Having a background in photography my interest in the Raspberry Pi started because I saw some potential for expanding my horizons in this field. I have previously worked with infra-red film so the development of the NoIR Pi camera module has given me even more reason to get involved with this wonderful new computer.
My initial areas of interest are security and surveillance, motion detection (for surveillance and wildlife/people/other image capture, time-lapse and special effects photography. Most of these mean working with low light levels so there was a need to start testing the camera to see what it can produce. When working with infra-red film I generally used a red and/or UV filter to boost the effect and reduce unwanted blue/green light. This lead me to thinking that if I use the camera with other coloured filters as well I would be able to produce some rather interesting effects - although usually with IR surveillance I would be producing black and white images.
My first trials with the camera modules - both normal and NoIR - involved the time-lapse function as this would be one of my surveillance methods. I was using available light but, although reasonably happy with the results, I wanted to see if using filters would improve the camera’s ability to capture light.
So, some testing was planned and the first test gave me some interesting and useful results.
The location chosen was a dimly lit street with one main light source and a large amount of greenery for someone to hide in. The results shown here are from using no filters and four different coloured filters including the blue filter that comes with the NoIR camera module - red, green, blue and yellow. I took colour and black and white images.
The following comes from various sources but I think originated from the Mag-Pi magazine and I used it as a guide for several of the images.
"The colour effects option (--colfx) is interesting. Internally, the image is represented using a YUV colour space. YUV represents colour using the luminance, Y, and the blue–luminance and red–luminance differences, UV. The colour effects option allows us to specify the values we are going to use for U and V (range 0 to 255) This gives us a quick and easy way to do black and white images, we just need to set UV equally to the middle of the range, which is 128. So
raspistill -t 5000 --colfx 128:128 -o image.jpg
Other values for U and V give varying blue and red differences from the middle."
Here are the resulting images (7 x 2) with corresponding histograms for comparison.
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Now this is where things became interesting as I altered the --colfx settings.
Notice the shift to yellow/green resulting from the change from --colfx 128:128 to --colfx 156:100 and 200:56
The second image here - --colfx 200:56 - shows are distinct increase in light captured. And finally this image converted to black and white, post production, and with some changes to the 'levels'. There is an increase in the amount of noise but also more detail to be seen which is useful when shooting in very low light levels.
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Future blogs will cover these same filters (and more) and possibly the 'normal' camera module in daylight and other well lit situations. I hope to show some special effects and/or abstract images and see what benefits there are, if any, in using these filters for surveillance images.
