SL and infrared

[svenb]

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With the M8, and to some extent the M240 in mind, are there any information on the IR-sensitivity of the SL?

[wlaidlaw]

I would guess that the SL sensor will have a high pass filter to cut IR or else you would get false colours like you get with the M8 on synthetic black fabrics, if you don't use an IR cut (high pass) filter. I have actually kept my M8 for taking IR photographs. In the UK, there are some companies, like my local camera service man, Kelvin at Protech, who specialise in removing the IR filtering cover glass from the sensors of Nikon and Canon DSLRs so that they can be used for IR photography. Now whether he would be brave enough to try that on an SL, I don't know. 

 

Wilson

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[edwardkaraa]

There is no official information but my educated guess is that it should have less IR contamination because the sensor cover thickness has been increased, allowing for more efficient filtering.

[wlaidlaw]

Edward,

 

I think the IR filtering is by a high band pass interference coating on the front face of the sensor cover glass, rather than simply by the thickness of the cover glass itself. I think it is this coating which corrodes on the M9. The M8 does not have the same coating, which is why it has not suffered from the corrosion that the M9 has. Someone has found out the spec of the coating on the M9 cover glass, which is from memory, a Schott product. 

 

Wilson

[scott kirkpatrick]

I don't know the current situation, but I recall that in the M8-9 days, the cover glass was just green glass, as interferometric filters had nasty angle-dependences (worked differently for light coming in at an angle), and as you know that was the problem to be avoided.

 

scott

[edwardkaraa]

Hi Wilson,

 

I think IR filtering is done by a proper glass filter stack with other brands. Leica could not do that for obvious thin cover glass requirements so they resorted to the coating solution.

[wlaidlaw]

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Hi Wilson,

 

I think IR filtering is done by a proper glass filter stack with other brands. Leica could not do that for obvious thin cover glass requirements so they resorted to the coating solution.

That is certainly what I understood that the IR filter on the M9 was by coating of the cover glass. The M8 has coating also but far less effective as a high pass filter. Now the Q/SL CMOS sensors may be more conventional and use the filter stack like others do but I don't know. 

 

Wilson

[scott kirkpatrick]

None of the previous notes sound very sure of the answer.  there was a published spec for the M8's chip from Kodak.  It used a Schott cover glass that is green all the way through (but very thin, something like 0.5 or 0.6 mm).  Not a coating.  I thought for the M9 used the same but made it a bit thicker, or maybe the color of the glass was darker.  The spectral response of the glass cut off rather softly and let quite a bit of near IR through.  An interferometric filter is a coating, since its thickness has to be similar to the wavelength of light, less than a micrometer, while the glass layers we are talking about range from a fraction of a millimeter to several mm.  Although interferometric filters cut off more sharply than absorption filters, the cutoff frequency is angle-dependent, and I think this would rule them out for Leica's application.

 

The person who would know what is done in the 240, 246 and SL is MJH.  ???

 

scott