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What are accepted figures for signal/noise (SNR) and dynamic rangefor CRT? LCD? Film? Human eye?



 
 
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  #11  
Old February 22nd 05, 07:45 PM
Jerry Avins
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Nicholas O. Lindan wrote:

...

The eye is comfortable with a brightness range of ~50-100x in one scene
with no pupilary/retinal adaptation, 1000:1 is viewable without noticing
much adaptation - after images or time for pupil to adjust. The eye can
accommodate a brightness range from reflected starlight on a field
(remember clean air and no streetlights?) to sun on fresh snow (except
in extreme cases, mountains, clean air ...). This about a 25 bit/zone
range or ~32,000,000:1 and has no relevance to viewing a CRT.


Yes. There's more light reflected from a lump of coal in sunlight than
from a bowl of snow indoors. The eye/brain isn't fooled.

Within a 2.0 OD range it can easily discriminate to .005 OD in the middle
of the range. However, OD is logarithmic and photocells and CRTs aren't.
Figure you will need about 12 bits for very good fidelity, but 8 bits
looks just fine on a CRT.

Numbers above are from memory, you should check with a good text on
human visual response if the values are important.

Negatives have close to no relation to photoptic response. Exposure
and development of film are such as to yield the thinnest (least dense)
negative that will yield a good print. This figure depends on scene
contrast, film contrast, paper contrast and the method used to image the
negative on the film.


For high-resolution work, we used to dye the emulsion black to avoid
exposing interior grains. After all, the image can't be in focus
throughout the emulsion depth. To make the film a little faster, we
would etch away the top few microns of the emulsion, leaving an exposed
layer of grains, like in sandpaper.

I have to confess I don't see much relevance in all this to an SEM. All
you want is the range of signal that presently goes to the current
analog display and be able to accommodate that. 8 bits resolution should
be adequate.


The question of which 8 bits can be important. I can adjust images from
my 12-bit flat-bed scanner in ways that just don't apply to 8 bits.

Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
  #12  
Old February 22nd 05, 07:55 PM
Richard Owlett
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Nicholas O. Lindan wrote:


I have to confess I don't see much relevance in all this to an SEM. All
you want is the range of signal that presently goes to the current
analog display and be able to accommodate that. 8 bits resolution should
be adequate.



But that is one of the attractive features of this group.
( OK, so I'm reading comp.dsp

By reading one learns so much more than one narrow area of knowledge!


  #13  
Old February 22nd 05, 08:20 PM
Richard Owlett
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Jerry Avins wrote:

Nicholas O. Lindan wrote:
[snip]
I have to confess I don't see much relevance in all this to an SEM.
All you want is the range of signal that presently goes to the current
analog display and be able to accommodate that. 8 bits resolution should
be adequate.



The question of which 8 bits can be important. I can adjust images from
my 12-bit flat-bed scanner in ways that just don't apply to 8 bits.

Jerry


I think that touches on a question I raised in another subthread.

Why a *FAST* *16 bit a/d* ?

Would it not be more suitable to have 2 A/Ds in "parallel"?

A very fast 6-8 bit A/D for use while positioning sample.
A 16+ bit A/D for analytical output.

Caveat Lector ;}

  #14  
Old February 22nd 05, 09:15 PM
Bart van der Wolf
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"Jerry Avins" wrote in message
...
SNIP
There's more light reflected from a lump of coal in sunlight than
from a bowl of snow indoors. The eye/brain isn't fooled.


See e.g.
http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html

Bart

  #15  
Old February 23rd 05, 12:14 AM
Roderick Stewart
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In article , Jeff miller
wrote:
How about for CRT's? Deos it depend in part on the design and
construction of the CRT, and do dedicated monochrome CRTs have a greater
dynamic range for greyscale than color CRT's used to display greyscale
images?


As others have said, you should capture and manipulate your original images
at the highest bit depth you can handle, but what is usually transmitted by
TV companies, recorded on DVDs, or saved in JPG files for display on CRTs
is based on 8 bit linear coding after gamma correction, and it generally
looks just fine. i.e. 8 bits seems to be quite sufficient for display.

From personal observation, dynamic range of monochrome and colour CRTs are
about the same, though the detail resolution can be slightly higher on a
monochrome CRT displaying a monochrome signal.

Rod.

  #16  
Old February 23rd 05, 12:37 AM
jeff miller
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Richard Owlett wrote:

Jerry Avins wrote:

Nicholas O. Lindan wrote:
[snip]

I have to confess I don't see much relevance in all this to an SEM.
All you want is the range of signal that presently goes to the
current analog display and be able to accommodate that. 8 bits
resolution should
be adequate.




The question of which 8 bits can be important. I can adjust images
from my 12-bit flat-bed scanner in ways that just don't apply to 8 bits.

Jerry



I think that touches on a question I raised in another subthread.

Why a *FAST* *16 bit a/d* ?

Would it not be more suitable to have 2 A/Ds in "parallel"?

A very fast 6-8 bit A/D for use while positioning sample.
A 16+ bit A/D for analytical output.

Caveat Lector ;}


Hmmm.... I'd thought about that but not to those extremes: in fact it
makes much more sense at the extremes you suggest and I may well
incorporate the idea. It will also give me a chance to jump-start the
project before I have the funds for the higher performance A/D.

Interestingly enough the 16 bit 5MSPS sigma-delta A/D I was thinking
about has a 10MSPS "turbo" mode with 14 bit resolution.

This thread is providing some very useful information about SNR of media
and human eyes... keep it coming!
  #17  
Old February 23rd 05, 12:40 AM
jeff miller
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Roderick Stewart wrote:

In article , Jeff miller
wrote:

How about for CRT's? Deos it depend in part on the design and
construction of the CRT, and do dedicated monochrome CRTs have a greater
dynamic range for greyscale than color CRT's used to display greyscale
images?



As others have said, you should capture and manipulate your original images
at the highest bit depth you can handle, but what is usually transmitted by
TV companies, recorded on DVDs, or saved in JPG files for display on CRTs
is based on 8 bit linear coding after gamma correction, and it generally
looks just fine. i.e. 8 bits seems to be quite sufficient for display.

From personal observation, dynamic range of monochrome and colour CRTs are
about the same, though the detail resolution can be slightly higher on a
monochrome CRT displaying a monochrome signal.

Rod.


Yes good point as there is no "dot pitch" issue.

-Jeff
  #18  
Old February 23rd 05, 05:23 AM
Roger N. Clark (change username to rnclark)
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jeff miller wrote:

This thread is providing some very useful information about SNR of media
and human eyes... keep it coming!


See:
Notes on the Resolution of the Human Eye
How many megapixels equivalent does the eye have?
The Sensitivity of the Human Eye (ISO Equivalent)
The Dynamic Range of the Eye
The Focal Length of the Eye:
http://www.clarkvision.com/imagedeta...esolution.html

Contrast thresholds data:
http://www.clarkvision.com/visastro/omva1/index.html

Roger

  #19  
Old February 23rd 05, 06:46 AM
jeff miller
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Roger N. Clark (change username to rnclark) wrote:

jeff miller wrote:

This thread is providing some very useful information about SNR of
media and human eyes... keep it coming!



See:
Notes on the Resolution of the Human Eye
How many megapixels equivalent does the eye have?
The Sensitivity of the Human Eye (ISO Equivalent)
The Dynamic Range of the Eye
The Focal Length of the Eye:
http://www.clarkvision.com/imagedeta...esolution.html

Contrast thresholds data:
http://www.clarkvision.com/visastro/omva1/index.html

Roger


Great resource. Based on that data, I'd conclude that images captured
from my microscope might benefit for 65K x 65K resolution, so I'll use
16 bit D/A's to drive my scanning coils. Of course that kind or
resolution can only be rendered in print. And represents an 8GB file
size. And will take 2 hours to collect.... but might as well build it in.

The figure of 10,000:1 dynamic range in any "one view" corresponds to
about 12 bits. That's pretty much exactly the nominal dynamic range I've
heard quoted for the front end of an SEM.

But it might seems no current reproduction technology acheives quite that.

-Jeff


  #20  
Old February 23rd 05, 12:30 PM
jim
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Hi Jeff
If you have a set of 40 cards with gradations of gray going from black
to white about 90% of the population will be able to lay them on a table
sorted into the correct order. If you increase the number of cards to 64
about 2/3 will be able to sort them correctly. If you up the number to
80 the number of people that can sort them drops to something like 20%
and by the time you get to 128 cards almost nobody can put them in the
right order. I'm recalling this from memory but the numbers are
something like that.
The sensitivity to brightness for film and the human eye is not
strictly linear. That is even when you give people 128 cards with linear
gradations of grayscale cards to sort there are certain ranges of
brightness wherein they will mostly get it right. So 8 bits (256
brightness levels) will pretty much handle the non-linearty of both the
display and the viewer. That is, most people will not be able to see the
difference between any two adjacent gray levels.

If your next question is - Is it spelled "grey" or "gray"? I don't know
:}
-jim

jeff miller wrote:

I'm converting my old analog scanning electron microscopes to digital/pc
based.

One of the important questions is how many bits of resolution ie: levels
of greyscale are required. I've heard the analog front ends of the
scopes have a maximum SNR of about 72 dB, or 12 effective bits, and to
give me some flexibility for digital image processing that's about what
I'll shoot for: a bit better if I can get it.

But on the other end of the scale, I'm wondering what are accepted
figures for signal to noise or dynamic range of various display
technologies, and of the human eye.

It seems to me the published "contrast" figures for LCD displays of
about 600:1 peg them at about 49dB dynamic range.

What is generally accepted for B&W film? Is it highly dependent on film
type and processing? Anyone know some typical figures for various
processes? Is "silver print," which in my mind is marked by high
contrast, a special technique or just a fancy word for black and white?

How about for CRT's? Deos it depend in part on the design and
construction of the CRT, and do dedicated monochrome CRTs have a greater
dynamic range for greyscale than color CRT's used to display greyscale
images?

And what about the human eye, for that matter? It's been suggested the
human eye has only about 6 bits or 36 dB dynamic range/SNR for greyscale
images. Is that about right? Bonus question: I know the sensitivity of
the human eye varies with color, being most sensitive at about 555 nM
green. How about dynamic range and SNR? Deos it vary with color, too?

Thanks!

-Jeff


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