Example: Raytracing Evaluation Clerestory Mirror Designs

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ExampleRay-tracing EvaluationClerestory Mirror
Designs

• By Gregers Reimann20 September 2005

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AutoCad based Ray-Tracing Tool

• Software conceptualized together with CK Tang
• Software written by CK Tang using the Lisp code,
  which can be run with AutoCad
• Software completed 15th September 2005

Excerpt of software code
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Analysis of Clerestory Reflector Designs

• Designs analysed
• No outer reflector
• Parabolic reflector 1 inner flat mirror
  (original design)
• 3 outer flat mirrors 1 inner flat mirror
• 3 outer flat mirrors 2 inner flat mirrors
• Circular outer mirror 2 inner flat mirrors

Screendump of AutoCad file
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Ray-Tracing Input

• Start angle 0 deg (horizontal)
• End angle 180 deg (horizontal from the other
  side)
• Angle step interval Every 5 deg
• Number of rays 40 at each angle step interval
• Rays evenly stretched along 2.6 meter line
  pivoted at top of window

Pivot point
2.6 m line
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Ray-Tracing Software Features

• 2D ray-tracing analysis
• Surfaces are either perfectly specular
  (reflective) or completely non-reflective (i.e.
  no rays reflected)
• Rays are reflected 100 times and are not reduced
  in strength for each reflection
• Parallel rays from user-defined angle intervals
  are emitted and the percentage of the rays that
  reach their desired destination (e.g. from
  outdoors to indoors) is automatically calculated
• The ray analysis is automatically put in a text
  file for easy import and analysis in Excel

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Example of Ray-Tracing(with only a few rays)
Pivot point
Innerreflec-tors
Outer reflectors
2.6 m line that is rotated
Opening to atrium 26 of rays reach here
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Ray Penetration at Different Angles(0
horizontal 90 vertical)
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Relative Ray Penetration
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Conclusions

• The two inner mirrors perform considerably better
  than the single inner mirror. A split inner
  mirror design is therefore recommended.
• The solution giving the highest light penetration
  is 3 outer flat mirrors 2 inner flat mirrors
• All the solutions with the outer reflector give a
  high light penetration for steep angles around
  vertical. This is where the sun often is
  positioned.
• The bulkiness of the frames holding the mirrors
  and grills inside the clerestory opening should
  be minimised not hinder light entry
• The internal surface below the clerestory window
  should be highly reflective (NB. For the
  simulations it was assumed to be spectrally
  selective).