In this section a brief description is offered of the optical systems and general performance characteristics of the basic types of luminaires utilized in cinematography.

Fresnel Lens Spotlights

Fresnel spotlights are made for standard incandescent and tungsten halogen incandescent sources, and also for the range of HMI, CID and CSI arc discharge lamps. The range of wattages, taking into account all types is from 200 watts or so to 12,000 watts.

flood position.
Figure 47. Optical system of Standard Fresnel Spotlight when in spot position.

These luminaires represent the most widely used motion-picture lighting units. They provide the means for changing the beam diameter and center intensity through a relatively broad range. Using standard incandescent lamps, the "spot" to "flood" ratio may be of the order of 6 to 1 or so, and with a tungsten halogen lamp, it may be

Coverage (Ft.)

Figure 49. Characteristic intensity curve of Fresnel Spotlights.

Coverage (Ft.)

Figure 49. Characteristic intensity curve of Fresnel Spotlights.

possible to extend this ratio to 8 or even 9 to 1 under some circumstances.

The optical system of these luminaires is the same for all the variations that may be presented. The light source and a spherical reflector are located in a fixed relationship to one another. This combination of light source and back reflector is designed so that the spherical reflector reflects the energy being radiated toward the back of the housing through the filament and towards the lens. The effect intended is that the energy being radiated to the lens appears to come from a single source. The combination of the reflector and light source are moved in relation to the lens to accomplish the focusing.

Figures 46 and 47 show the optical system of the fresnel in the spot and flood positions. Note that the flood position is accomplished by moving the light source/reflector combination very close to the lens. When the tungsten halogen light sources are utilized in these systems, due to the fact that the envelope is much smaller, it is possible to move the light source/reflector combination even closer to the lens resulting in a wider flood beam distribution. This is shown in Figure 48.

This is a very attractive feature, since the highest efficiency is achieved in the flood position, and there need be no sacrifice in the spot performance. Typical efficiencies in the beam (the portion of the pattern that is within 50% of the center intensity) in "spot" focus for fresnels would be from 7% to 9% and in the "flood" position from 307.» to 40%.

One of the most important features of the fresnel lens spotlight is its ability to barndoor sharply in the wide flood focus position. This property is less apparent as the focus is moved towards a spot (at spot focus it is not effective at all). The barndoor accessory used with this spotlight provides the cinematographer with the means for convenient light control. The sharp cutoff at the wide flood is, of course, due to the fact that the single-source effect produces a totally divergent light beam. The action of the barndoor then is to create a relatively distinct shadow line.

Occasionally it may be desirable to optimize the spot performance of these units, and for this situation "hot" lenses are available. These tend to produce a very narrow beam with very high intensity. It is important to remember that the flood focus is also narrowed when these lenses are used. Figure 49 shows characteristic intensity curves for fresnel spotlights.


The Dedolight, introduced within the last several years, is a lighting instrument whose concept is unique, and which offers a remarkable range of performance combined with small size, and low power requirements (see Figure 50).

The optical system is shown in Figure 51. Note that the moving element in the system is the light source with a collection mirror behind it, and meniscus lens opposite. To change the focus of the unit, these three elements, which are fixed with regard to each other, are moved as a unit relative to a clear fixed condenser lens.

Figure 50. The Dedolight.

Spherical Reflectors Filament

Condenser Lens

Spherical Reflectors Filament

Condenser Lens

Figure 51. Dedolight Optical System.

The performance of the light is shown in Figure 52, where the 25:1 focusing range can be seen, and the unusually flat, even and soft-edged illumination fields are evident at all focus positions.

When fitted with an accessory projection attachment, the beam can be controlled further by the use of an iris or framing shutters. It projects Rosco "M"-size gobos and will project patterns with hard edges and without color fringing. Where a diffuse or soft-edged pattern projection is desired, the front lens of the projection accessory can be adjusted to accomplish this effect.

The Dedolight is made as either a 12-volt or a 24-volt 150-watt unit. The 100-watt unit can utilize a family of lamps including (at 12 volts) 20,50 and 100 watts. The units can be battery operated or can be used from 120- or 240-volt AC supplies offered for use with these luminaires which permit selection of 3000° K, 3200°K or 3400°K operation.

30° 25° 20° 15° 10° 5° 0° 5° 10° 15° 20° 25° 30°

Figure 52. Dedolight performance with lOOw source, 10 ft. distance, spot 3.4°, flood 40°.

30° 25° 20° 15° 10° 5° 0° 5° 10° 15° 20° 25° 30°

Figure 52. Dedolight performance with lOOw source, 10 ft. distance, spot 3.4°, flood 40°.

Open Reflector Variable Beam Spotlights

These are typically the tungsten-halogen open reflector spotlights. There are also some low-wattage HMI-types available. These non-lens systems provide "focusing" ac-

Figure 54. Characteristic intensity curves of non-lens spotlight (variable beam).

tion, and therefore a variable diameter beam, by moving the light source in relationship to the reflector (or vice versa). These types of units are available for sources ranging from 400 to 2,000 watts. Refer to Figures 53 and 54. One of the drawbacks of this system, when compared with the fresnel lens spotlights, is that there are always two light sources operative. The illumination field produced by these systems is the sum of the light output directly from the bulb and the energy reaching the field from the reflector. The use of the barndoor accessory with these lights does not produced a single shadow, due to this double-source characteristic. Typically a double shadow is cast from the edge of the barndoor. Figure 48 shows the optical systems of these open reflector spotlights in both the spot and wide flood positions.

The great attraction of these luminaires is that they are substantially more efficient than the fresnel lens spotlights. Typical efficiencies in the spot position give 20 to 25% of the source lumens in the beam (50% of the center intensity area) and in flood, efficiencies of 45 to 50% are not uncommon. Figure 49 shows typical intensity distributions for these units.

Typical spot to flood intensity ratios for these types of units is between 3:1 and 6:1.

Figure 55. Characteristic intensity curve of tungsten-halogen floodlight (broad) (horizontal axis).

Tungsten-Halogen Floodlights

A variety of tungsten-halogen floodlighting fixtures have been developed, taking advantage of these compact sources. Two of the more typical forms are treated here. These fixtures are available in wattages from about 400 through 2,000 watts.

The so-called "broad" normally uses a linear source and represents a relatively high efficiency system. Barndoor control of the light is effective with the edge of the door that is parallel to the light source. Typical characteristic intensity curve for the broad is shown in Figure 55.


Was this article helpful?

0 0

Post a comment