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White shading

The conversion of light into electrical signals involves many processes that can affect image quality. Correct camera alignment helps minimize the impact of these variables. This article examines white shading — a vital function that is often overlooked in camera setup, or even worse, performed by an unqualified operator using less than adequate equipment.

Every camera operator should at least be familiar with basic white shading techniques. For example, overall brightness must be even across the whole image. To be sure, for studio cameras with fixed lenses and shooting in the same controlled environment, white shading will likely need to be checked less frequently than a digital filmmaker on-location changing lenses frequently. Still, these measurements should be done in a controlled environment on a regular basis.

Electronic error compensation

White shading is defined as the process of electronically compensating for errors in brightness or color, primarily generated by the optical system of the camera. A frequent problem is that the center of the image may appear brighter than the edges. This type of aberration is caused by differences within the camera's optical system components and is more commonly found within older lenses. Differences may also occur when changing from one filter to another.

The basic tools required for white shading are an HD waveform monitor and an even light source such as the diffuser panel of the DSC Ambi Illuminator. For optimum signal accuracy, it is better to use an HD-SDI output from the camera, but the process can be performed in SD.

White shading measurement

The Ambi Diffuser has a vertically adjustable light source. For maximum precision, measure the light output from the device with a luminance spot meter. Aim the meter at the center of each rectangle, and record luminance and color temperature values in each quadrant. Adjust as required until the light output in each quadrant is as even as possible.

The gain on the camera should be set to 0dB. Also, ensure that the knee and other camera gamma controls are set to off. Position the camera in front of the Ambi Diffuser panel, and zoom in to overshoot in order to use the center of the panel. Ensure the camera lens is set to manual iris mode and the zebra setting is adjusted to 95 percent. The lens aperture of the camera should be adjusted to between f4 and f5.6.

Once the setup is complete, perform a white balance of the camera, and use the light meter and waveform monitor to check that the white balance has been properly performed. The color temperature readout after white balancing should show 3200K, and it may be necessary to adjust the camera's red and blue gain settings. Slightly defocus the lens to further soften any diffusion error. In the shading menu of the camera, adjust the vertical and horizontal saw to 50. The vertical and horizontal pars should be set to zero as a baseline.

Select the monitor mode on the waveform monitor. On the camera, adjust the white shading settings in the following order: green, red and blue. Initially, view just the green channel on the waveform monitor by turning off the other two channels while viewing the RGB display (see Figure 1) and adjusting the H and V saw so that the trace is a flat as possible. Then make adjustments of the H and V par so that the green trace is as flat as possible. Repeat the adjustment process for the red channel and then the blue channel, with each channel showing individually on the display. Once these adjustments are complete, switch to a vector display on the waveform monitor, and adjust the gain to maximum. At this point, a small circular trace should be visible directly in the center of the display. (See Figure 2.)

On some cameras, it's not possible to adjust RGB shading parameters individually, and an absolute master control is used. In this case, use just the vectorscope display in maximum gains, and adjust the master control until a circular trace is displayed. An oval shape, as shown in Figure 3, indicates that the shading is incorrect. Deviation from the center of the vector display indicates a color cast to the image; the type of hue depends on the angle of the deviation.

Once these adjustments have been completed, store the setup as a new lens file preset. The above procedure can be repeated with different lenses and filter configurations and saved.

The waveform monitor's diamond display can be used to aid in the adjustment of white balance and camera shading. The diamond display processes the signal in the RGB color space just like the input processing of the camera. When the value of R, G and B are equal, a gray value for the signal is produced. A resulting gray scale camera chart or evenly illuminated white field will therefore produce a vertical line in both the upper and lower diamonds if the camera is correctly aligned. (See Figure 4.) Any deviation can easily be seen with the diamond display.

Conclusion

There are many ways in which a camera can be erroneously white shaded, just as there are as many ways to incorrectly white balance a camera. White balancing to a piece of paper that has blue spectral tones or uneven lighting is one problem. The same can be said for white shading. The characteristics of the saws and pars in each RGB channel require a perfectly illuminated surface, and any fluctuations in evenness may cause geometric distortions.

By using these techniques to shade and align the camera, and becoming familiar with the correct use of the Ambi Illuminator and waveform monitors, operators can ensure that the image captured by each camera has proper white shading and image quality. The user should also monitor the output of these images from the camera with waveform monitors to ensure that the camera is producing the correct video levels, is properly color aligned and matched to other cameras.

Jon P. Hammarstrom is the senior manager of global marketing, video, for Tektronix.