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photo:infrared_digfalsecolour

digital false colour infrared photography

Introduction

  • false colour IR imagery with digital cameras requires an IR filter which blocks out most but not all of visible light so that there are sufficient data to work with in the green and blue channels of the image.
  • if one uses too strong an IR filter (eg. 840nm / Wratten 87C filter) which blocks out too much visible light then the only data will be in the red channel and you will end up with only a monochrome IR image
  • most digital cameras have a sensor sensitive to near IR light up to ~1100nm, but also have a special IR-blocking filter in front of the sensor as the IR light would otherwise degrade the image, thus sensitivity ranges are usually:
    • normal digital cameras with IR & UV blocking filter:
      • 400nm to 750nm (this roughly correlates with the range our eyes can detect)
    • normal digital camera with IR & UV blocking filter removed:
      • 280nm to 1200nm, but because sensitivity is reduced at the extremes, the practical range is approximately 325nm to 1100nm.
  • the usual recommended IR filter for false colour digital IR imagery is a 720nm filter (alternatives are R72 or Wratten 89B)
  • placing such a filter on a normal digital camera will result in the need for relatively long exposure times as digital cameras have IR blocking filters on their sensors to give sharper imagery (IR light focuses slightly differently to visible light so a sensor receiving both will have less clarity)
    • in addition, if your camera is a dSLR and you add such a filter to the lens, you will not be able to see much through the viewfinder, nor will the camera autofocus well, and you will need to resort to Live View mode and manual focus. Furthermore a strong IR blocking filter on the sensor may make false colour IR less effective and some may find in this case a deep red filter will work better than an R72.
  • a much better alternative is to have your camera modified with removal of the IR blocking filter and replacing it with a 720nm filter
    • this means you can have much shorter exposures, you don't need to buy IR filters for your lenses, the AF will work (it will need to be re-calibrated for dSLR cameras though - mirrorless cameras are not an issue in this regard), and you can compose through the optical viewfinder or the Live View
  • when taking your photo, check the red channel on the histogram and adjust the exposures to ensure it does not get blown out / over-exposed
    • auto exposure on cameras will generally try to get exposure optimised for all 3 color channels but this is NOT what we want - we want the red channel only exposed correctly.
  • ensure you shoot in RAW file mode

Post-processing for false colour IR images

post-processing with On1 Photo RAW

  • in Edit tab, go to Filters and Add the Channel Mixer filter
  • there is a preset for BW IR as well as one for IR SWap for false colour IR (this just swaps the red and blue channels)
    • NB. at present On1 does not support LAB color space so you can't use these techniques to tweak your IR images
  • then go back to the Develop tab to tweak the white balance settings to optimise the false colour
  • in addition you can add further filters in Edit tab as desired such as:
    • LUTS filter eg. Venus
    • Glow filter eg. Rich Glow for the traditional light and airy IR look
    • Sunshine filter
    • Curves to adjust the mid tones and highlights in particular
    • Dynamic Contrast to add a bit of detail etc
    • Color Balance to tweak the hues of the midtones and shadows

post-processing with Photoshop

  • post-processing method 1:
    • selecting the color image, selecting the entire image (ctl-A), clicking on the “Channels” tab and selecting the green channel and copying this (ctl-C), selecting the blue channel and pasting (ctl-V). Repeat the process to copy the red channel into the green channel. Select the IR image, select the entire image, copy the red channel, return to the color picture and paste it into the red channel.
    • this may produce “crazy” colors
  • post-processing method 2:
    • Take the color image and convert it into “Lab Color” mode. Take the IR red channel and copy into the “Lightness” channel. This creates color images that “pop” very well and have very dramatic skies.
  • post-processing a Sony green nightshot image with R72 filter on:
      • create duplicate layer
      • open hue/saturation and set green saturation to minus 100%
      • then change master to: hue = +124 and saturation = +50 to 60
      • use Gaussian blur of 3-5 pixels radius on background copy layer to reduce noise
      • change background copy layer to layer method “colour”
      • use auto-levels on background layer
      • open hue/saturation on background layer & set red saturation to minus 100%
      • flatten the layers
  • post-processing via Daniella (“zylen)” method using a Dimage 7:
    • take a good photograph with good subject, white balance to tungsten, color saturation to -3 or -2 and contrast to -2.
    • copy the layer and apply auto-level. Adjust the slider to adjust the result. Once the result is ok, merge them.
    • copy the layer again, then adjustment, channel mixer and swap the blue and red channel. Keep the unswapped image underneath. You can add a layer mask and blend the swapped and unswapped image, and reveal or hide part of the swapped image that you want or don't want.
    • create a new layer and use “color” mode for the blend. Then use the color that you want and spray the area to tint the area that you want to correct.
  • post-processing via Ralph Croning's method:
    • take a photograph using custom WB on sunlit grass through the R72 filter
    • Image > Adjustments > Channel mixer:
      • in RED channel: red = 0% and blue = 100%
      • in BLUE channel: red = 100% and blue = 0%
      • Click OK
    • Image > Adjustments > Hue and Saturation
      • From the drop down menu choose Reds and desaturate to minus 100
      • From the drop down menu choose Magentas and desaturate to minus 100
      • If the pic is dark choose Blues from the drop down menu and adjust lightness to plus 70 or to your taste
      • If your photograph is overexposed all the overexposed areas will show up as yellow.
        • In this case you will then also choose Yellows from the drop down menu and desaturate to minus 100 or according to your taste
        • While in yellow also adjust the lightness slider to the positive side till you achieve the desired brightness
      • When done click OK
  • Photoshop actions for digital IR post-processing:
photo/infrared_digfalsecolour.txt · Last modified: 2021/10/03 14:05 by gary1