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POVRAY Files required to  make these images are:




































































































      CalpovZMX.zip



























































      
        pipebend.zip
 

























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                            BENCHMARKING POVRAY AGAINST ZEMAX

POVRAY is clearly capable of forming real images inside photorealistic representations of the optics creating the images.  Although as observed in the index page it was originally conceived as an artistic tool,  it is worthwhile testing the accuracy of the POVRAY images using ZEMAX as a calibration.

This first test-scene is a darkroom with a lightpipe experiment as a test object.  The lightpipe is formed by just two optical components – a glass cylinder merged with a spherical end forming a lens.   Light from an LED is collected over a 90deg cone angle by the cylinder and is guided by direct raypaths and total internal reflection TIR to the lens.  The glass material of the unit is Schott SF6.  This glass was selected, using ZEMAX as a design tool, to have the sharpest image from a 10mm diameter 60mm long cylinder and 10mm diameter lens at an image distance of 1.0mm.  The lens focusses the light to a diffuse 10mm x 10mm square diffuse white screen.  The screen can also be contrived to be an image source glowing with the site logo.

The assembly is mounted on a steel topped scraped texture laboratory optical bench.  The table is drilled with countersunk holes for fixing components.  The scene is lit by a single overhead point light source mounted from a 30% reflective ceiling. The camera is located about 100mm from the pipe and has a field of view 35degrees wide.

                                                   

                                                                           

In this first image the LED is off - the scene is lit only by the overhead light.
Notice how a virtual image of the web-site logo on the screen is visible through the end of the cylinder, and the optical bench is visible through the wall of the cylinder.  But notice also the shadow under the cylinder.  This is counter-intuitive – any optical engineer would expect to see a strip patch of focussed light under the cylinder.  This was the type of image that was created by early versions of POVRAY - it was the introduction of 'photon mapping'  by Nathan Kopp following its creation by Henrik Wann Jensen that transformed the artist's toolbox into a serious optical analysis tool.


                                                                             


In this image photon mapping has been applied - the image is now much more realistic in that one sees that the cylinder and sphere have formed a strip image of  the overhead light  source. The image is irregular - because the steel table is not uniformly diffuse spatially.

The scene is rather stark because of the single lightsource - by mounting an array of
luminaires on the ceiling the lighting becomes much more diffuse - stark shadows are softened and highlights appear in the countersunk holes in the bench.


                                                                                 



                                 Steel tables are rather dreary - so replace it with a shiny checkered  reflective plastic.


                                                                                                                

The darkroom lights are then turned off and the LED turned on.  POVRAY was then used to calculate the image profiles on the display screen.
 
                         
                                   

POVRAY creates a *.tga file representing the image brightness as a solid object. Because the image is rotationally symettric it is convenient to display the profiles through a centrally cut section. In the image above the cross section has been textured with the 'jet' palette to indicate the height

                                                        
                         

This is the ZEMAX style image in which the ray intersections in the image plane are counted and displayed as a graph.

These images are clearly of a very similar structure, so we can be confident that POVRAY is yielding reliable results.

We now extend the lightpipe application to the much more complex task of a bent lightpipe.  The lightpipe is now arranged to relay the LED through to a screen  100mm diagonally displaced. The bend is achieved by with a quarter torus and two cylinders arranged at 90deg.  But it's not known what should be the radius of the torus to achieve the maximum brightness on the screen. Nor is it known what will be the position of the brightest image.

By animating the bend radius and observing the screen image we can answer both of these questions.


The time required to create the 32 frames in this animation was greatly reduced by omitting the holes in the optical bench and enclosing the lightpipe  in an opaque plastic cladding.

To view the 0.64MB animation:
 

  pipebend1.m1v 
 


The image to the left is a textured heightfield of the intensity at the screen  for the value of the bend radius which maximises the intensity. The peak intensity is 0.8mm from the axis. The values were determined from the following  0.64MB movie:

pipebend3.m1v


 



   
 
© Don Barron 2005