Getting the most out of your camera HD Camera Measurements Parts 1 and 2 covered the theory and actual measurement of white shading, making sure that the camera non-linearities introduced by various different lens systems on a particular camera are compensated. In this issue, Part 3 will investigate setting up colourimetry and exposure as well greyscale adjustments using a waveform monitor. Courtesy of Tektronix EMEA Getting the most out of your camera, Part3 camera converts this light image via, the CCD (Charged Coupled Device), to digital voltage levels. The camera then converts the signal to the HD-SDI output. This signal can then be measured using a waveform monitor such as the WFM7120 or WFM5000 to quantify the characteristics of the signal. An accurate test chart does not negate, but complements an electronic test pattern generator by including the camera's `Taking Characteristics'. `Taking Characteristics' is a term frequently used to describe the many elements of the conversion of light by the camera to an electrical signal. These include the lens and adapters, the prism block, dichroic and trim filters and the colour characteristics of the CCD or other image devices. Taking Characteristics will also be affected by changes in the colour temperature and spectral distribution of the scene lighting. Accurate evaluation requires a precision test chart such as the Combi DX-1 (rear-lit Figure 8a) or (front-lit Figure 8b) ChromaDuMonde test patterns from DSC Laboratories. These unique test patterns provide meaningful test signals that facilitate camera alignment and image control. The characteristics of grayscale and colour steps contained within the charts can be used to adjust the camera's set-up controls. Optimising the image largely depends on the adjustment capabilities of the individual camera. The DSC colour patterns are designed to the latest (International Telecommunications Union) ITU-R BT.709 colourimetry standard used with HD standards (SMPTE 274M and 296M). Note the excellent wide colour gamut of the NTSC standard as shown in Figure 9. When television sets used this standard they did not sell well because the picture was too dim and had to be viewed in a darkened room. For this reason the colourimetry component of the NTSC standard was replaced by SMPTE C. This greatly reduced colour gamut but provided a much brighter image. It is important to understand these differences in colourimetry between HD and SD when viewing the waveform monitor display. In Figure 10, a 100% colour bar test signal for both HD and SD formats is shown, in a YPbPr waveform display. Notice the difference of the Y-channel Green-Magenta transitions between the HD and SD signals. Users who are familiar with SD signals may initially consider the HD signal to be incorrectly adjusted. This is not the case. This difference in levels of each colour component is normal and due to the different colourimetry equations used between HD and SD. Setting up correct colourimetry and exposure S hading, as discussed in the previous issue, primarily deals with the luma adjustments of the signal, but it is also important to ensure the colour fidelity of the image. Typically the aim is to reproduce the image on a monitor or television set as closely as possible to the original scene. To achieve this, an accurate test chart is required that produces a reference pattern. The >> Figure 8a Figure 8b Page 52 of 100 Figure 9. 1931 Chromaticity Diagram