In this section we will be using photoshop or gimp to measure how the values of each of our zones printed in gum bichromate. This will give us a set of information which we can then use to create a scale that should give us a predictable print every time.
First you will probably want to set up a chart in a spreadsheet like Excel. this will help keep track of all the numbers and make ease of doing any math. Although you can do it all by pencil and paper if you should desire.
The first column is our zone numbers, the second column is the corresponding rgb values of those zones. The third column is the inverse of those rgb values. To get these numbers you subtract each rgb value from 255. Effectively this is what happened when we made our transparencies earlier by inverting the values to a negative image. In the columns 'A' and 'B' we will input the values we measure. And in the last column we will average the values of both A and B.
First you will need to make a scan of your print once it is dry. Most scanners have automatic settings to correct brightness and values. Since we are using this scan to measure we don't want each of our tests getting scanned differently so turn off any automatic settings. The scan may look dull and flat, but that is perfectly fine, we're after data, not pretty scans.
This is a scan of my 4-8-2 print.
No matter what our prints will have a slight color cast that can interfere with our measurements. Remove all the color from the scan by going to Image>Adjustments>Desaturate... in photoshop or in Gimp go to Colors>Desaturate... and choose lightness.
Zoom in a bit. Using the rectangular selection tool, select a portion of one of the zones. You can move the selection by clicking and dragging inside of it.
Go to window>histogram to make sure the histogram window is open. A histogram if you are unfamiliar, is a graph of how many values of each pixel there are. In this case it is showing us the values within our selection. You will notice a bell curve in the graph portion showing the slightly different values of gray within your selection. We are after the average (or mean) of all these values, which you can find just below the graph. Don't worry about being to specific, just round the value to the nearest whole number.
Enter the measured value into your chart. Make certain you are entering the data in the correct place. Consider column A as being your left scale and column B being the right scale. My zone 10 printed as 182. Proceed to measure all of the zones on both scales until your chart is full.
(note that sometimes values may seem to out of order as you are measuring them. Don't worry about it. Just record the values as they are measured)
Use the spreadsheet to average the values to the nearest whole number. If you are not sure how to use math in spreadsheets you can look it up online. The function I used in the cells was: =ROUND(AVERAGE(D5:E5),0)
