While I understand that color sensing with camera and sufficiently powered processor running image histogram logic (or other such algorithms) can determine presence of certain colours fairly reliably.
However, are there other, significantly more cost-effective mechanisms to determine presence/absence of certain color (or it's close shades), at close range, using simpler/cheaper sensors and lower computational requirements ?
I am guessing that things like pH sensors, or other chemical sensors might use such methods. In my case, the application is such that I need to detect presence/absence of a specific colour (a shade of light blue), in a small area, at close range.
Edited: By 'close range' I mean something between 1-5cms, though this isn't a very strict requirement. I was thinking of "close" relatively, i.e. there is no direct irradiation from light source to sensor, kind-of a double barrel, such that only reflected light hits the sensor. So closeness is a function of physical sensor placement, and I am open to suggestions (including completely alternative / orthogonal approaches).
Answer
Since LEDs used as photodiodes are most sensitive to the colour they emit during normal operation, a basic colour sensor can be made using the LED in reverse and an opamp:
The circuit above came from this page. It can also be done the other way round - a much more detailed look at colour sensing using LEDs and light sensors is available here - this page details using a normal light sensor and different colour LEDs.
I couldn't find the app note mentioned in the comments, but this page seems to be quite a thorough treatment of the subject of transimpedance amplifiers. You can alter the bias across the LED to change response time/sensitivity.
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