Cinematography-Theory-And-Practice

Figure 9.8. SMPTE 75% color barsas displayed on the vectorscope.As a check on system alignment,each primary and secondary coloris placed in boxes. If they are notin their boxes, then something iswrong.The vectorscope is really just thecolor wheel (see Figure 12.1 in thechapter Color) represented electronically:around the circle is hueand how far away from the centeris chroma saturation. If the camerais photographing a black-and-whitescene (zero saturation), then thevectorscope will display only a dot inthe center.THE WAVEFORM MONITOR AND VECTORSCOPETo see the various elements of the video signal, two special test oscilloscopesare used: the waveform monitor and the vectorscope (Figures 9.7and 9.8). Usually combined into one unit, these tools are invaluable;it is essential to understand what they are telling you. On a videoshoot, the waveform monitor is your light meter and the vectorscopeis your color meter. Color monitors, even sophisticated ones,can be unreliable, but information for the waveform and vectorscopecan almost always be trusted.Waveform MonitorsThe waveform monitor displays the luminance video signal information.It allows you to analyze the information from an entire frameor from just one line of video. The waveform monitor displays thesignal on a scale seen in Figure 9.7. The scale is from zero to onehundred or in milivots. It can be measured in IRE units (InternationalRadio Engineers) or as percentage, which is generally used in HDvideo. The waveform is merely measuring the voltage of each partof the video picture: voltage represents the brightness of the image.Zero represents pure black, and one hundred represents pure white.It is possible for the video signal to go below zero or above one hundred,but the final image will still only show pure black at zero andone hundred will still be pure white.The VectorscopeUsed in conjunction with the waveform monitor, the vectorscopemeasures the chrominance (color) of the video signal (Figure 9.8). Thescale of the vectorscope is a circle overlaid with the color amplitudeand phase relationship of the three primary colors (red, green, andblue). The vectorscope displays color information only; there is noinformation about exposure — although you can see the effects ofover or under exposure in how they affect color saturation.The vectorscope screen for calibration shows a display of a SMPTEcolor bars test signal with all of the dots in their boxes and the colorburst correctly placed on the horizontal axis. Chroma phase errortranslates as “the color is off,” as shown in Figure 9.9. Notice howcolor is conceptualized as a circle and we rotate the circle around itscenter to adjust the hue (phase). How far it is from the center indicatedchroma saturation. At zero saturation (black-and-white) thedisplay is just a dot in the center.cinematography156