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Science Forum Index » Optics Forum » need help with ray tracing
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| pereges |
 Posted: Tue Apr 22, 2008 8:06 pm |
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Guest
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Hello
I'm a computer science student and I am doing a project where I need
to calculate the radar cross section of an 3D object. I have to use
the ray tracing approach. So far I'm able to read the object from the
geometrical specifications given and store it in appropriate data
structures.
But now I am having problem in generating the rays given the
location of the source. I have to generate parallel rays(plane waves)
which hit the object I cannot figure out a way to do this ? Can any
one please give me some idea ? Is there some optical equation involved
which I can use ? Also, I am calculating energy as E = h * f and
attenuating it by 1/r ^2 depending on distance travelled. IS this a
right approach ? |
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| Helpful person |
| Posted: Wed Apr 23, 2008 1:45 am |
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Joined: 22 Jun 2004
Posts: 677
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On Apr 23, 2:06 am, pereges <Brol...@gmail.com> wrote:
Quote: Hello
I'm a computer science student and I am doing a project where I need
to calculate the radar cross section of an 3D object. I have to use
the ray tracing approach. So far I'm able to read the object from the
geometrical specifications given and store it in appropriate data
structures.
But now I am having problem in generating the rays given the
location of the source. I have to generate parallel rays(plane waves)
which hit the object I cannot figure out a way to do this ? Can any
one please give me some idea ? Is there some optical equation involved
which I can use ? Also, I am calculating energy as E = h * f and
attenuating it by 1/r ^2 depending on distance travelled. IS this a
right approach ?
The easiest book to understand and read regarding ray tracing (in my
opinion) is "applied optics and optical design" by A E Conrady,
written (I think) back in the 1930s. Get a copy out of the library
and read the section on ray tracing. It is refreshingly well written
and the methods are the same as the ones used today. (The whole book
makes wonderful reading.)
This book was written when ray tracing was performed using mechanical
calculators. You can ignore (for computing purposes) the
constructions that were required back then to ensure accuracy. You
will find much more than you need in this book but it will contain the
equations you do need.
Regarding radiometry, the equations to use depend on your system
geometry. But in general, power falls off as the inverse square law. |
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| Helmut Wabnig |
| Posted: Wed Apr 23, 2008 9:30 am |
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Guest
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On Wed, 23 Apr 2008 04:45:56 -0700 (PDT), Helpful person
<rrllff@yahoo.com> wrote:
Quote: On Apr 23, 2:06 am, pereges <Brol...@gmail.com> wrote:
Hello
I'm a computer science student and I am doing a project where I need
to calculate the radar cross section of an 3D object. I have to use
the ray tracing approach. So far I'm able to read the object from the
geometrical specifications given and store it in appropriate data
structures.
But now I am having problem in generating the rays given the
location of the source. I have to generate parallel rays(plane waves)
which hit the object I cannot figure out a way to do this ? Can any
one please give me some idea ? Is there some optical equation involved
which I can use ? Also, I am calculating energy as E = h * f and
attenuating it by 1/r ^2 depending on distance travelled. IS this a
right approach ?
The easiest book to understand and read regarding ray tracing (in my
opinion) is "applied optics and optical design" by A E Conrady,
written (I think) back in the 1930s. Get a copy out of the library
and read the section on ray tracing. It is refreshingly well written
and the methods are the same as the ones used today. (The whole book
makes wonderful reading.)
This book was written when ray tracing was performed using mechanical
calculators. You can ignore (for computing purposes) the
constructions that were required back then to ensure accuracy. You
will find much more than you need in this book but it will contain the
equations you do need.
Regarding radiometry, the equations to use depend on your system
geometry. But in general, power falls off as the inverse square law.
The OP wants to use E = h * f inverse square damped.
Someone should tell him what to do.
w. |
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| Fritz |
| Posted: Wed Apr 23, 2008 5:32 pm |
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Guest
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Pereges
The Planck equation, E=h*f, gives the value of the energy delivered
per photon. Working with very rarified quantities of photons such as
when dealing with PMTs this may be useful, but generally one wants to
deal with the total power (energy over time of a large number of
photons) delivered for an optical system. The power of the system
indeed may fall off as 1/r^2 as you mention, however the energy
delivered per photon is still given by h*f and in no way reduced.
This was the whole theory of the photoelectric effect studied by
Einstein and Planck et. al. and led to the quantum theory of matter.
Thus IF an aggregate of photon's propagates as a spherical wavefront
through space, the indigenous geometry of 3-space causes the photons
to spread out as 1/r^2. Each photon carries the same amount of energy
h*f, but the power, energy over time, is reduced as 1/r^2. Note the
duality here-dealing with a continuum of power and wavefronts on the
one hand and discrete photons on the other.
When applying ray tracing to these situations I am not so sure you
want to reduce the power as 1/r^2. I am not an expert on the "ray"
concept but I think it is simply a vector perpendicular to the
wavefront and not generally associated with a power value. (A poynting
vector is associated with energy-I hope others will jump in here).
However, I believe that the power for each ray remains constant and
the density of rays of the front will decrease IF they propagate in a
non parallel fashion e.g. they spread out. So-you need to be careful
here-if you consider an aggregate of collimated rays leaving a laser,
the power does not fall off as 1/r^2. The energy of the photons in the
beam still remains, or course, h*f.
PS Helpful Person- the book you cite sounds interesting-Dover still
prints it at low cost so I ordered a copy. I often turn to early 20th
century books to gain an understanding of physics-often these books
are very lucid and not overwhelmed with mathematical niceties. Thanks
for the tip.
Hope this is of some value,
Fritz |
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| Helpful person |
| Posted: Thu Apr 24, 2008 2:43 am |
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Joined: 22 Jun 2004
Posts: 677
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On Apr 23, 11:32 pm, Fritz <sonni...@berkshire.net> wrote:
Quote:
PS Helpful Person- the book you cite sounds interesting-Dover still
prints it at low cost so I ordered a copy. I often turn to early 20th
century books to gain an understanding of physics-often these books
are very lucid and not overwhelmed with mathematical niceties. Thanks
for the tip.
Hope this is of some value,
Fritz
If you like books of this form you must buy a copy of Twyman's "Prism
and Lens making". It is (I believe) the first (and certainly best)
book about grinding and polishing glass. It was written originally
about 1920. Twyman describes in detail the mechansims involved in and
the methods then used in making and testing optical components. You
do not want the first edition but the "expanded one", updated a few
years later. They come up occasionally on Ebay.
Conrady is considered the father of lens design. All future work is
built on his work. |
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| Fritz |
| Posted: Thu Apr 24, 2008 5:55 am |
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Guest
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OK and thanks for the tip!
I guess lens grinding has come a long way since the days' of Spinoza
(;->
FS |
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