Huawei P9 monochrome camera - red filter

Having a "monochrome" camera, i.e. a camera that records luminosity only, rather than RGB colors, allows for the good old color filter tricks, without reducing image quality, just as in the days of B&W film. Personally, I would prefer to call the camera filterless, rather than monochrome.

There are a lot good articles what happens when using which filter on B&W film. Here is one of those, please have a look.

Now, what about the loss of quality I was indicating, which does not occur when using a filter-less imaging array?
A regular color image sensor has a filter array printed on it, letting pass red, green or blue light on a respective pixel. In a so called Bayer filter array, the distribution of filters is RGGB. Consequently, only every 4th pixel is sensitive to red light. When using a red filter, for high contrast B&W photography (see article above), the 12 Mpx color camera is turned in a camera having effectively 3Mpx only, since the green and blue pixels will contribute to noise only, potentially creating JPEG artefacts.
In the absence of the RGB filter array, every pixel will be sensitive to all wavelengths, therefore, using a red filter wont affect the resolution or noise levels of the image.

It happens to be the case, that I got some red (LEE Filters 164 Flame Red) gel for spots. Although not perfect, this is pretty good stuff for improvising filters. The filter cuts somewhere in yellow, so, it will result in a little less contrast than a pure red filter.

LEE Filters 164 Flame Red

Those gels are usually just cut to size. So, this is what happened to a small portion of my roll.

A small piece of Scotch tape and the filter gel sits in the P9's case. Mind you, the RGB camera should not be covered!

Small piece of filter gel attached to case

So, what can you expect?
As mentioned above, the filter is a low pass which cuts somewhere in the yellow wavelengths. So, the photographic effect to be expected is in the range between a red and a yellow filter.
To give you an idea, I shot the city hall building, once with the desaturated RGB camera and once with the red gelled filterless camera. As a reference, I added color images from the RGB camera.

Desaturated RGB camera

Red gelled  filterless camera

JPEG converted from RAW (DNG) file

A slightly different angle and one face the shopping mall in Rijswijk. Same test, the effect might be even more obvious here. I metered for the 'IN' sign.

RGB camera

Red gelled filterless camera

JPEG converted from RAW

For the time being, I will keep the Fire Red gel in my phone's case. Should I require all wavelengths, I can easily flip the filter gel away from the camera lens and still keep it inside the case.