history o f filters blending toning and colouring is way over my head.. very scentific hahaha, thank god for modern technology.. aslo a shortage of good quality dark rooms and chemicals back in the early days meant it made sense to add the filters first again . just brief.. answer here.. physics and chemistry and all the jazz is all way over my head..

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