HD Camera Measurements Getting the most out of your camera, Part 2

Recap Part 1:
In part 1 of this knowledge series we discussed the requirements for making correct white shading measurements in terms of the equipment used. Part 2 now will focus on performing the actual measurements and making sure that we achieve optimum linearity and dynamic range.
White shading measurement
Power up the camera, waveform monitor and DSC Ambi Illuminator and then connect the HD-SDI output of the camera to the waveform monitor. Allow a few minutes for the light source and the system to stabilise to normal operating temperature before starting your measurements. The Ambi Diffuser has a vertically adjustable light source, Figure 1a. However, to achieve greater precision, measure the light output from the device with a luminance spot meter and adjust the AmbiMirror and light source. The optional AmbiGrid (Figure 1b. Equal rectangles on a transparency, sit in front of the diffuser) facilitates separation of the diffuser area into equal quadrants. Aim the luminance spot meter at the centre of each rectangle and recording the luminance and colour temperature values in each quadrant. Adjust the barn doors and vertical distance from the DSC AmbiLight to the AmbiMirror as required until the light output in each quadrant is as even as possible. Measurement deviations of +/- 0.5% are typical; however deviations of 1% or more are not uncommon.
The gain on the camera should be set to 0 dB. 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 a bit in order to use the centre of the panel. Ensure the camera lens is set to Manual Iris mode and the Zebra setting is adjusted to 95% so that the zebras cover the entire image. Note that if a spot meter has not been used to ensure evenness and the zebras do not cover the entire image then the light may not be even and the above steps should be repeated until the zebras cover the entire image. The lens aperture of the camera should be adjusted to between f4 and f5.6. To do this, adjust the physical distance of the camera to/from the AmbiLight to achieve the f-stop range. In doing this process it may be necessary to adjust the AmbiLight source to produce an evenness of the light and the above steps may need to be repeated.
Once the set-up 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 colour temperature readout after white balancing should read 3200K and it maybe necessary to adjust the camera’s Red and Blue gain settings to achieve this. 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 a 0 as a baseline. If these numbers are off, the gamma controls may be on and must be disabled.
Select the waveform mode on the waveform monitor. Push and hold the WFM button to display the menu and configure the display for RGB parade mode, ensure that all channels are selected on as shown in Figure 2.
On the camera, adjust the white shading settings in the following order: Green, Red and Blue. Depending upon the controls of the camera, you maybe able to make individual adjustments on the Green, Red and Blue channels. Initially, view just the Green channel on the waveform monitor by turning off the other two channels and adjust 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 with the waveform monitor viewing just the Red channel on the display while the other two channels are turned off. Then turn on only the Blue channel on the waveform display and turn off the Green and Red channels.
The Saw and Par adjustment should then be made on the Blue channel, so that the waveform trace is as flat as possible. Once these adjustments are complete select the Vector display on the waveform monitor and adjust the gain of the display to maximum. Push and hold the Gain button on the waveform monitor to access the menu. Select the 5X gain setting and enable variable gain. Then rotate the general knob in the vectorscope display until the lower left of the display indicates a maximum gain of 20X. The small circular trace should be observed directly in the centre of the display as shown in Figure 3.
On some cameras, it is not possible to adjust the RGB shading parameters individually and an absolute master control is used instead. In this case use just the vectorscope display in maximum gains and make adjustment of the master control so that a circular trace is displayed. An ovular shape, as shown in Figure 4, would suggest that the shading is incorrect and adjustment of the master control should be made to make it as circular as possible, as shown in Figure 3. Note that a deviation from the centre of the vector display indicates a colour cast to the image. In this case the white image would have a tint of a colour; the type of hue would depend on the angle of the deviation. By adjusting the camera controls this error should be corrected. Once these adjustments have been completed store this setup as a new lens file preset. Now the camera should be properly white shaded using this specific lens. The above procedure can be repeated with different lenses and filter configurations and these configurations can be saved for later use. Figure 5 shows the final paraded waveform display of the RGB signals after camera shading has been completed.
The 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 colourspace just like the input processing of the camera. When the value of R’, G’ and B’ (R’=B’=G’) 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 as shown in Figure 6. Any deviation can easily be seen with the Diamond display. In Figure 7 there is a slight deviation in the upper Diamond representing the Green and Blue channels and a significant deviation in the lower diamond for the Green and Red channels. The camera should be adjusted such that the levels on Red, Green and Blue are equal and produce a more vertical trace as in Figure 6. Once the camera operator becomes familiar with this display, they will find it easier to see the interaction of all the channels within the one display.
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 a problem often overlooked by operators. The same can be said for white shading. The characteristics of the saws and pars in each RGB channel require a perfectly illuminated or lit surface, as any fluctuations in evenness may cause geometric distortions.
In the next issue: Setting up correct colourimetry, exposure and grayscale adjustments using the waveform monitor
Tektronix waveform family
Tektronix waveform family

Tags: tektronix | hd camera measurements | white shading | white balance | waveform monitor | colourimetry | exposure | grayscale | iss027 | N/A
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