This is intended to extend the range of tones and colors to more closely match the range of tones in the real world, which means less interpolation. This has resulted in more accurate color profiles. Another feature of the SG target is the series of black, gray, and white patches around the perimeter. There are used to check the evenness of light and useful for determining exposure.

With one light, it is more difficult to evenly illuminate the target. Differences in color temperature with two lights can also cause problems with the capture. With that in mind, it is easier to evenly illuminate the target with two lights if you know your lights are carefully matched for color temperature. When using either one or two lights, it is important that the lights are placed far enough to the side of the target to eliminate any glare which is more problematic with the SG target.

With the target evenly illuminated, the next step is to set up the camera so that it is relatively square to the target, since later we can fine-tune the cropping. Other considerations include, but are not limited to properly adjusting the exposure and setting the white balance. You know your camera, your lighting, and your expectations. Follow a consistent approach when setting up the capture.

 

The quality of the digital capture, and the resulting profile is directly related to the following three items

1. Proper white balance
2. Proper exposure
3. Evenness of illumination on the target

White Balance:
Most digital cameras have a variety of options for setting the white balance. White balance allows the camera to be matched to the color of the light source. Different light sources have different color temperatures, varying from warm, such as tungsten and incandescent lights, to cooler light sources such as daylight and shade. For even more accurate white balance results, we suggest using the custom white balance if your camera has this option. Depending on your camera, you will need to photograph a white or gray card and use all or part of the frame to set the custom white balance. Review your camera manual for the proper procedure.

Gray Balance:
If your camera allows you to select part of a frame and the gray patch of the SG ColorChecker is large enough, set your gray balance on this area. Many digital cameras only let you white balance the camera by filling the frame. Typically it is better to balance a camera using gray instead of white. Gray generally works better for two reasons. The first is that there are many different shades of white ranging from yellow to blue, which may not be discernable to the eye. The second reason, and more important reason, is due to the non-linearity of most camera sensors. For example, lets say we are able to balance the Red, Green, and Blue (RGB) values of the white card to be an even 240R, 240G, and 240B. Since the values of all three colors are equal, we have a neutral color at the highlight area. However, since many cameras are non linear, if we were to read the gray patch, the RGB values would not be equal, therefore will not be a neutral gray. Since our eyes are most sensitive to colors, especially neutrals from middle gray to light gray; we would achieve better neutrals if we can balance the white balance to a gray instead of a white. The SG target gray is a good neutral to balance the camera, but if your camera doesn’t allow you balance on this area, there are several other options available to you. Two options include the Expo Disk and the other is a light gray card sold by RM Imaging.

Exposure and Illumination:
Once the proper white and gray balance is achieved, the next step is correct exposure, and finally, evenness of illumination. We will use the white squares around the perimeter of the target both to check exposure and evenness. Using the manual exposure mode of the camera, determine the proper exposure with the built in light meter, a separate handheld light meter, or the histogram if the camera has one. Shoot one frame at this exposure and then vary the exposure by shooting additional frames by changing the shutter speed or lens opening. It is recommended that you incrementally vary the normal exposure by two stops more and two stops less.

After opening the capture, we are looking for the frame with proper exposure. What is the proper exposure? By using the probe or eyedropper in an image-editing program, we are looking for exposure where the white value of the perimeter patches is approximately 240 for RGB We will also read a variety of white squares on different sides of the target and make sure that are similar to each other. Anything greater than 5 points in each direction is unacceptable making the range from 245 to 235.

The black squares are an even better indication of evenness of illumination with the SG target since they show glare better. Again we are looking for values not to vary more than 5 points from side to side and top to bottom. Anything greater indicates that the lights need to adjusted and the target shot again.
It may take several adjustments of the lights to achieve the desired evenness.
If you are using Adobe Photoshop to check for even distribution of lighting, another trick to use is to change the Info Palette to display Lab. Move the cursor over the white, gray, and black patches and check for nearly equal values within the L channel, which is Luminance. Also check for a and b values which will show a color cast depending upon the values displayed.

The ColorChecker SG has the ColorChecker built in as shown in Figure 1.
We have found some cameras respond more appropriately to the ColorChecker than to the ColorChecker SG when creating the camera profile. Crop the ColorChecker out of the capture and experiment with both options.

Figure 1
 

 

As a quick review, lets outline what the software for creating digital camera profiles is actually doing. The software is used to compare the capture to the data file of the target. The capture is the way the camera sees color. The data file describes the target content. The profiling software compares the capture to the data file, defines where the differences are, and creates a profile, optimizing color and tone, much like a set of eyeglasses do for us.


This overview was written by Rick Lucas and Roy Bohnen of Digital Attributes.
 

Digital Attributes is a group of consultants providing national and international coverage for color management consulting, assessment, installation and seminars.