FILMS ­ TYPES AND TECHNICAL DATA 9 with a densitometer, reading through a blue filter to measure yellow dye density, then a green filter for magenta and a red one for the cyan layer density. All three plots must coincide in shape along most of their length, as in the graphs in Figure 9.16(c) and (d). It is vital that the same filters and densitometer colour response be adopted worldwide when making comparative measurements of this kind. Certified conditions known as Status A are used for positive colour film images. Three slightly different filter values are needed for evaluating colour negatives because the dye images here are geared to best suit the colour sensitivity of colour printing paper, not your eyes. Conditions are then called Status M. Looking at Figure 9.16(c), you can see that characteristic curves for a typical colour negative film, read through correct blue, green and red filters, are similar in contrast to regular monochrome negatives. They are also offset vertically. This displacement is due to masking built into the dye layers, designed to compensate deficiencies in negative and print dyes. Provided there is no displacement horizontally, and all curves match in shape, colour printing will cancel out this apparent discrepancy. Programs for quality control of colour processing make similar readings of test strips which are put through the system at regular intervals to check any variations in solution chemical content, temperature, etc. (see page 287).