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