More on the IR hotspot

[Stan-42]

To the group,

I posted this yesterday, but never saw it appear. Here is a second try...

Stan Patz   NYC

www.PatzImaging.com

----- Original Message -----
From: "Stan" <Stan@...>
To: "Infrared" <infrared@...>
Sent: Monday, September 24, 2007 3:44 PM
Subject: More on the IR hotspot

> To the group,
>
> I emailed Dan Llewellyn at LDP about what glass he used to replace the
> "internal correction filter". His company converted my Sony DSC-R1. Part
> of his reply is below. He doubts reflection off this glass is the source
> of the IR hotspot. His opinion is that internal  reflections in the  lens
> cause the hotspot and points out that Sony cameras with "NightShot", an IR
> mode, does not have this problem. Sony may properly coat these models so
> the NightShot mode would be unblemished.
>
> His reply suggests that HIE would show the hotspot with some lenses.
>
> The XDP filter he mentions is a narrow bandpass filter for IR light.
>
> Stan Patz   NYC
>
> www.PatzImaging.com
>
> *******************************
>
> The glass we use in the camera is a Schott WG280 material.  A normal glass
> like BK7 would be too restrictive for those doing UV photography.
>
>
> Keep in mind that IR film has a different response curve than a CCD/CMOS.
> I am not certain, but I seem to recall that the IR film doesn't go up all
> that high.  Something like 850nm comes to mind.  That may be why the XDP
> helps - the IR reflections may be occurring between 850nm to 1100nm.
>
> The Fuji IS-1 is exactly the same camera as our Fuji S9100.  Fuji was
> considering using us for their conversions.  We have converted a number of
> cameras for them though they now have copies of our cameras with different
> model numbers (and higher prices).
>
>
> Dan Llewellyn
> President
> LDP LLC
> http://www.MaxMax.com
>
>
>

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[Keith Davison-2]

> > I emailed Dan Llewellyn at LDP about what glass he used to replace the
> > "internal correction filter". His company converted my Sony DSC-R1. Part
> > of his reply is below. He doubts reflection off this glass is the source
> > of the IR hotspot. His opinion is that internal  reflections in the  lens
> > cause the hotspot and points out that Sony cameras with "NightShot", an
> > IR mode, does not have this problem. Sony may properly coat these models
> > so the NightShot mode would be unblemished.

I have converted a couple of Sony F717's to full time IR function by replacing
the IR block filter with a piece of glass, and the modified cameras do show
hotspot problems. It is deceptive to claim that Sony cameras with nightshot
do not show hotspots because of enhanced coating, as the lens aperture is
locked wide open in nightshot mode. If, as people on the list report, the
hotspot phenomenon is aperture sensitive, it may well be that it does not
manifest when the Sonys are wide open in nightshot. The Sony's lens is an
f2, faster than most, and this may also have an effect.

Even if the idea of true IR coatings on the Sonys was reality, I can confirm
that replacing the coated IR block filter with plain glass DOES cause a
hotspot problem (I wish it did not ;o). The CCD has to be removed to replace
the filter, so I can tell you that the sensor itself is highly reflective, at least, it
is in the visible wavelengths.

Keith

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[Stan-42]

>> > His opinion is that internal  reflections in the  lens
>> > cause the hotspot and points out that Sony cameras with "NightShot", an
>> > IR mode, does not have this problem. Sony may properly coat these
>> > models
>> > so the NightShot mode would be unblemished.
>
> I have converted a couple of Sony F717's to full time IR function by
> replacing
> the IR block filter with a piece of glass, and the modified cameras do
> show
> hotspot problems. It is deceptive to claim that Sony cameras with
> nightshot
> do not show hotspots because of enhanced coating, as the lens aperture is
> locked wide open in nightshot mode. If, as people on the list report, the
> hotspot phenomenon is aperture sensitive, it may well be that it does not
> manifest when the Sonys are wide open in nightshot. The Sony's lens is an
> f2, faster than most, and this may also have an effect.
>
> Even if the idea of true IR coatings on the Sonys was reality, I can
> confirm
> that replacing the coated IR block filter with plain glass DOES cause a
> hotspot problem (I wish it did not ;o). The CCD has to be removed to
> replace
> the filter, so I can tell you that the sensor itself is highly reflective,
> at least, it
> is in the visible wavelengths.

To Keith and the group,

In my conversation with Dan Llewellyn at LDP, he expressed some opinions of
his own. He was not trying to deceive anyone when he said he never saw the
hotspot on a Sony in the "nightshot" mode. He then opined that the camera
may have an IR anti-reflective coating.

Thank you for pointing out that in "nightshot" mode, the lens aperture is
locked wide open. In IR tests on my Sony DSC-R1, a camera without
"nightshot", the hotspot did not form until about f5.6 and was sharpest at
f16. (Also, the hotspot only formed around middle focal lengths on my
non-changeable zoom lens.)

We know the IR hotspot is a phenomenon that appears on some (all?) digital
IR systems. But we are still no closer to knowing if it effects film
shooters also.

Stan Patz   NYC

www.PatzImaging.com

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