What you’ll need:
- The setup described in the preceding page.
- Photoshop, or another image editing program that lets you create images with arbitrary colors.
- Rawdigger, or another program that lets you look at the average values of the ref, green, and blue pixels in raw files.
- Excel, or another spreadsheet program that can operate with Excel files.
- This spreadsheet (Right-click on it and select “Save target as” to download it. Add the suffix “.xlsx” if necessary). Here’s an “.xls” version if your version of Excel is old: OneStepUniWB
Here’s the procedure in brief. There’s a detailed explanation at the end of this page.
- In any RGB color space within the gamut of your monitor for which you know the gamma (sRGB is probably the safest choice), make three exposures: one at R=255, G=0, B=0, one at R=0, G=255, B=0, one at R=0, G=0, B=255.
- Bring the images into Rawdigger. Enter the average values of the red, green, and blue raw pixels into the appropriate columns in the spreadsheet.
- Enter the color space gamma into the spreadsheet.
- Enter the desired raw value for the UniWB target.
- Read the UniWB target RGB values from the output cells, and fill the test target with that color in Photoshop.
- Balance to the test target.
- Take a picture of the test target.
- Bring that picture into Rawdigger and look at the EXIF WB coefficients and the raw values to see how well you did.
The details follow.
To make the red, green, and blue images in Photoshop, create an image about the shape of your format. Make three layers, on for each color. Then make a fourth layer for the white balance target color. When the colors are all filled in, the layers will look like this:
Click on the foreground color and enter the values for full red as follows:
Use Edit>Fill… to fill the red layer with the foreground color:
Do the same with the green and blue layers. Photograph each of the three layers with an exposure that’s about what your built-in meter recommends for the green layer and bring the files into your computer. Open the spreadsheet.
Here’s what the spreadsheet looks like when you open it:
First, clear all the input cells so you don’t get confused about what you’ve entered and what you haven’t. Don’t worry about the error messages; they’ll go away when you fill in all the input cells:
Bring up the red image in Rawdigger, and look at the average for the R, G, and B channels. Ignore the G2 channel:
Enter the red channel information into the first column of the monitor measurements field in the spreadsheet:
Bring up the green image in Rawdigger, and look at the average for the R, G, and B channels. Ignore the G2 channel:
Enter the green channel information into the first column of the monitor measurements field in the spreadsheet:
Bring up the blue image in Rawdigger, and look at the average for the R, G, and B channels. Ignore the G2 channel:
Enter the blue channel information into the first column of the monitor measurements field in the spreadsheet:
In the entry cell at the Desired camera values column, put a number that is less than the smallest number in the monitor measurements matrix diagonal. It’s not important what this number is, as long as it’s not so small that there’s a lot of camera noise to deal with, or so large that the monitor can’t get bright enough to reach it. I entered 4000:
Enter the gamma of the color space you’re using in the image editing program. In this case, it’s Adobe (1998) RGB, which has a gamma of 2.2 except near 0.
In Photoshop, click on the foreground color square to open up the color picker dialog. Enter the trial monitor values into that dialog.
Fill the white balance layer of your test image with the foreground color:
White balance to the test image, then photograph it and bring the raw file into Rawdigger. Look at the average values for R, G, and B.
Enter those values into the result column of the spreadsheet and read the coefficients. If they’re within 5%, you’re done.
You can check the EXIF data to make sure.