From: ctlant@lims01.lerc.nasa.gov (CHRISTIAN LANT)
Newsgroups: rec.audio.high-end
Subject: Re: Laser Illusions Spatial Filter
Date: 15 Dec 1992 09:20 EST
Organization: NASA Lewis Research Center
NNTP-Posting-Host: 129.89.7.4
News-Software: VAX/VMS VNEWS 1.41    
Originator: tjk@csd4.csd.uwm.edu

In article <1gi3klINNfb3@uwm.edu>, deanaj@elec.canterbury.ac.nz (A. J. Dean) writes...
>: Kevin Sutton (suttonk@lan.lincoln.cri.nz) writes:
>: 
>: Has anyone tried the Laser Illusions 'Spatial Filter', a
>: supposedly jitter-reducing lens which fits over the CD laser
>: lens?  I would be interested to know how these devices are
>: supposed to work, how much they cost and where I can get one from
>: (they are not in the shops here).
> 
>I have absolutely no idea what they are...
>Antony. (deanaj@elec.canterbury.ac.nz
>

I have no experience with this particular product, but maybe I can 
shed some light (sorry) on what it does.

A wavefront of light can be (and usually is) a complicated sum of 
"spatial frequencies" which make up a pattern that one can see (you 
are looking at one now, if your eyes are open :) ).  This is analogous 
to the bandwidth of sonic temporal frequencies we are used to talking 
about on the net, except instead of time being the domain of interest 
(in units of Hz = 1/sec), space (distance) is the domain (units of 1/mm).
Every detail we see in an image has a size, which is thought of as its 
characteristic wavelength - the inverse of this wavelength is then its 
spatial frequency.  The sum of all these spatial frequencies is the 
spectrum of the image.  

Just as sounds can be low-pass filtered to get rid of noise, light images 
(wavefronts) can also be filtered to cut noise.  This is done by taking 
the Fourier transform of the image with a lens (a lens is a real-time 
Fourier transform device for light - the transform appears at the focal 
point) and physically blocking the high spatial frequency light by 
placing a pinhole in front of the spectrum.  The DC component of the 
wavefront is what you want to end up with, which is the component that 
passes straight through the pinhole.  

I gather that Laser Illusion's premise is that since the light from a 
laser diode and the sending optics is very noisy, due to diffraction 
from the laser device, optics, dirt, etc., then the noise can contrubute 
to jitter and/or read errors.  So this spatial filter process would 
ideally give you a smooth, clean beam of laser light shining on the CD 
surface.  I have read that a properly implemented spatial filter can 
contribute to better sound, but it also can increase tracking errors 
(sensitivity to bumps), so a trade-off is used in the design.

I am interested in why Laser Illusion's spatial filter works at all. 
For an optical system with a high numerical aperture (N.A.) such as 
a CD optical system, one needs a very small pinhole (on the order of 
10 micrometers) to block the spatial frequencies containing any 
appreciable power.  It's like amplifier distortion - most of the 
distorted power appears in the first few harmonics.  From the post 
describing one reader's problems with the fit of the Laser Illusions 
filter, the pinhole actually has a diameter of 60 mils!  This is more 
than two orders of magnitude larger than the ideal, which should let 
virtually all of the light's bandwidth through the filter, if 
traditionally designed.  

I suspect that the filter isn't placed at the laser waist (focus) since 
this would require additional focusing optics ( or placing the filter 
within the CD surface :) ) and would make very tight alignment 
necessary.  Therefore, this spatial filter design must 
have many tradeoffs involved in order to make it user-installable,
inexpensive, etc. 

I hope this post helps.

Chris Lant - ctlant@lims01.lerc.nasa.gov

			Disclaimer:  I speak for myself.