Printer light points were designed to create a precise way of color timing film. The concept uses a series of dichroic filters that split the light into Red, Green, and Blue values. Each color then passes through ‘light valves’. These are metal vanes that open and close in precise increments to allow the exact amount of light through to replicate the exact value for each light point. The 3 colors are then recombined back into full spectrum light and output to the film. This is complicated by the fact that it is not a linear system but rather a logarithmic system.
Each 12 points is twice the amount of light (1-stop), therefore the amount of change is significantly greater as you go up the scale of light points. Considering that the valves only open about 3/8” in total this is a very precise process.
There have been many systems used for the control of the light valves to determine the exact time that the change occurs in the printer. The earliest systems used notches in the side of the negative to trigger a sensor on the machine. Later, they used foil metal tabs on the side of the negative to trigger the changes.
Some time in the early 70s, Mike Chewey at MGM labs developed the FCC (frame cue count) system that used a digital counter to determine the point at which lights change, which is the system still in use today.
The system uses light points from 0 to 50 to cover the range of exposure available. In addition, there are ‘trims’ that are used to set up the basic control on the machines, usually about 24 points on the trims giving an overall value of 74. The trims are set at the beginning of the shoot and do not change during the printing of a roll, while the regular light points change with every scene. Many printers have the capability of making fractional light changes in as little as a 10th of a point.
In addition, there is a system known as DLC (Dynamic Light Change), which was pioneered at FotoKem. This allows you to make gradual changes in light over a long period rather than instantaneously, so you can make a gradual change over many feet.
Most of the modern color grading systems have some way of referencing printer points.
Printer point adjustments affects the density
When making printer point adjustments, you need to take the density into account as well.
A combination of one printer point on all the three primary colors is equal to a full 1/12 of a stop, also known as a full point of global density.
Because it takes all 3 colors to get the 1/12th stop difference, then each color would be a percentage of that value. Film response tends to show more in the red exposure, slightly less in the green, and slightly less in the blue. Based on the measured values on a densitometer I would estimate that:
- Red has a total density value of 37% of 1/12th of a stop or 3% of a full stop
- Green has a total density value of 33% of 1/12th of a stop, or 2.7% of a full stop
- Blue has a total density value of 30% of 1/12th of a stop, or 2.5% of a full stop
However, to say how the eye would perceive these would be different as we have our greatest sensitivity in the Green spectrum and our eye is more sensitive to changes in density than color.
In most nomenclature, it is considered to be 8 points to a stop instead of the usual 12. This is correct when changing stops in cameras that took the original film and assuming the gamma of original camera negative stocks, which respond at 2/3 value to the exposure (gamma .66).