a camera which records images at different wavelengths using optical filters

Question

I need to create images at a certain wavelength range(around 850-900 nm). For this a frequently cited solution is using optical filters. However I am not being able to understand the concept behind those. A camera has image sensors which store charges, and then convert it into colored images using RGB optical filters. Now if I am to put up an optical filter that transmits wavelengths of only 850-900 nm, then what kind of image shall be captured?

From what I understand, since there is no RGB based wavelength in 850-900nm light, so the pixels will all record values (0,0,0) and we'll have just a black image.

Am I right? If not, please explain what exactly will happen and why? And is this method to capture a specific wavelength based image using optical filters the best one or even a possible one or I will have to go to the basics of image sensor development itself?

Answer 1

You use a black/white sensor that has no RGB filters.
Then, you put your IR filter in front of the camera. Now you have a camera that sees infrared only.

Most CMOS camera elements are IR sensitive and have an extra filter to remove IR so as not to make a mess when using RGB.

You can get black and white CMOS cameras commercially, some even have built in IR LEDs that can be used to "light up" the view field with IR.

Take a look around. You may even find a commercial unit with the correct IR filter already built in.

Answer 2

There are two issues here.

1. The sensor must be sensitive at 850-900 nm. Else you will "see" nothing.
2. If the sensor picks up MORE than 850-900 nm, then you must use an optical filter to limit the bandwidth to the frequencies of interest.

This is no different than an RGB color imaging device operates. The imaging device is sensitive to a broad range of wavelengths. Perhaps from <300 to >800 nm. But then optical filters are used to make certain spots "see" only Red or Green or Blue.

Many imaging devices used in color video cameras have response above visible red (~750 nm) So optical filters must be used to cut off response to avoid polluting the red video with invisible IR radiation.

There are commercially available products and components which image IR. They convert the image into a monochrome (black and white) video picture. In many cases, a microcontroller is used to "colorize" this monochrome image to better see subtle changes in "brightness" (heat). A company here in my area (FLIR) is one of the leaders in IR imaging products and they offer pickup components for sale to make your own devices. At this point they are still pretty expensive.

Example, FLIR Lepton: http://www.flir.com/cores/content/?id=66257

Answer 3

The camera sensor has a wide bandwidth, all pixels are equal. Then there is a filter RGB where every 3rd pixel gots the same coulor filter R,G,B,R,G,B.... If you need to record only a selected spectrum, you would need to remove the camera RGB filter and use an external filter of your choice.

Answer 4

I strongly suggest you get a black and white camera (either a digital camera sold for astronomy or a security camera if you can still find one that does B+W). JRE's answer explains what to do there, and there's plenty of further reading from amateur astronomy sites.

When it comes to colour cameras, I've used ordinary webcams to view 785nm and 830nm (even 1064nm) lasers. The sensitivity is reduced compared to visible light because the filters do attenuate these wavelengths significantly (even more so at 1064nm, where the response of the silicon sensor itself is falling off). These cameras tend to come with an IR-blocking filter (often on the back of the lens so easily removed). The IR blocking filter is quite effective.

The filters don't have to be perfect at dividing the light into bands - the human eye isn't, and given the IR-blocking filter they don't need to be specified outsie the visible range. If you google "bayer filter spectrum" you'll get a good idea of what I'm talking about. Here's an example (no copyright info so linked not embedded).