Leica's just announced the M Monochrom - essentially a Leica M9 with a monochrome sensor, although apparently calling it a "M9 Monochrom" is bad. Very bad!

Now the M Monochrom isn't for me - I do a lot of monochrome photography, but the way I work is  by using channel mixing to get the same effect as the red, yellow, etc filters that I used to use back when I first learned photography using my father's old rangefinder camera. Nostalgia is great, but I don't feel that the gain in resolution would justify going back to physical filters.

However, the M Monochrom still fascinates me - it's very much a purist's camera, and those that know me will probably confirm that I have a large purist streak.

There does seem to be quite a bit of confusion around the M Monochrom's sensor. Leica haven't been very forthcoming on the topic, and none of the initial reviews tell us much.  The assumption seems to be that the sensor is an M9 sensor with the Bayer array removed. That may be the case, but I doubt it's that simple.

You could have a "naked" sensor, but there are at least two reasons why there probably would be a filter on the M Monochrom's sensor:

1. Varying sensitivity to wavelength of the sensor itself (usually corrected for by the matrix in raw processing of a CFA sensor, but there's no matrix here). 
2. Also human's perception of luminance differs by color. 

Now, note that the TRI-X curve is in log scale, and the CCD is in a linear scale. But it's still pretty clear that there is a huge difference in response. The CCD rises to a peak at about 650 nm, which is deep red. TRI-X has a "brick wall" cut off at just about 650 nm. In addition, TRI-X responds deep in the ultra-violet, down to 300 nm (which is why UV filters were a good idea on film cameras, btw).

The M Monochrom's sensor may respond differently to the one above, but the point remains. If you don't filter to get the sensor output to conform to what us humans perceive as luminance, the images will appear "muddy" or "wrong". So you would probably need a single color filter (versus a 4 color bayer filter) on a mono sensor. It should certainly pass more light in total than a bayer array, but not 100%. This would be a design trade-off for the manufacturer - more sensitivity versus a more natural color response. You would also hopefully have a IR filter, else you'd end up with out-of-focus IR contamination blurring your otherwise beautiful high-res image.

In fact, somewhat ironically, the most important characteristic of a monochrome sensor is probably its color response. You can't adjust the response in post as you can with a regular sensor, so you really need it to be built right.

So, I certainly hope that the M Monochrom has a filter of some sort. Looking at the curves, I would think that Leica would want a red/infra-red cut filter, and probably a green cut as well if they wanted to match something like TRI-X. If they wanted something that matched a theoretical luminance equation, which is green heavy, they might only use a red/IR filter.

Hopefully Leica have tuned the M Monochrom to be similar to some of the classic B&W films. Certainly, after the M8 IR debacle, and the M9 red edge debacle, one would hope that they're sensitive to special response as an issue. Slightly scary that many of the sample images look a bit grey and muddy - like the filter isn't too great.

Sad that this aspect was entirely missed in the reviews. This need for a filter, BTW, was probably what Leica were alluding to when they talked about it being the M Monochrom being more than just a M9 without the bayer array.

Note to Leica: I get it that you want to be loyal to reviewers that supported you back when all that Leica had to sell was the M7. I also get that the PR department wants reviewers that are prepared to stick to the company line. But could you at least invite one reviewer that is digital imaging savvy enough to be able to at least ask the right questions? Please? It's a bit frustrating to have to read through pages about sensor resolution (yes, no bayer filters, better resolution. Got it!!!), but never get to any useful information.

Update: Above I made reference to "theoretical luminance equation, which is green heavy, they might only use a red/IR filter". I should probably make that a little less cryptic - the classic formula is

 Y = 0.2126 R + 0.7152 G + 0.0722 B 

In other words 71% of luma is green, rather than a flat response. That is approximately what you would get in a conventional color to mono conversion in LR or C1. But a bare sensor would, I'd suggest, have too much red sensitivity to do a good job no matter what view you take on what the ideal should be.

Second Update: Leica have published the M Monochrom's actual spectral sensitivity - see this post.