Colours in UV-photos

Recently, I have been made aware on a forum for ultraviolet photography (http://www.ultravioletphotography.com) that the colours in my UV-photos are not “balanced”. I had noticed that before, but I was not sure why I could not remove the colour-cast by white-balancing with a PTFE (Polytetrafluoroethylene) target. My solution was to convert the images to black and white, since we do not see UV-light anyway, and therefore there is no “right” colour or set of colours for it.

All digital cameras (unless they are monochromatic) have a set of RGB filters in front of the individual light collecting pixels. The dyes in these filters are not equal in terms of UV-transmission. In the case of my Panasonic Lumix G1 it looks like the blue channel collects the highest amount of UV-light, followed by the red channel, while the green channel is usually the darkest. Phrased differently, the green dye seems to absorb the highest amount of UV-light and the blue channel seems to have the lowest absorption. So, in order to get images without a strong colour cast, the blue and the red channels are down-regulated and the green channel is boosted. In my camera this can be done by setting the white balance manually against a PTFE target.

However, since I use to shoot in RAW-format I was hit by the fact that Photoshop Lightroom (ACR) does not allow the values for the white balance to fall out of a certain range. Therefore, my final images looked always different from the previews on the rear screen of my camera. Using the DNG-profile-editor (http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/products/photoshop/pdfs/cs6/DNGProfile_EditorDocumentation.pdf) can change that behaviour by creating a custom profile, as I learned in an Adobe forum. After creating a suitable profile I was able to reproduce colours similar to those described here:

http://www.ultravioletphotography.com/content/index.php?/topic/81-how-to-make-uv-colours-reproducible/

I’m going to show my UV-photos using this profile in the future, mainly for aesthetic reasons. However, these “UV-colours” should not be confused with “bee-colours”, since here we have just a small part of the spectrum that the eyes of bees can detect displayed in the colours that we know. In one of my next posts I’m going to elaborate more on “bee- colours”, today I will show mainly some UV-shots and the colours they can produce in a modified digital camera.

First a Spring Cinquefoil (Potentilla neumanniana) in visible light:

Now the UV-photo of the same flowers:

Next Coltsfoot (Tussilago farfara) in visible light:

And the UV-image (I had shown this before with the colour cast introduced by the RAW conversion. See here: http://bee-colours.blogspot.de/2013/04/the-gear-i-use-for-uv-photography.html):

This is what I call “simulated bee-colours” for the Coltsfoot (Tussilago farfara):

In the examples above we have seen yellow flowers that also turn yellow in the UV-photo. Now an example of the white flower of the Common Daisy (Bellis perennis):

Interestingly, in the UV-shoot the petals turn blue:

Finally for today the blue flower of the Lesser Periwinkle (Vinca minor), as we see it:

Below, the UV-photo of Vinca minor. The petals appear in a blue, similar to the colour that we see, but a bit darker. Note also that the background colours are very different in this image:

The colours of the UV-photos are not (completely) predictable, since some yellow flowers can appear entirely black and blue flowers can have different intensities of UV reflections. The latter is also true for white blossoms. However, until now I haven’t found a flower that we see as blue which would turn yellow in the UV-image or vice versa.

I've posted some more examples here:
http://www.ultravioletphotography.com/content/index.php?/topic/258-white-balancing-uv-photos-in-lightroom/