EQUIPMENT

Colour grading and timing

The light in printers can be controlled to allow for the density and balance of the negative and differing film stock sensitivities, and to adjust the relative values of the colours in the printed image. This process is called grading, timing, or colour correction. It is an area of subjective intervention and experimentation which can be used to creative ends, but it can also be a closely monitored process with predetermined aims for quality and repeatability.

LINKS
GRADING AND TIMING

What is grading?

Trims and aims

Exposure and light valves

Colour printing

Printer lights

Grading

One-light printing

Printers work with either an additive or subtractive light head, additive systems are most common. Dichroic (beam-splitting) elements in the printer reflect and transmit part of the white light from the lamp house, and only transmit light of specific wavelengths to the film. The dichroic elements separate the light into red, green and blue (RGB) components, and these in turn are filtered by light valves which can be set to allow different intensities of each light onto the receiving film stock.

Each (RGB) colour primary’s light valve, or vane, has a combined total of 75 settings, each with an exposure value of 0.025 log E - combining to a total exposure rating of above 1.80 log E, a ratio of 64:1. The logarithmic exposure (log E) scale means that each step increases exposure by 6%, and after 12 steps, the film’s exposure to light is doubled.

in addition to light valves, the printer has trims which are monitored and set to the daily operational aims of the laboratory, to control for changes in the print-process workflow. In a commercial lab, the printer operator usually sets up the printer trims to the best, last-used lights on a given stock, before printing test material for processing and measurement on a densitometer, to set the new trim values according to lab aims. Lab aims might be determined in-house, or follow Kodak LAD (lab aim density) standards. When reading a Kodak LAD, the Status A density aims for Kodak Vision (3383) colour positive stocks are: 1.09 red, 1.06 green, and 1.03 blue. Experimentation and deviation from these aims can lead to different, sometimes pleasing results, casting a warmer or cooler colour balance across the print.

Trims set process controls, but grading determines the light valve (or vane) positions for a print (numbering 0-50, for each of the three primary printing lights - RGB). These positions or ‘printer lights’ are chosen in grading to match the exposure needs of a particular scene or negative. This can be done on a video color analyzer, or over a lighted bench with light filters and pre-made test strips to judge the colour values of each printer light setting.

Grading (also known as colour correction) allows you to predict the results of printing your film. In grading, the negative is analysed scene-by-scene on a winding bench, analyzer, or flatbed viewer. Printer lights are given for each scene, counting from the film start (0) to the first frame of picture, and then each subsequent scene after that. It is common practice in commercial labs to insert a LAD strip, or a few frames of ’China girl’ or ‘leader lady’ at the head of each negative print roll in order to check the aims, these would be graded at 25-25-25 (in the middle of the light range). Black frames or leader are graded 0-0-0. Therefore the cueing count from the film start typically runs with a section of 0-0-0 for black frames, a section or 25-25-25 for the LAD, and then the first cue for the film image.

Depending on the printer setup, the light valves can be controlled either electronically in a local area network between printer and grading terminals, or via punch-tape mechanisms, or (rarely) by manual means. The movements are triggered by frame count cues (FCC) or, in older systems, by edge clips and silver tabs that signal to the printer head the cue to change lights.

The vanes themselves do not move instantaneously but can take a fraction of time to adjust to their new position, which can mean a few frames of visible light change (and therefore colour shift), depending on how quickly the film is travelling through the printer. A big change in grading light can lead to either a flash at the top of the frame, or increasingly, as printing equipment grows older and harder to maintain, it can cause a vane to become stuck and unresponsive to other cues to open or close. Care should be taken when grading to avoid overly fast (between a few frames) or overly large light changes. One way to avoid the need for big light changes is to assemble the negative printing roll based on the grading lights required. This would, however, lead to splices in the reassembled film and so it is best avoided in projection prints. Another way is to stagger big changes over a few (minimum 8) frames each. This will be visible as colour shift (mostly visible across neutrals and greys), but reduces splices – which makes the final print safer for projection and better for archiving needs.

A one-light print is when printer lights and trims are set at the best common light for exposure of a particular stock or project. Once graded intermediate material (otherwise known as a ‘fully balanced’ duplicate negative) has been generated from the original camera negative, or if material has gone through a digital intermediate (DI) process and software grading before laser-out, then a one-light print should be sufficient to create release copies. At least, very few printer light changes should be needed.