ASC - [1993] by American Cinematographer Manual.pdf

Author:American Cinematographer Manual.pdf
Format: epub

Lamps The most widely used of the new types of photographic enclosed-arc AC discharge lamps are known as HMIs. This term is a trademark of Osram, but has become very much the generic term for this family of lamps. Some of the other trademarked brand names for these sources are BRITEARC, DAYMAX, TRU-ARC, MSR and DiLite. An assortment of these lamps is shown in Figure 20. These are fundamentally mercury arcs with metal halide additives to adjust the color balance. All of tlie various sizes of this lamp are rated by the manufacturers at approximately 5600K (see Figure 21). This is normally stated as having a plus or minus 400°K tolerance. Color Rendering Index (CRI) of the lamp is greater than 90 for all types. As will be noted from the color temperature and its tolerance, there can be some variation in the color rendering characteristics from lamp to lamp. Also, accoimt must be taken of the age of the lamp since this tends to result in a reduction of the color temperature. In normal daylight fill applications, these variations are probably not significant. 200 WATTS Figure 20. Comparative sizes of some HMI lamps. Where more than one light will be used as key in a scene, and these are likely to be seen in a single shot, it is strongly recommended that these keys be measured with a three-color type of color-temperature meter. Appropriate filtering materials are available for application to these units that allows correchon of green-magenta shifts as well 342 as adjustment of the color temperatme. With the proper meter, and the right filter materials at hand, it is literally a matter of minutes to balance lights to an extremely close match. If this prachce is not followed, it is possible to have significant variation in color rendering from two keys in the same scene. Refer to the section on '"Filters for Arc Sources." 400 500 600 700 nm 800 wavelength X - ► Figure 21. (a) Relative spectral power distribution of radiant energy of HMI 575-W and spectral radiance distribution (b) of daylight at 6500 K. Page 345 is a brief summary of the electrical and physical characteristics of the lamps comprising the full range of HMI sources. Figure 22 is a graphic presentation of the various parameters of HMI amps expressed in terms of percentage changes in the supply voltage. It is of particular interest to note that the color temperature increases with decreasing voltage. Like all metal vapor lamps, HMI lamps require a certain period after starting imtil final operatiiTg conditions are reached. The warm-up period varies with the lamp wattage, but typically is of the order of a minute or two from a cold start. Figure 23 shows curves of the electric and photomeh'ic data during warming-up of the lamp in operation with a standard inductive ballast. After ignition the lamp current at first iiicreases. Power consumption, operating voltage and luminous flux, however, are lower during the warm-up stage than when in full operation.

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