Design and evaluation of daylighting applications of holographic glazings Page: 3 of 22
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Scope and Objectives
According to the contractual agreement, BTP would develop a computer model of the POC holographic structures
and then simulate the performance of alternative designs using the RADIANCE lighting and rendering computer
program [Ward 1990]. The RADIANCE model would then be used to evaluate the daylight performance of
alternative designs of holographic glazings in a prototypical office space. The simulation process would be
validated against actual photometric measurements of holographic glazing samples developed by POC. The
results would be used to evaluate the potential for increased electric lighting savings through increased daylight
illuminance levels at distances more than 15 ft - 20 ft (4.6 m - 6.1 m ) from the window wall.
Algorithmic Evaluation
In collaboration with POC, LBNL developed algorithms to model the behavior of holographic glazings. These
algorithms were coded into subroutines that were used with the RADIANCE lighting and rendering computer
program to simulate the daylight performance of holographic glazings when applied to side apertures in
prototypical office spaces.
Diffraction Modeling
The modeling of the radiative behavior of holographic glazings was based on the equations provided to LBNL by
POC. These equations are used to compute diffracted direction and efficiency, based on a given incident
direction of radiation. The initial sets of equations provided by POC were not compatible with the way that
RADIANCE computes light propagation. The equations were approximating diffraction direction, while
RADIANCE needed exact directions of each ray in order to find a light source as small (in terms of angular size)
as the sun. To overcome this limitation and to avoid the cost of numerically averaging over many incident
polarizations, POC developed a new set of equations providing a closed-form solution to the randomly polarized
average.
Using the second set of equations, LBNL developed two versions of the basic hologram model. In the first
version, one enters the exact wavelength of light to simulate, and the computation is carried out at that wavelength
only. In the second version, a range of wavelengths is averaged to get a better (though slower) approximation to
the continuous spectrum of the solar radiation.
Daylighting Modeling
RADIANCE employs a variant of ray tracing, where light is followed backwards from the point of measurement
(usually a camera) into a scene and to the light sources. Because of this, re-directions such as diffraction by
holographic glazings must be treated specially. In a preprocessing step, RADIANCE identifies all redirecting
surfaces and the light sources that may be redirected by them. The provided formulas for new ray directions and
coefficients are then used to determine general behavior for each surface, and a set of virtual light sources is
created. In the case of our holographic glazing, one new sun is created for a total of two light sources. One is in
the same position as the original sun, representing the transmitted component, and one is in the position of the
diffracted component (Figure 1).2
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Papamichael, K.; Ehrlich, C. & Ward, G. Design and evaluation of daylighting applications of holographic glazings, report, December 1, 1996; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc722894/m1/3/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.