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Model Planning and Construction

Beauty is only skin deep - General Principles

Light measurements scale perfectly which allows the representation of real-life lighting conditions in scale models as long as attention is given to certain parameters. A daylighting model is constructed specifically for testing and may or may not be useful for making other general design decisions and may not be a particularly "attractive" model, if done properly however, it will provide valuable information regarding overcast sky performance of a daylighting design.

How big should I make it? - Scale/Size

The scale of the model is influenced by a variety of factors that can result in different model scales/sizes dependant upon project and testing parameters.

Sensor Size/Work Plane Measurement Level

An important influence is the physical size of the Li-Cor measurement sensor. Each sensor is a cylinder 1" in diameter by 1" in height with the photocell at the top center of the cylinder. The 1" height is important in the context of the scale height of the collected measurement. For example, in a 1/4" = 1' scale model, the 1" high sensor will take a measurement at a height corresponding to 4' above floor level in the proposed building design. Likewise, a 1/8" = 1' scale model will result in measurements taken at 8' above the floor level in the proposed design (perhaps not a particularly useful measurement).In practice, sometimes a scale of 3/8" = 1' is convenient to use because it places the 1" high sensor top at a 32" scale height which is close to the standard workplane height of 30". A1/2" = 1' model is also sometimes practical in that, with a 1/4" thick base for the sensor, the top of the sensor's scale height would be exactly 30".

While the dominant plane of evaluation for lighting performance is the thirty inch high work plane across the entire dimension of the floor plate area, the uniqueness of activities in a space, (for example a children's play space in which the users are on the floor) the differentiation of the space into sub-zones of activity (a conferencing area as distinct from a desk area or a lounging/seating area) all suggest that one must be clear-minded about the intended need for measurement. It is not uncommon to construct models in which the sensor is recessed into the floor or placed in other ways to account for the intended plane of measurement—e.g., wall surfaces in museum studies; floor level placement in sports facility studies; or elevated placement for factory settings in which the work plane is at standing height.

It is possible to select a model scale based upon other influences and then either raise the sensors on a base or recess the sensors into the model floor to bring the top of the sensor to the desired measurement height. Be advised however that this technique often adds to the complexity of the model and testing process.

Is there daylight in there? - Areas of Interest

Often the scope of a model's construction can be limited to a particular area of interest. If an area or component of a design does not receive daylight or does not influence the daylight received by an adjacent area of the design, it is not necessary to include it in the model. This can influence the size and scale of the model to be constructed and tested. Often it is not necessary to model the entire building. Individual rooms and/or particular zones of the building can be independently modeled and tested if this approach proves convenient. Modeling and testing only representative or prototypical areas of the design can also be an approach to efficiency in a daylighting investigation.

Do I need to build that? - Economy of Materials and Detail

Simplicity with accuracy is a desirable goal for the model. While components, materials and details that are influential to daylighting performance should be accurately included, non-influential elements may be simplified or omitted. At smaller building model scales even influential elements may be simplified to represent their overall effect if known. For example, at small scales, furniture might be modeled as simple block elements of appropriate reflectances to simulate the overall effect of the furniture's presence without including unnecessary detail.

Will it fit in the box? - Testing Chamber/Table Size

There are physical limits that restrict a model's size and resulting scale. The artificial sky's testing zone volume is approximately 7' long by 7' wide by4'-6" high. In practice however it is best to keep well back from all the edges (at least l'-6")andtop(at least 2') of the testing zone surfaces for accuracy and to minimize mirror influences and leave room to work around the model during testing. In addition the door opening into the artificial sky is 4' wide by 4'-6" tall. The model table surface on which the model must rest is a 2'-6" diameter circle. These physical limitations must be considered when planning model construction.

Can I get my hand in there? - Physical Access

As was just mentioned, it is necessary to make the model small enough to leave some room around the model in the artificial sky to allow access to the model exterior when needed. As a counterpoint, the size and scale of the model will need to be large enough to accommodate access to the model's interior for alterations, adjustments, repairs and sensor placements. Usually this requires that will allow the tester to reach in to the mo eel interior for adjustments.

Oooh, a picture of that would've been nice - Photographic Access

In addition to data collection, sometimes it is desirable to photographically document the testing configuration and/or general daylighting effect. While this must be done carefully to avoid light distribution influence from the presence of the camera and/or any photographic portal this consideration also has an influence on model size and scale. The model must be at a size that is able to accommodate any photographic lenses/equipment needed to photograph the model interior.

Light Leaks

During testing light entering the model from unintended openings and directions must be eliminated. During model planning and construction care must be taken to seal material joints and edges to eliminate hairline cracks where light can enter the model. Opaque tape (like black electrical tape) is sometimes used to seal all joints in the model from its exterior. In cases where interchangeability of model parts and surfaces is desired, sometimes the model design and construction will include overlapping joint techniques that will seal light leaks and yet allow the removal or exchange of model components and surfaces.

Translucent Modeling Materials

Unintended light can also enter a model directly through some materials used in the model's construction. Some materials, while at first glance appearing opaque, turn out to be translucent when exposed to high illumination levels like those found in the artificial sky. The popular modeling material called Foamcore is one such example. Models must be constructed to be truly opaque. This means either carefully choosing only opaque materials or layering materials to achieve opacity. Layering is sometimes used to address both translucency and reflectivity (see next section) in the model. Sometimes a model is constructed of one material and then the interior surfaces are lined with a second thin but opaque layer of material (paper) that has the proper reflectivity. The layering of materials can also add complexity to a model's construction and details.

All white is not right - Accurate Reflectivity

Different colors and reflectance
without light source.


It is critical that the actual reflectances of all proposed interior surfaces are modeled correctly to achieve meaningful testing results. While it is not necessary to represent the actual color of surfaces, the equivalent tonal reflectances must be represented by modeling materials, regardless of color. For example if an interior wall will be painted with a blue paint that has a 45% reflectance characteristic then a material must be used in the model that has the same reflectance characteristic, perhaps a grey chipboard that also has a reflectance of around 45%. As was mentioned in the previous section, sometimes secondary materials are layered onto a model's interior to achieve proper reflectance representations.

Reflections from the ground plane should also be considered.

Same colors and reflectance with light source.

The entire chamber below the horizon is purposefully painted a 27% reflectance tone of grey. This is the approximate reflectance value of grass and vegetation. If the immediate vicinity surrounding the proposed building design is a different material (such as asphalt, water or sand) a ground plane of an appropriately reflective material should be constructed and attached to the model adjacent to any daylight apertures. In certain situations, the reflectance of roof materials can also affect the amount of light bouncing into a roof aperture. In this case, the exterior building reflectance of the adjacent roof should be accurately modeled as well.

Windex anyone? - Visible Light Transmission

In real buildings light entering through a daylighting aperture is reduced by the presence of glazing materials. This reduction can be significant ranging from a 10% reduction to nearly a 90% reduction in daylight. Glazing systems are measured to determine their ability to transmit visible light. Each glazing system should have an associated specification called Visible Light Transmission (VLT). VLT is a measure of what percent of visible light will pass through the glass. A glazing assembly with a VLT of 75% allows three-quarters of the available light outside through to the inside. Decisions about glazing are clearly important when considering daylighting design and testing. The proper modeling of the effect of glazing in a physical model is very difficult due to the extremely limited palette of modeling materials when compared to the vast palette of glazing possibilities. Because of this, gross approximations are sometimes necessary. Usually models are constructed without any glazing or framing members in the daylight apertures. Instead the measured values are universally adjusted downward by the appropriate percentages after testing to account for the glazing and framing influences. This is acceptable practice if all of the glazing types and framing are similar within the building design being modeled and tested. If drastically different glazing systems coexist then attempts must be made to include the glazing in the model using manufacturer's samplesor other representative transparent materials.

I can't even see through that - Transparent vs. Translucent

When only transparent glass is present it is acceptable, within the stipulations stated above, to use the universal VLT adjustment technique in lieu of including actual glazing materials in the model. However, any translucent daylight apertures should be physically modeled and should use layering to achieve the appropriate VLT value for the proposed glazing system. Drafting velum and tracing paper are good materials to use when modeling translucent glass. While variations in clear glass VLT affect only the amount of light that is transmitted through the glass, the introduction of translucent glazing also changes the distribution pattern of that daylight and must be physically modeled. This presents complications when both translucent and transparent glazing is present in a model because the universal post-measurement adjustment technique cannot be used and all glazing materials must be physically represented.

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