The purpose of this section is to explain simple general rules for dealing with illumination data. In particular, it will provide the means for interpreting data offered by manufacturers and for interpolating readings based on measurements made by the cameraman.

1. Lighting Quantities — Intensity

Intensity is measured in units of "candelas." An earlier term for this is candlepower. Normally, a value for candelas is also accompanied by directional information. In former times the intensity on axis was referred to as center beam candlepower.

The unique property of intensity relative to the source of light in a given direction is that it is not dependent on distance from the source. The intensity is the same no matter how far away. The only restriction is that it has reduced accuracy if measurements are made closer to the source than approximately ten times the maximum diameter of the lighting unit. For example, for a 12 fresnel lens spotlight, tine intensity figures are only accurate at a distance greater than about 10 feet.

There are two ways that the intensity information is normally shown. Examples of these are shown in Figures 7 and 8. The only difference between these is that in one case the data is presented in a rectangular coordinate format, and in the other polar coordinates are used. Most lighting manufacturers supplying instruments to the motion-picture industry tend to present their data in a rectangular format. The polar presentation is more likely to be encountered with commercial/industrial type fixtures.

Where the intensity distribution of a lighting source is known, the illumination produced by the unit can be calculated using the inverse square law. This is expressed as follows:

Intensity (candelas)

Illumination (foot candles) =

Illumination (Lux) =

D2(D = distance in feet)

Intensity (candelas) D2(D=distance in meters)

(Example: A fixture is described as having a center intensity (or center beam candlepower) of 50,000 Candelas. What is the illumination at 25 feet? What is the illumination at 10 meters?

50,000 50,000

25 x 25 625

50,000 50,000

10x10 100

2. Lighting Quantities—Coverage

All lighting fixtures have a lighting distribution which may be visible as projected on a flat wall. Often this is expressed as shown in Figure 9 and defined as an illumination distribution curve. The important standard measuring points for such a distribution are as follows:

Beam Coverage: This is described as the limit of the area covered to within 50% of the maximum intensity.

Field Coverage: This is described as the area covered to within 10% of the maximum intensity.

Of the two areas described above, the beam coverage is the more important photographically. It describes the area that is illuminated at a level that is not lower than 1 stop down from the center intensity. The assumption is made, where a single distribution is shown, that the distribution pattern is essentially circular.

Calculating Coverage from Beam Angle: The following expression allows the computation of the coverage diameter (W) for any distance (D) and a given beam angle (Refer to Figure 10). The expression is:

Maximum Intensity

Maximum Intensity t

Beam Coverage --Field Coverage

Figure 9. Definition of intensity distribution curves.

Maximum Intensity

50% of Maximum Intensity

Maximum Intensity

Beam Coverage --Field Coverage

Figure 9. Definition of intensity distribution curves.

(Example: For a distance of 50 feet and a known beam angle of 26 degrees, what is the coverage diameter of the beam (50% of the center)?

D = 50 feet; Beam Angle = 26 degrees. x/i Beam Angle = 13 degrees Tangent of 13 degrees = .231

3. General Comments on Calculations

Most manufacturers are now offering both candela information and angular coverage. This is actually sufficient information to make some approximations of what to expect from the lighting fixtures using the procedures outlined above.

In the event that it is necessary to convert from foot-candles to lux, the value of footcandles should be multiplied by 10.8. To convert lux to footcandles, divide lux by 10.8.

Usually, lux values will be associated with distances measured in meters, and footcandles with distances measured in feet. In the case of the illumination calculations above, the use of feet or meters as the units of distance will automatically yield illumination values in footcandles or lux respectively.

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