Example:Ray-tracing Evaluation

Clerestory Mirror DesignsBy: Gregers Reimann20 September 2005

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

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

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

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

Example of Ray-Tracing(with only a few rays)

2.6 m line that is rotated

Pivot point

Outer reflectors

Innerreflec-tors

Opening to atrium: 26% of rays reach here

Ray Penetration at Different Angles(0 = horizontal; 90 = vertical)

0%

20%

40%

60%

80%

100%

0 30 60 90 120 150 180

ray angle from horizontal (degrees)

pe

rce

nta

ge

of r

ays

pa

ssin

g th

rou

gh

No outer reflector

Anidolic design (original)

3 flat outer + 1 inner

3 flat outer + 2 inner

circular + 2 inner

Relative Ray Penetration

Increase in light penetration in comparison to having no outer reflector

0%

41%

15%

68%

45%

0%

20%

40%

60%

80%

100%

No outerreflector

Anidolicdesign

(original)

3 flat outer +1 inner

3 flat outer +2 inner

circular + 2inner

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).