What to know about ambient HID lighting design.

Apr 1, 1996 12:00 PM

Are there easy-to-use reference materials that can be used to design ambient HID lighting?

You can design and specify an HID lighting system by using charts and tables, and making a few calculations. A Room Cavity Ratio table and a Footcandle/Spacing/Area chart are used for preparing an ambient lighting system design of regularly shaped areas. The charts assume that the interior space to be lighted has standard reflectance values for the room surfaces: 80% for the ceiling; 50% for the walls; and 20% for the floor. These are the most common reflectance values found in interior design today.

The footcandle (fc) levels are stated in initial horizontal fc at a work-plane height of 2 1/2 ft, and reflectances or absorption from wall panel systems are not considered. [TABULAR DATA FOR FIGURE 2 OMITTED]

A particular Footcandle/Spacing/Area chart is based on the use of a specific lamp type and wattage. The metal-halide (M-H) and high-pressure-sodium (HPS) lamps typically used are seen in Fig. 1.

Room Cavity Ratio

The Room Cavity Ratio (RCR) is a numerical value, or factor, representing the effect of the room proportions on the light level in the space.

For example, in a small room using a single, suspended, indirect HID fixture over a desk, the light is reflected from all fairly close room surfaces. During each reflection, some absorption occurs. In a larger room, less light will strike the room surfaces and be lost. Light contribution from a number of fixtures in the vicinity reaches the desk or work plane.

Thus, a small room has a higher RCR than a larger room. For example, a 14 x 14-ft room has an RCR of 5 while a 30 x 30-ft room, with a depth cavity of 9 ft, has an RCR of 3. The cavity depth is found by subtracting the height of the work plane (2 1/2 ft), from the overall ceiling height. A portion of an RCR table that covers rooms from 8 x 8 ft to 500 x 500 ft is seen in Fig. 2.

Using spacing charts

The Footcandle/Spacing/Area charts provide a simple means of laying out fixture locations for a regularly shaped room. Fig. 3 (on page 74) shows the plot curves for a single-lamp (250W M-H source) suspended fixture having a symmetrical wide distribution. The three plot curves represent a small, medium-sized, and large room.

As an example, let's consider the lighting-system requirement for a 30 x 30-ft room (RCR = 3) in which an initial illuminance of 40 fc is needed on the work plane. The first step is to locate the 40-fc level at the left side of the chart and extend this point to the right until it intersects the medium-size room curve (RCR = 3). This point is then projected down to the bottom of the chart where the spacing requirement is read. In this case, the spacing will be slightly greater than 17 ft, and each fixture will cover about 290 sq ft. Then the total area of the room (900 sq ft) is divided by the 290 sq ft per fixture; thus, 3.10 fixtures are required. This fractional number is rounded off to three 250W M-H fixtures.

The notes beneath the chart indicate that by substituting a 250W super M-H lamp, the fc level will increase by a factor of 1.18. So, by using the 250W super M-H lamp, it may be possible that the 40-fc illuminance level can be achieved with fewer fixtures. The new fixture count is found by dividing the previous fixture count by the given proration factor. Here, 3.10 fixtures divided by 1.18 equals 2.63. Thus, three fixtures again would be required using the super M-H lamps. However, a slightly higher lighting level will be achieved.

Irregularly shaped areas

The previously described chart is not suitable for planning irregular or unusually shaped spaces. Instead, an isofootcandle-curve reference plot is used. Because each particular HID lamp/fixture combination has a defined candlepower distribution pattern, it can be reviewed or studied the same way as the isofootcandle-curve reference plot of a roadway or outdoor-area lighting fixture.

For an indirect luminaire, this method involves the use of an isofootcandle-curve plot that shows the lighting level on a horizontal work plane at 5-ft increments from the location of the HID source. Fig. 4 is a computer-generated plot for a 250W super M-H lamp in a ceiling-suspended fixture with a reflector system having a wide symmetrical distribution. The tabulation is based on the fixture at a 7-ft mounting height above the floor, or a ceiling height factor of 1.0. A proration factor is used for other mounting heights such as 5 1/2 ft, which is the mounting height for a freestanding unit.

The plot has 2-ft grid increments, and the dark square on the lower left-hand corner in Fig. 4 represents the fixture location, while the curved lines represent points of equal illumination. The curve closest to the fixture indicates points of 60 fc; the next curve out indicates points of 50 fc. From this single quadrant, a total symmetrical (or asymmetrical) plot can be reproduced.

To show how a plot can be used, the distribution pattern for two wall-mounted, asymmetrical distribution luminaires in a 30 x 30-ft room is given in Fig. 5. The plot indicates that every point in the room receives fc contributions from both of the fixtures, each of which have forward throw optics. The shaded rectangle indicates the location of a desk or workstation. Each fixture has a 250W M-H lamp and a 250W HPS lamp. In such an installation, the brightness ratios of the room would be within acceptable standards as per the Illuminating Engineering Society (IES).

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