Return to moorhen home page

Return to section index

Images in Ultraviolet (UV) Light

Technical detail of Survey Techniques

What is the Survey looking at

Many creatures, especially insects, can 'see' in ultraviolet and some flowers take advantage of this to provide attractants and navigational information for nectar collection and pollination.

We are NOT talking here about UV fluorescence - visible (often single colour) light produced by materials when irradiated by UV light such as in Discos and some visual presentations.
Instead we are talking about UV as a 'colour' reflected by parts of living species and only visible to creatures with the appropriately equipped eyes.

Handling of Live subjects

Plants were simply cut and mounted as appropriate with no backing material and a black sheet sufficiently far behind that the inverse square law renders is black. A convenient mounting medium is 'Bostik Blue Tak' - where this appears in images it is 'bright' in UV.

Photographing insects introduces all sorts of problems - they won't keep still and we didn't want to 'mount' them in any way.
Our technique has been to keep them in the dark so they don't panic in their boxes, and then chill them to a few degrees above freezing (being very careful not to the injure them by freezing any part). At temperatures of about 4C (measured on the insect body by non-contact thermometer) all but a few moths are torpid for long enough (typically 4 minutes) to photograph them at room temperature. The camera is mounted looking downwards at black velvet on which the torpid insect is placed. An ice pack under the black velvet was a later addition that makes room temperature less critical.

Insect recovery takes from 0 (i.e. the insects become active during photography - very evident in some images) to 5-15 minutes in sunshine for large insects. So far all have flown off apparently unharmed as we had hoped would be the case. We (and others) took advantage of the unusually placid state while warming up to take more attractive and naturally presented images.

Types of Image

The images presented consist of up to 4 types:-
Included for most insects but unnecessary for plants.

Where appropriate 'Identification' image on a light background (Cream coloured 'art' paper). This shows up details that may not be apparent against black, such as damselfly wing patterns, dark antenna and legs etc. This image was usually taken using the camera's built in flashgun.

Most Identification images are on cream coloured 'art' paper that has a ridge pitch with 1.25mm spacing. You can use this to 'calibrate' the size of the subject. A few are taken against black - particular images of opportunity - e.g. wing position change as insects become active.
Usually taken before the UV image, only missing on very few occasions when an insect was moving during photography.

An image in natural (photographic flash) light intended to register exactly with the UV image to provide a reference when the outline of the original is not obvious in UV. The quality of this image is not as good as any 'ID' image because it is taken under similar conditions to the UV. See description of technique.
The primary image.

An image as far as possible limited to the ultraviolet. Its is shown in black and white - colour sensors in cameras tend to show the image in Blue with variable amounts of red according to camera model. The actual colour is meaningless. See description of technique.

Depth of focus on this image is very poor - see description of technique. Sometimes more than 1 image is included showing different items in focus.

Two adjacent grey-scales appear at the right of the image. The one at the edge is a full range scale in 17 brightness levels 0, 16, 32 ... 255 This is a comparative & display check - every rectangle should be visibly different - if it isn't you may want to adjust your monitor brightness and contrast. The scale next to it shows the effect of contrast adjustment on a scale 17 levels scale 0, 8 ... 128 (not 255 to compensate for all UV images being 'underexposed'). The pair of scales provides a quick visual check on how much artificial lightening has been applied.

The numeric brightness of the UV reflectance can be determined by the information entry White=nnn. See description of the embedded text information.

From 31 Aug 2008 small subjects were taken 32cm from focal plane. Closer flash distance required F 5.6 for UV (no change to RGB which uses auto-exposure flash).

On the camera setup used the Blue channel provides noticeably lower resolution than Red and Green. Blue is also the most sensitive, but using the standard paint-package channel weightings means it makes limited contribution to the monochrome image. At the resolution presented on the web montages the loss is barely detectable. Processing before 19 July 2009 used channel weightings
Red 0.29 Green 0.588 Blue 0.114
Images processed after this date used the weightings
Red 0.4 Green 0.6 Blue 0.0 (No contribution)
Unlabelled Image to Right of or Below the UV

A monochrome negative of the 'UV' image with separate contrast adjustment (but not greyscale which are not helpful). Included to highlight features that may not show in the normal image - in particular low reflectance UV.

How the images are presented

Each set of images is presented as a single .jpg file containing a montage of the various images selected. The bottom of each image contains embedded human readable text including the following information:-
xx% of
How much linear reduction from the original frame size has taken place in preparation of the montage. Some measure of how much more resolution is available in the original image.
Original Filename with the general form

        XXX_yyyymmdd_hhmm_ccc Description.jpg        
XXX ----------------------------- Camera ID
yyyymmdd_hhmm --------------- Date and time taken
ccc ----------- camera card frame count
Description Text description & type info

Type info includes on of (ID), (RGB) or (UV) as described above. (orig) means processed from untouched camera original file. (Q) means query - indication of uncertainty of identification at the time of archiving the original images.

Adjusted Black=nnn White=nnn Gamma=n.n
These show how much brightness & contrast adjustment have been applied. These figure are VERY important in the UV image. Many subjects have very little UV reflectance and the image would appear black if not enhanced.
What original level 000..255 (decimal numbers) is now used for the Black (000) level.
This almost always stays at 000.
What original level 000..255 (decimal numbers) is now used for the White (255) level. Any value above about 160 means that UV reflectance is quite high. Values below 80 mean substantial enhancement was required to make the UV image visible.
Black and White level adjustment is performed using linear scaling.
A correction used to adjust contrast at the dark end only.
1.0 is 'Neutral'.

Specular Reflections
When interpreting images beware of the effect of 'specular' (mirror like) reflections giving the impression of brightness when non exists. It is usually obvious in ID and RGB images that you are seeing the reflection from a shiny surface or edge (such as dragonfly / damselfly wing membranes, beetle carapaces and the like), but in UV it is easy to misinterpret. In particular the slightest movement of the subject may substantially change the specular reflections.

When interpreting the images note that the light source is consistently on the left just above centre. For insects it is usually safe to assume that any feature symmetrical about the insects body is 'real'.

The orientation of the images is 'as photographed' even when circumstances would have made rotation or mirroring more aesthetically pleasing. This means that the flash lighting direction is always determinable as follows:-
ID Images: Top of camera RGB &
UV images
: Left & slightly above camera

Warning: All the images are individually 'watermarked'. You are welcome to use the images for free for research, in research papers (with adequate credit of origin), but not for profit without prior arrangement.

Photographic Technique.

Canon D10 SLR (3072 x 2048 Pixel) with focal plane approximately 40cm from the subject
Canon 50mm Macro Lens (Filter ring does NOT turn with focus) with registration mark added to manual focus ring.
Cokin A series filter holder using to insert/remove 52mm dia Schott Glass SG1 UV pass filter 3mm thick mounted on a slide without otherwise disturbing the settings.
External Flash: Mecablitz 36C-2 Initial settings:- Zoom 50m from same distance as filter holder. Used in both automatic for RGB images and Maximum output for UV images

ID Images: 100 ASA, 1/180 Sec (Flash Sync), F 22, Image size 'L', White Balance = Daylight, Single frame, LENS on AF, One Shot Auto-focus. External flash OFF and built-in camera flash used. (ID images of small subject taken at about 20cm).

RGB Images: 800 ASA, 1/180 Sec (Flash Sync), F 22-F32, Image size 'L', White Balance = Daylight, Single frame, LENS on MF after AF, One Shot Auto-focus. External Flash on Automatic exposure (various settings used to suit distance and colour of backing).

UV Images: 800 ASA, 1/180 Sec (Flash Sync), F 4, Image size 'L', White Balance = Daylight, Single frame, LENS on MF successively moving systematically clockwise to provide spread of focus options, One Shot Auto-focus. External Flash on Full Power. Schott glass 3mm thick SG1 filter in the optical path (rendering the viewfinder black and focus performed by starting at that used for RGB. Direction and amount offset of the UV image varies with lens and zoom setting (where relevant) and has to be determined initially by trial and error. Very large aperture lenses introduce so much glass into the optical path that they may perform worse than the more moderate equipment used here.

UV-negative: An alternate display of the UV image to help show up detail otherwise difficult to see in dark areas.

Plant RGB and UV images were mostly exposed with camera looking horizontally (a few taken laying flat as described for insects but without ice packs). Subject mounted in free space with large black backing sheet about 40cm behind subject (provides almost perfect black).

Insects RGB and UV images were exposed with camera looking vertically down on the subject resting on a black velvet, often with an ice-pack underneath. Sometimes rolls of velvet or other items are used to stop the torpid subject toppling - these are sometimes visible.

Some Findings

Lepidoptera (Moths and Butterflies) which have small white spots or marks (e.g. Meadow Brown, Gatekeeper, Comma) show these marks to be bright in UV even if other white marking are dull in UV. So far there are no exceptions.

Dragonflies are generally dull in UV on wings and top side, but include a variety of marking on their underside.

Some of the daisy family are dull on one side of the petal, and bright on the other. Oxeye daisies are only bright in UV on the underside/outside while a yellow variety is only bright on the upper/inside surface.

Lilies and other asymmetric flower shapes show some navigation patterns, or at least upper and lower petals having different UV reflectance even though similar in white light.

Last updated 13 Oct 2009


Comments and requests for image use

To make a comment, ask for information or to request a full resolution image, send us an email including the reference or the date for the image you are interested in.

All initial contacts should be made using the icon below:-

Mail Us