Re: WHICH FILM/SLIDE SCANNER TO BUY????

On Wed, 26 Oct 2005 15:12:15 GMT, "Peter D" <please@.sk> wrote:

>"mp" <mpapet@xxxxxxxxx> wrote in message
>news:1129671566.484786.273930@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>>>Do you think the mirror is a significant problem ?
>> Yup. They lose 30%+/- of their light.
>
>How do you know this? What actual evidence do you have that this is a fact
>and that it is a significant fact? And what exactly is "30% +/-"? "+/-"

http://www.celestron.com/starbrightxlt/coatings_overview.htm

OK... Going to the telescope industry which has more critical light
absorption/loss requirements than the scanners: I say that as in
astronomy they need every photon that can be captured. In scanning
you only need enough light to keep the signal to noise in a state
where it's not a problem.

We use "front coated" mirrors and I'd assume the photography industry
does so as well. Front coated mirrors using an Aluminum coating have
*about* 86 to 88% reflectivity. So on the low end (worst case) they
aren't quite as bad as 30% loss, but close. Silver coatings will
reduce that loss to around 10% for 90% reflectance. Coatings as
mentioned in the above article can reduce the loss to the point of
95% reflectance per surface.

I would assume (which is never safe) that the photographic industry
uses good quality front coated mirrors with good protective coatings.
It's also easier to make small mirrors optically flat compared to
large ones.

"I doubt", but don't know that the photo industry is probably running
around 90% reflectivity and not using the really expensive coatings,
but using some sort of coating for protection.

With mirrors there are two problems as far as the light loss.
The first is reflectance which *should* be of a relatively low
importance as long as enough gets through to give a satisfactory
signal to noise ration. The other problem is light scattering. This
is the important one as it can cause softening of the image (loss of
contrast) and distortion of the image, or parts of the image.

>what? Sorry if that comes across as hard on you, but those of us who are
>trying to learn have a hard time with figures tossed out here and there to
>support specious arguments, and this sure sounds like one. All it does is
>distract from the potentially valid point you are trying to make.

"In my opinion" it makes little difference as to what they do with the
light _as_long_as_the_image_ is clear and sharp, or does not suffer
from the circuitous light path. I believe the light loss would have
to be very high to become a problem depending on the source and
sensors. OTOH if you have to fix one the alignment of the optics
could be a major problem. If they are reliable, I wouldn't worry
about it.

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

>
>
.

Relevant Pages

  • SCT coating specs, take with a grain of salt?
    ... Seems like I remember both Celestron and Meade ... only regular aluminum on the mirrors (88% reflectivity ... Celestron claims their standard coatings ...
    (sci.astro.amateur)
  • Re: STUNNING image of M51
    ... > PSF is a Gaussian curve and using Nyquist limit is wrong. ... > right next to the mirrors. ... Coatings acquire pollution and coatings ... of the scope and the amount of backfocus desired. ...
    (sci.astro.amateur)
  • Re: How to use an optical flat
    ... Those are usually multi-layer coatings which improve the throughput for visual or color photography at the expense of throughput in the UV and IR. ... I usually specify aluminum with a slightly thicker than normal single layer SiO2 coating which has it's first peak in the red and a secondary peak at 325 nm. ... I've had best luck with mirrors coated by Navitar in Newport Beach CA. ...
    (sci.optics)
  • Re: lowest reflectivity in mirror
    ... glass before it produces acids that can damage the glass ... the coatings used on refractive optics and on mirrors are ... But absent some chemical that is hostile to the mold, ...
    (sci.astro.amateur)
  • Re: Canon 5D Pentaprism or Pentamirror
    ... >dielectric mirrors, which show efficiencies>99.9%. ... It's a multilayer coating similar to ones put on transmissive optics. ... I'm simply saying I have never seen a prism (apart from ... Four mirrors means a 40% light loss, ...
    (rec.photo.digital.slr-systems)