Figure 1. The PG&E Heliodon with a residential model (38K jpg)

Over the years heliodons or "sun machines" have been built in a variety of configurations. In each case, the device creates the appropriate geometrical relationship between an architectural scale model and a representation of the sun. By allowing adjustment for solar declination (season), the earth's rotation (time of day), and site location (latitude) a heliodon can simulate sunlight penetration and shading for any combination of site location and time. The result is a useful representation of solar patterns for clear sky conditions. Other techniques are often used in concert with heliodon simulations to account for variations in the strength of the sun (due to weather, angle of incidence, and atmospheric attenuation) and local horizon shading. Heliodons provide an effective tool for the visualization and calculation of solar effects at the window, building, or site scale.

Figure 2. Participants in a heliodon seminar (58K jpg)

Companies have periodically produced manufactured heliodons. Commercially-produced devices have been largely marketed to architectural firms and, unfortunately, are often compromised by the criterion of low first cost. The products have typically been less robust than desirable and the companies are often short lived. A more serious generation of devices has been produced in building science research and academic institutions around the world. These devices are fabricated to custom designs and support a more demanding duty cycle. The Building Science Group's heliodon, located at the PG&E Energy Center in San Francisco, belongs to the latter category.

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the PEC heliodon design | heliodon construction | heliodon accouterments

The PEC Heliodon Design

The heliodon we designed for the PG&E Energy Center uses the fixed-sun moving earth approach. The sun is represented by a theater spotlight mounted near the ceiling at a distance of 32 feet. Adjustments for season, time, and weather are built into an articulated "table" with the top of the table representing the building site. Advantages of this approach include:

• The light source is relatively inexpensive. The light striking the heliodon, like the sun it represents, should have parallel rays. The fixed-sun, moving-earth scheme approximates parallel rays by placing the light source at a distance. If mounted at 30' from the table, light will diverge less than 2 from the table centerline to its edge.
• The fixed-sun, moving-earth heliodon demonstrates the actual motions of the earth relative to the sun. In our design the moving-earth table has a visible north pole-south pole axis that tilts toward and away from the fixed-sun for seasonal variation and around which the table top rotates.
• The fixed-sun, moving-earth scheme requires relatively little space. If the light source is ceiling mounted and has a clear path to the table location then a working area of ten feet square is sufficient.

There are two disadvantages traditionally associated with fixed-sun, moving-earth heliodons and both are associated with the action of moving the table and the model attached to it:

• The tilting table will place the model it carries at various angles during a simulation. The model must be constructed to withstand tilts up to 90 from horizontal. In practice, this disadvantage only arises when examining models developed for presentation rather than analysis. It is our experience that the great majority of presentation models can be easily tightened up for heliodon analysis and, in fact, we have yet to turn a model away from the PEC facility.

• It is somewhat difficult to observe patterns of light and shadow when they occur on a moving object. At the PG&E Energy Center we use a variety of techniques to work around this disadvantage. The most powerful of these is a small video camera that moves with the moving model and provides a constant-point-of-view depiction of light and shadow on an adjacent video monitor.

Heliodon Construction

The heliodon at the PG&E Energy Center was designed and built by a three person team. Professor Charles C. Benton of the University of California, Berkeley served as the design expert with principal responsibilities in review and management. Paul LaBerge was the lead designer for the device and resolved the basic issues of its operation and production. Finally, Ian Melody, a machinist specializing in prototype devices, provided technical design assistance and the actual construction. 

All parts of the heliodon are custom machined and fitted, a process involving over three hundred hours of machining. The PEC heliodon is fabricated with high-quality ball bearings for all movements, a high-strength stainless steel axis shaft, and self-locking steel worm gears for declination and latitude adjustments. The table top, base, supporting structure, axis tube, and cross-axis tubes are all T-6061 aluminum. The main axis has a substantial stainless steel counter balance. A pinch collar surrounds the main axis shaft to temporarily secure the device at a specific hour. 

The legs, main axis tube, cross axis tubes, and handles are bead blasted and then covered with a clear acrylic lacquer finish. The top and base were prepared with a rotary sander and have the same finish coat. The heliodon is fitted with low profile rubber casters and non-marring feet. When repositioning or storing the device it is moved using a custom-fabricated lifting mechanism. 
The heliodon installation at the PG&E Energy Center is specific to its particular niche. The table's legs are set to an angle perpendicular to the light coming from the ceiling mounted fixture (the ecliptic plane.) We have recently built a second heliodon of the same design for the University of Hawaii and this too is custom designed for its setting.

Figure 4. Robert Marcial at the PG&E Energy Center Heliodon (50K jpg)

Figure 4 shows the heliodon and its accessories. The video monitor (upper left corner of image) displays an image of the interior of the residential model mounted on the heliodon. The image from the point-of-view camera is also routed to the Macintosh computer below the monitor. The shelves behind the heliodon contain the videotape recorder and camera electronics.

Heliodon Accounterments

Figure 5. Toshiba Point-of-View Television Camera (18K jpg)

Our heliodon is supported by a range of commercially available equipment. As mentioned before, the video imaging system is particularly effective in presenting and archiving the results of heliodon studies. The following roster outlines this supporting equipment.

• Strand Lighting Fresnelite Spotlight, model #3480 with a BVT 1000-W lamp and a 200-mm fresnel lens. This is a source with a 1 angular width that provides sufficient light for video recording. A less-expensive theater spotlight can fixture is a good alternative.

• Toshiba IK-M40A color CCD Point-of-View camera head and camera control unit. This delightful unit provides SVHS resolution (460 horizontal lines) video, S-video output, and interchangeable lenses in a camera the size of a lipstick holder. The unit achieves its small size by placing the electronics in a small remote box. The camera does not have an auto-adjusting iris but the steady light of the heliodon light fixture makes this unnecessary. 
  
• Toshiba 4-mm wide-angle lens for the Toshiba IK-M40A. This lens is roughly equivalent to a 20-mm wide angle lens for a 35-mm still camera. If offers substantial advantages in near focus, depth of field, and wide coverage.
  
• Toshiba EXC-405C cable. A 5-meter cable to connect the Toshiba IK-M40A camera head to its camera control unit. The cable is available in different lengths.
  
• Mitsubishi HS-U65 SVHS video cassette recorder. This unit is used to record heliodon test sessions. In our opinion the higher resolution of an SVHS or Hi-8mm recorder makes an appreciable difference in picture quality. The deck we use requires an external microphone. Many other VCRs are available.
  
• Mitsubishi CS2010R television monitor with S-video input. We wall-mount this 20" color monitor adjacent to and above the heliodon for ready reference during tests.
  
• Macintosh IIci computer with a video digitizing card by Supermac for the computer manipulation of heliodon video images. This hardware has been useful but is now out of date. You can purchase the equivalent for less than $200.
   

For further information on the PEC heliodon please contact the author. We intend to expand this site with additional pages on the type of models examined on the heliodon and a historical view of heliodons. If you know of useful heliodon references we would be interested in hearing about them. The earliest heliodon reference we have is a 1931 report by the Royal Institute of British Architects.