File 21-Step Gray Calibration

The 21-Step Gray Calibration command lets you calibrate black and white output workflows. It is particularly useful when the print or transparency that comes out of your printer is the first stage in an alternative photographic process such as Platinum/Palladium printing or photo-etching.

The calibration procedure requires the use of a black and white densitometer such as the Heiland densitometer shown below -- used ones are often available on eBay at a reasonable price. If you are creating prints, you need a reflection black and white densitometer; for transparencies you need a transmission black and white densitometer. Some models can take take both reflection and transparency readings.

The calibration process works as follows:

1) Create an image with 21 patches that run from 100% black to 100% white in steps of 5%. This can conveniently be done using the Step Wedge transformation. Note that 100% corresponds to black and 0% to white.

2) Output the image using the process you calibrating.

3) Using a black and white densitometer, measure the densities of each of the 21 patches, first zeroing the densitometer for the white (0% black) patch.

4) Run the 21-Step Gray Calibration command and select a target gamma (usually 2.222) and enter the 21 density values. A valid gamma value is required to convert from density to gray level. If there are target patches beyond the maximum density (Dmax) you want to exclude, leave them blank. The example below is for a photopolymer-etching process that generates full black at around 80%. Above 80%, the output is unusable, so those density levels are left blank.

5) Click OK to generate the response curve and the correction curve. The response curve represents the measured gray levels vs. the gray levels in the image, assuming a specific gamma value. The correction curve is the curve you need to apply to the image to get the final output to represent each gray level accurately. The correction curve is the inverse of the response curve (i.e. flipped around the diagonal line from the lower left corner to the upper right corner).

6) Using the Settings Menu, save the settings in case you need to edit them later so you won't have to type them in again.

7) Click Save Printer Curve... to save the correction curve in the Printer Curves folder. To produce calibrated results, select the saved printer curve when printing and otherwise follow exactly the same procedure to create the final print that you used when you created the calibration test print in step 2. The printer curve automatically adjusts the gray levels in the image you are printing so they come out right in the final print.

8) Close the 21-Step Gray Calibration dialog box using the OK or Cancel button or the close icon in the upper right corner.

This type of calibration works best with 16-bit black and white images.