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Hi All, June 14, 2011
With regard to the missing dark band, and shooting from the hip, it is
logical to assume that the ideal display (crisp primary rainbow, dark band
and vivid secondary rainbow) will be seen only when the water droplets are
small enought to be perfectly spherical.
Based on spectra seen around waterfalls and sprinklers this size would
be drizzle to mist. Presumably these small droplets that give rise to
rainbows are produced by splatter when larger raindrops collide with
slower-falling smaller raindrops. Thus rainbows are seen just after a rain
event. [In contrast, small droplets produced by condensation would have poor
optical properties due to a high content of condensation nuclei; e.g. the
dull 360o rainbows seen when one flies above clouds. ]
Rainbows are transient and small droplets without condensation nuclei
would also be transient due to increase in vapor pressure with decrease in
size.
As droplet size increases and shape becomes less spherical then the
change in display would logically form a continuum from ideal, through e.g.
clear primary rainbow with faint secondary rainbow and no dark band to faint
smeared primary rainbow only followed by no rainbow at all.
Yt, Dave Webster, Kentville
----- Original Message -----
From: "Stephen R. Shaw" <srshaw@Dal.Ca>
To: <naturens@chebucto.ns.ca>
Sent: Tuesday, June 14, 2011 3:34 PM
Subject: Re: [NatureNS] Desaturated 180 Degree Double Rainbow Photo
> Hans: Agreed -- I actually looked at your colour picture last night and
> thought the same thing (great picture as always by the way, as others
> have said).
>
> Another potential 'enemy', though, is non-linear processing. I don't
> know if consumer digital cameras use a non-linear algorithm to compress a
> scene's extended brightness scale from the CCD chip into the 256 steps
> usually available in the conversion from light energy to pixel memory
> record. I became concerned about this when taking pics of fluorescence
> down a microscope, and had to go to some length to check that the fancy
> camera/software there was giving a linear output up to CCD saturation, so
> we could make proper relative brightness estimates from the images.
> Don't know the answer for consumer cameras. In addition, some of
> Photoshop's filters must act non-linearly to distort the initially
> recorded image, for instance the wonderful Unsharp Mask: wondered if you
> may have used it on the colour picture of the rainbow (the telephone
> cables have unexpectedly bright white adjacent dots).
>
> Another perhaps curious feature is that the few clouds seem quite bright
> when present just below and extending into your primary rainbow, but wash
> out where they look to extend beyond into Alexander's (putatively dark)
> band. But that could just be in the cloud distribution itself, not in
> the processing.
>
> It would be interesting to know if you or anyone on the list knows
> whether consumer cameras (or some of them) operate linearly, that is
> (roughly), give out twice the number (0-256) to the memory pixel record
> where the scene is twice as bright objectively (not visually). Or do
> some/all use intensity compression to render scenes more acceptable or
> manageable?
> Steve
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Quoting Hans Toom <htoom@hfx.eastlink.ca>:
>> When analyzing photographs colour can sometimes be our enemy so I've
>> desaturated the first rainbow photo to black and white. The visible
>> brightness of the image is clearly brightest below or within the primary
>> rainbow and darkest between rainbows and the sky above the outer
>> rainbow. The visible brightness in the space between rainbows and the
>> background sky above the rainbows appears to my naked eye as identical.
>> No dark band is visible to my eyes anywhere.
>>
>> The overall effect of the desaturated image is much like a giant eyeball
>> with the space from the inner rainbow skyward becoming the imaginary
>> eyelid. The visible brightness below the lower rainbow is stunning and
>> much brighter than the background sky as seen above the outer rainbow.
>>
>> http://www.hanstoom.com/StockPhotos/Visions3/0179.html
>>
>> Hans Toom
>> Portuguese Cove, Nova Scotia, Canada
>> http://www.hanstoom.com/
>>
>
>
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