Mono Camera Magic

[Cpaulo]

Hi there everybody,

There is something that is bothering me with the process of using a mono (short for monochrome I presume) camera to produce colour  images. It's not something I've tried, the results look amazing but it bothers me how effective it is because after all these colour pictures are taken with a camera that sees in black and white.

I have read up on the subject a little and I know about colour theory, how the 3 prime colours make up all other colours, I know these are replicated by an LRGB filter set in the imaging process and the theory of it makes perfect sense; I know some people use different filters like the Hubble palette for example but I am keeping it simple. 

So my point is why should putting a filter in front of a camera that sees in black and white make a difference to colour, surely if the camera is monochrome and you put a green filter in front of the camera lens then the only change in what the camera returns should be a change in the contrast or to make the overall picture seem a little darker? For some reason this all reminds me of my Granddad watching snooker on a black and white TV.

If the simple answer is that Mono cameras are not strictly monochrome cameras and they do actually see in colour, which I figure they must, then I'll go away and keep my thoughts to myself. But then it does beg the question what's the difference between a colour CCD and a Mono CCD?

I do not wish to come across as a bit simple but it does honestly occupy my thoughts more than perhaps it should.

[skybadger]

All colour cameras are mono internally. They use filters at the individual pixel level to reconstruct a colour image from the matrix. Each filter allows only its colour to pass through. A camera without these filters sees all colours between uv and near-Ir. Picking filters at R, G and B passbands allows colour images we recognise to be reconstructed.

So using a mono camera with external filters just allows us to have the control of which filter is in use at any one time. Its a trade-off between control and ease of use.

Mike

[swag72]

As I understand it, a mono camera is a mono sensor - There's no colour jiggery pokery involved. By using RGB filters in front of the mono sensor you are only allowing light to pass to the sensor within a specific bandwidth. So you pick up red light with a red filter for example. Then you merge the three filters in Photoshop and you get a colour image.

If you look at a mono image of each RGB filter, you will see that there is a difference in the light captured, not just the contrast of the image.

[Earl]

Mono Cameras see all light when used without a FIlter. and IIRC the intensiy has a value of 0 to 65536 (if 16bit data)

A filter only allows a certain bandwidth of light to bass through so you filter out the rest to give a clear image of that specific colour. a single visable channel will never be as strong as the full no filter data this is why Luminance (normally filtered to take out UV and IR data) is used to boost the intenstity on the strenght of the RGB data (although this does not always give visually pleasing results for various reasons).

Doing this allows you to construct an image based upon these colours.

A "One  Shot Colour" camera essentialy has a filter built into it ontop of the chip which is called a bayer matrix.

[Cpaulo]

Thank you guys. Thanks for your patience and taking the time to explain this perplexing issue of mine; I'm glad there is a bit more to it than my initial interpretation of what is going on. Things are more clear to me and a couple more things drop into place.

[Shiinsuh]

As others have said previously. A colour camera is simply a monochrome camera with RGB filters at the pixel level (one colour per pixel usually in a chequered pattern) and with out the filter the camera will capture all colours. If the colour camera simply had one colour per all the pixels, lets say all green, then it would be equivalent to a mono camera with a green filter over. 

Now because on a colour camera all the total amount of pixels are sheared between three colours only 1/4 of Red 2/4 of Green and 1/4 of Blue light hits the sensor and is converted into an image. (There are twice as many green pixels as each of the other colours). With a mono camera with a filter, 4/4 of the pixels are now getting the light in the specific colour as they don't have individual filters.
 

However this doesn't mean that you will simply get a bright picture with a mono camera, due to the fact you will have to run 3 separate exposures, one for each colour, which, if you spent that long of time with a colour camera, you will still get the same amount of light gathered. Difference is with the over all process instead of getting 25% Red, 50% Green and 25% Blue light from a colour camera, you get 33% Red, 33% Green and 33% Blue light from the mono, or you can spend more time on a specific colour such as red and this will give you more data in the red part of the spectrum (if you did this with a colour camera 75% of the light will be wasted as you don't want it).

Finally with a colour camera, the generating of a colour picture is done by the camera its self and when it comes to manipulating it, there isn't a hard stop of where one colour starts and another ends. With a mono with filter, each colour is added to the RGB channel in the image editing software, manual (colour cameras add them automatic) and also it has a hard stop when the light wave length is cut off, giving more control over manipulating the image.

I hope that helps and clears something up for you. If you want to learn more, its always best to do research on the subject. Really that's about the basics and is as far as I understand the subject, so I maybe missing things or could be wrong on something. 

Regards

[lensman57]

Hi there everybody,

There is something that is bothering me with the process of using a mono (short for monochrome I presume) camera to produce colour  images. It's not something I've tried, the results look amazing but it bothers me how effective it is because after all these colour pictures are taken with a camera that sees in black and white.

I have read up on the subject a little and I know about colour theory, how the 3 prime colours make up all other colours, I know these are replicated by an LRGB filter set in the imaging process and the theory of it makes perfect sense; I know some people use different filters like the Hubble palette for example but I am keeping it simple. 

So my point is why should putting a filter in front of a camera that sees in black and white make a difference to colour, surely if the camera is monochrome and you put a green filter in front of the camera lens then the only change in what the camera returns should be a change in the contrast or to make the overall picture seem a little darker? For some reason this all reminds me of my Granddad watching snooker on a black and white TV.

If the simple answer is that Mono cameras are not strictly monochrome cameras and they do actually see in colour, which I figure they must, then I'll go away and keep my thoughts to myself. But then it does beg the question what's the difference between a colour CCD and a Mono CCD?

I do not wish to come across as a bit simple but it does honestly occupy my thoughts more than perhaps it should.

To put in simple terms, when a filter is used the filter blocks all other wavelengths except the passband that the filter was designed for. So if you use a Red filter it will only allow the specific Red portion of the spectrum to get to the sensor. The same is true for G and B. As an object is being imaged in R, G, and B what you are doing is recording the R, G and B dataof the target and you would combine these post capture to get a true full spectrum image of the target. R,G and B are in effect broadband  filters as against Ha, Oiii, Ha and Sii which are narrow band colour filters only passing through a very narrow passband of only a few nano Meters.The L filter will allow all R,G and b photons of the target to hit the Mono sensor and it is therefore more sensitive and also much sharper. Hope this helps.

A.G

[D4N]

The biggest advantage for mono is its efficiency when using narrow band filters.

When you use a Ha filter you pick up only a very narrow wavelength of light that's emitted in most nebulae, this weak signal is easily drowned out by light pollution and other sources of light but if you block most of that you can take long exposures of just the light you want.

A colour camera would only capture a quarter of this light as the green and blue filters will block it. An un-modified DSLR will only capture a small percentage of this light as the IR blocking filter will remove most of it before it even gets to the bayer matrix.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

[glowingturnip]

chiming in with a simple analogy - back to watching snooker on the black and white TV.

With the simple mono camera, it is sensitive to all wavelengths (for simplicity), so it will see all the balls on the table as varying shades of grey.

However, if you put a green filter in front of the camera, it will be able to see the green ball, but will now struggle to make out the reds or the blue ball.  Similarly if you take a red filter, it can see the reds, and not the others, and so on for blue.

If you were able to take each of the three resulting mono video channels, and project them onto the wall on top of each other, with their appropriate filter in place in front of each projector, you'd get a colour picture of the snooker, even with a mono camera !

that's how they took one of the first ever colour photos, here - http://en.wikipedia.org/wiki/File:Rgb-compose-Alim_Khan.jpg