Re: Colorimetric Accuracy in the Field
Re: Colorimetric Accuracy in the Field
- Subject: Re: Colorimetric Accuracy in the Field
- From: Ben Goren <email@hidden>
- Date: Tue, 04 Jun 2013 06:31:54 -0700
On Jun 4, 2013, at 3:49 AM, Ernst Dinkla <email@hidden> wrote:
> On 06/04/2013 05:16 AM, Ben Goren wrote:
>
>> Some variations I'd suggest...well, hesitatingly, I'd suggest using a light tent (outdoors at noon in the middle of summer) to illuminate the chart.
>
> Pure teflon cloth preferably then or build an Ulbricht Iglo with fresh polystyrene foam.
The nylon (or whatever) in a standard commercial photographic light tent is sufficiently spectrally neutral for these purposes, and the resulting illumination is sufficiently even. I don't think, in practice, it's all that different from the integrating sphere you suggest.
The biggest problem you'll face with a light tent is with (diffuse) specular reflections. And, if you tried to use an IS, you'd have the exact same problem with specular reflections.
An IS is great for reflective spectrometry when set up in d/8° geometry...but only for a very, very narrow field of view. And even then you need the light trap at the opposite 8° position on the sphere. Whatever the size (and shape) of that light trap is the size of a sample you can get good measurements from.
You could probably build a rig using an integrating sphere that mimicked a spectrophotometer, but it'd only work for small samples at a time and you'd have to reposition either the sphere over the chart or the chart under the sphere and only shoot the chart in small sections at a time. Considering how rapidly the Sun moves in the sky and how fast atmospheric conditions (and thus exposure) can change, I don't think that'll be very practical.
Oh -- and polystyrene is a good material to use for constructing an IS, but I'd recommend first spray-painting it with a high-quality flat white home interior paint and, while the paint is still wet, liberally dusting the interior with the purest reagent-grade barium sulfate powder you can get (which will still be quite inexpensive). The result should be not that far off from a commercial IS. And you can buy hollow polystyrene spheres (in halves) from a hobby / crafts store. (Or you can spend many thousands of dollars on a commercial IS, of course.)
What I'm working on is something not entirely unlike a monitor hood that will hold a chart and will have baffles that block everything but direct Sun and skylight at angles that won't produce specular reflections. The dimensions will depend on the final dimensions of the new chart I'm working on, so I won't really start on this gadget until after I'm done with the chart. But I'm probably just going to make it out of foamcore and glue some black flock velvet to the insides, so it shouldn't be expensive or hard to make...the hard part is going to be figuring out the geometry, and maybe assembling some odd angles.
I should add: this is all for building a profile of sunlight. In the studio, you just have to set your lighting up right. But even there, proper lighting isn't 45°/0...it's more like 45° + ([angle of view] / 2) / 0. For a 50mm lens on 135 format, if you fill the frame (not necessarily a good idea), that means 68°/0.
Cheers,
b&
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