Colour - Summary of UV Imaging

The facts and some serious speculation

In Birds and UV Light I summarised how birds see in UV and also how they display ultraviolet reflection and absorption patterning in their plumage.  Most research looking into these phenomena to date have been through the medium of spectrophotometry rather than digital photography.  At the time of posting there are very few avian UV digital images readily available online so, up until now it has been difficult to imagine what a UV image of a bird actually looks like.  While researching this area I felt it might be possible to address this with minimal modification to some of my existing camera equipment.  The postings HERE and HERE outline how I have achieved this and in this posting I will reveal some of my preliminary avian UV imaging results.  

Imaging techniques using UV

Firstly, I think it is important to discriminate between two UV imaging techniques which actually have very little to do with each other.  UV reflectography (UVR) is a technique used mainly in the art world for forensic analysis of paintings.  The method used is phosphorescence not UV reflectance.  Phosphorescence is a phenomenon in which certain materials (phosphors) absorb UV light and re-emit it as visual light. So, phosphorescence is visible to the human eye and can be photographed by any standard digital camera.

UV reflectance is quite different.  UV and UV reflectance is invisible to the human eye.  It requires using a specially modified digital camera to record monochromatic UV images.  There is no phosphorescence involved.

What are Ultraviolet reflectance Images and how are they made?

A typical ultraviolet image is no different to any other digital image except that it is exposed entirely using light from the ultraviolet (UV) portion of the light spectrum, and it is monochromatic (like a B&W image) rather than a full colour image.  UV consists of wavelengths of light which are shorter than those of the visible spectrum.  As they are outside the range of the visible spectrum they are invisible to the human eye under normal circumstances.  However all digital camera sensors are sensitive to at least some wavelengths of UV and therefore, in theory at least, we should be able to create images from ultraviolet light alone using a standard digital camera.

The illustration above helps to explain how an ultraviolet image is formed using the example of a CCD image sensor.  UV must jump a number of hurdles to reach the sensor and generate the image, and these are depicted below.  Firstly, for comparison, here is a diagram showing how a normal colour image is created using a digital colour sensor in most modern digital cameras.

In the case of a camera modified for UV digital imaging, the configuration looks like this.

Firstly, moving from left to right, the Baader U filter blocks out both the visible and infrared, leaving only the ultraviolet portion of light to pass through to the lens.  Most modern lenses have coatings which absorb UV.  However, provided there are minimal lens coatings and few lens elements, a reasonable amount of  UV light should make it right through the lens.

Normally, at this point an IR/UV blocking filter absorbs any UV that has made it through.  However, in the case of the Sony Handycam which I have been using, that camera's IR night vision feature requires that there should be no IR/UV blocking filter present to restrict the image.  UV gets a free pass thanks to the lack of an IR/UV filter.

The UV light is now reaching the outer layers of the sensor.  On the surface of the sensor are microlenses that help focus the light onto individual colour filters in a mosaic structure called the Bayer filter array.  The Bayer is a honeycomb of tiny green, red and blue colour filters.  The purpose of the array is to give colour to digital images.  Each colour filter will only allow light from it's own portion of the visible spectrum through while blocking other wavelengths.  These colour filters do not block out IR and UV very well however, hence the need for a separate, dedicated IR/UV blocking filter.

Light which makes it through to the photoactive region of a CCD (or CMOS) sensor, beneath the Bayer filter array will now register as an image.  Each tiny photo receptor or 'photosite' is a capacitor or 'photodiode' which accumulates an electrical charge due to the photons of light striking it.  This charge, together with a record of the colour Bayer filter sitting above it ultimately transmits as raw image data to the image processor.

Note each pixel in an image is created from the accumulated data of a small grid of four neighbouring photosites, i.e. an individual pixel's hue, saturation and luminosity value are calculated based on multiple data points, not just from one photosite.  This is done through a process called de-mosaicing, as explained HERE.  This process accounts for a digital camera's wide colour gamut.

Avian UV Reflectance Images - some preliminary results

Compared with the amazing results obtained with flowers (nectar guides) and butterflies (butterfly UV signalling)  HERE and HERE, the avian UV reflectance results obtained so far have been uninspiring to say the least.  Almost invariably, there is very little notable difference between colour monochrome images (B&W or greyscale) and UV monochrome images of birds.  It has to be said however that, with it's rather drab avifauna, the Western Palearctic may not be the best place on earth to be looking for UV patterns in birds.  Most of the interesting UV finds to date have been in the Neotropics.

One of the few results of note so far - in Moorhen, the yellow tip of the bill is invisible in monochrome UV.  The remainder of the bird matches colour reflectance images more or less exactly.

This Mallard and it's surroundings effectively look identical in both VIS and UV in monochrome.  Even the structural iridescence in the secondaries is preserved perfectly in UV.  

Rather than put up a whole host of avian UV images that appear to show very little I think it's best to look beyond.  I have yet to record one of the more celebrated UV reflective species, the Blue Tit (Paris caeruleus) as they are keeping a low profile it seems, post-breeding.  The sexes of that species are indistinguishable in VIS but in UV the males have a more reflective blue cap.  It would appear that blue pigmentation is more likely to be associated with strong UV reflectance than many other colours.

So what is going on with birds and UV?  Presumably UV reflectance and absorption is by design rather than by accident.  If the majority of birds are similarly reflective in VIS and UV it might suggest that the primary purpose of UV reflectance in birds is to simply ensure no net loss of plumage brightness.  Does the story end with that dull discovery, or, are we missing something here?  I think we might be.

Limitations of monochromatic imaging

In the colourful, visual world that we inhabit our eyes are masterfully adapted for studying our environment.   Our eyes have three different colour receptor cells (red, green and blue cone cells) plus one receptor adapted for low light or night-time vision (rod cells).  We are programmed to respond differently to different colours.  Our eyes are most sensitive to green.  Red and yellow are used widely in nature to denote danger and our brains are pre-disposed for registering these colours in our environment.  They appear somewhat more vivid than other colours (even if they are not necessarily so in reality).  It is also known that we have a very good ability to discern subtle tonal gradients and we even modify what we see to improve acutance (edge contrast) eg. the phenomena of Mach bands and Cornsweet Illusion.  We are not typically used to studying the world around us in monochrome, so when it comes to the study of UV reflectance we are not really utilising all of our visual acuity and senses. 

Many birds have three cones, just like us but some species have a fourth cone that appears to have a function in UV vision.  The big question is, what is this cone registering?  Is it purely used to record UV light intensity and reflectance, like our night-vision rods.  Or, does it record UV hues in much the same way as our colour cone cells?

It is very important to stress a key limitation in our study of UV patterning in birds.  Whereas with colour images we can study hue, saturation and luminance, when it comes to monochrome images, we only have luminance to work with.  Studying images in monochrome is somewhat akin to humans trying to study plumage colouration in birds under moonlight.

Because we cannot currently discriminate between different hues and saturation's while imaging in the UV spectrum, we should not discount the possibility that we may be missing some fundamental UV plumage patterning in birds.  There could for example be a fringe on the edge of a feather with a UV hue peaking say at around 350nm, while the centre of the feather may have a UV hue peaking at 380nm.  For birds able to visualise and discriminate between UV colour wavelengths, this fringe would appear just as obvious to them as the coloured shapes in the example given above appear to us.  If however the reflectance and therefore the luminance of the feather is uniform throughout, this pattern will be completely invisible to the UV camera, as illustrated by the colour example below.

Spectrophotometry offers some advantages over UV photography in that the spectrophotometer can detect peaks at different wavelengths of UV.  However the method has it’s own limitations, because the data gathered by such a device is based on point sampling.   If the data is all collected from only one or two points on each feather, chances are that subtle markings around the edges or tip of the feather may be missed.  Scientists have tried to search for hidden UV patterning in feathers during avian UV research using spectrophotometer's so what I am saying here is not new.  It is just possible that variations in UV within feathers is rare or non-existent.  Have we another way of looking for it?

I think the ultimate UV imaging solution would be a bespoke UV imaging camera with false colour gradients set at discrete UV wavelength increments.  This type of device is already in existence for far Infrared work – eg. the IR thermographic camera.  Until such a device is readily available for UV we will have to make do with UV reflectance and spectrophotometry, but we should remember that we may be only seeing a small part of the whole picture.  UV imaging is very much at the stage that photography was before the dawn from B&W to colour, with the added disadvantage of course that as humans we can’t see in UV!  

Here is how a bespoke UV imaging camera might work if it were based on the same principals as normal digital cameras.

Here is a mock-up of what an image from such a device might look like.  So far I have not found any evidence that a camera like this has been manufactured but it should certainly be possible.  Unfortunately UV imaging doesn't have the same uses or mass market appeal as an IR thermographic camera.  IR thermography is a growing business thanks to its application in energy management, fire prevention, preventative maintenance, security and the rescue services.  It is hard to envisage too many uses for a UV imaging camera outside of very specialist industrial applications.  Then again, when such a camera is developed, I have no doubt there will ingenious uses found for it.

The 'colour' of UV light (pure speculation)

Considering that the UV spectrum (100nm - 400nm) is nearly as wide in range as the visible spectrum (400um - 700um), surely animals and birds that see in UV actually see a range of UV hues and perhaps even perceive colours which we simply cannot comprehend?

Perhaps a clue can be found in the design of the avian eye.  Some birds possess a fourth cone cell and this seems to function in UV vision.  So, if the bird uses three cone cells for the visual spectrum (green, blue and red) and an additional one for UV, it seems reasonable that birds with a UV cone cell should be able to discriminate between various hues of UV light in much the same way as we distinguish subtle hues in the visible spectrum.  Put another way, it would seem like an waste of valuable resources to equip oneself with a receptor for UV only to use it for UV reflectance purposes.

If we could see what birds see, what would the colour of UV look like?  Okay, so the next statement has no basis in fact but bare with me.  Could UV be magenta?

It seems weird that we have this one broad set of hues (the magenta scale) that don't have a place in the visible spectrum and yet we are surrounded by pinks and magenta's in nature.  It also seems highly convenient that magenta sits right where UV should be, beside violet on the majestic colour wheel, the foundation stone of colour theory.  Could it be that while most animals were busy losing the ability to see in UV they were at the same time gaining the ability to see in magenta, and without the need for an additional receptor in the retina?  Lastly, an evolutionary reason.  UV is damaging to life on earth - why waste time on a visual system that encourages us to look at UV, a potentially life-limiting form of radiation.