||Start
Contents

What is IRIS
Copyright and Use
Installation
Running a simulation
Control Panel
   Droplets
		Droplet size
		Size distribution
	Light source
		Spectral distribution
		Source size
	Simulation type
	Quality
	Sectors drawn
	Clear sectors
	Redo (re draw)
	Tune
Saving a simulation
Tuning a simulation
Zoom
Advanced settings
Tools
Acknowledgements
Contact

||Iris
What is IRIS

IRIS simulates coronas, glories and fogbows.  These are produced by light scattering from water droplets in clouds and fog.

Cloud droplets are so small that the wave nature of light is significant and geometric ray optics do not apply. The droplets scatter, diffract, sunlight in all directions but mostly into multiple concentric rings. They form a brightly coloured CORONA around the sun, a coloured GLORY directly opposite the sun and a large ghostly white FOGBOW. 

IRIS uses Mie scattering theory to produce accurate simulations. Formulated by Gustav Mie in 1908 for scattering by a sphere, this theory derives without compromise from Maxwell's equations for electromagnetism.  However, Mie calculations are so lengthy and complicated that they were not really practicable until large mainframe computers were developed. These rarely provided other than tabular output which hardly enabled the computed glories to be visualised.   Now, PCs are not only fast enough to do all the arithmetic in a reasonable time but can also display the results as full colour simulations. 

Glories and fogbows are polarised.  IRIS assumes an unpolarised light source.  It computes each polarisation component for the scattered radiation and finally averages them to present a simulation as would be seen by eye or camera without a polarising filter. 

IRIS computes ideal Mie glories, fogbows and coronae in the sense that the simulations correspond to what would be seen if each light ray reaching the eye had only been scattered once.  In Nature, there is inevitably some mutliple scattering and the phenomena are viewed against or through cloud or bright backgrounds. IRIS has an empirical facility to combine the simulation intensities with those of cloud background light. There are also standard image processor functions to adjust contrast, brightness or gamma. These are particularly useful for corona simulations where the tremendous brightness range between the aureole and outer rings is well beyond the puny 256 intensity levels of present day PC monitors.

More details and photographs of coronae, glories and fogbows are on my Atmospheric Optics website at www.sundog.clara.co.uk/droplets/light.htm accessed by clicking the lower left button.

||Copy
Copyright and use

IRIS may be freely downloaded and run for personal, research and educational purposes. 

Published simulations should acknowledge the use of IRIS and quote the URL www.sundog.clara.co.uk/atoptics/phenom.htm in captions.

IRIS is copyright 2002 Les Cowley. All rights reserved.  It may not be altered redistributed or resold.  Commercial copying or resale is forbidden. The author accepts no liability for any consequences of downloading and use.

||Inst
Installation 

IRIS requires a PC running Windows95 or higher.  A 500 MHz or faster processor is desirable.  Display must be 24bit 800x600 or larger.  Hard disk memory usage is minimal.

If HALO is NOT already installed:

(1) Download "iris1.zip" from www.sundog.clara.co.uk/droplets/dload.htm 
(2) EXTRACT (do not RUN) into a temporary folder
(3) RUN "setup.exe" and when prompted chose a folder for the files or use the suggested default. (4) RUN "Iris.exe"

Installation inserts only standard Microsoft systems files in other than the designated folder. All files were scanned for virii using NortonAntivus and the  latest virus definitions.


If HALO is already installed:

(1) Download the much smaller "irisup.zip" file
(2) UNZIP (do not RUN) into a folder of choice
(3) RUN "Iris.exe"


||Run
Running a simulation

Click the green "Start" button.  Within a few seconds the screen will show a full colour glory produced by 8-micron radius cloud droplets.

Select a corona by clicking the drop down list on the control panel.  Click start and a corona will be simulated within a few seconds.  

The corona simulation is "tuned" so that it is realistically displayed in spite of its tremendous brightness range.  Most of the tuning controls are the same as those in image processing applications. Adjust the sliders and click "Display" to view the effects. The original simulation information is always retained.

Select a fogbow and try a different zoom setting from the top menu.

||CPan
Control Panel

The floating control panel contains most of the 'everyday' controls. These are supplemented by further options accessed from the top menu.

DROPLET PROPERTIES
------------------

DROPLET SIZE
In the top left box insert your own value for the drop radius or select on of the presets.  The range is currently restricted to 1- 100 micron radius.  Larger drop sizes take longer to calculate.

SIZE DISTRIBUTION
The default setting is to for all cloud droplets to have the same size. The sharpest coronas, glories and fogbows are produced when in nature the droplets are similar sized.  

What is seen when a range of sizes are present is a superposition of the effects from each size.  This usually results in a blurring and reduction in contrast.  

Examine these effects by selecting a droplet size distribution.  The distribution width is selected by choosing a value (or entering your own) of the "std dev%" which is the percentage standard deviation or root variance of the distribution about the mean value.


LIGHT SOURCE
------------
Sunlight or moonlight are the usual sources.  IRIS can also simulate narrower band and truly monochromatic illumination.  The source can be a solar-sized disk or a point.

SUNLIGHT
Wavelengths from 380 - 700nm are included in the simulation.  The solar spectral radiance is that of Professor Raymond Lee.  Select a SOLAR DISK or POINT source.

NARROW BAND
A central wavelength and bandwidth can be selected. The bandwidth can be set encompass the whole visible spectrum, if desired, by setting the central wavelength to 540 nm with a bandwidth of 320.  The result will be a simulation for radiation NOT weighted by the solar spectral radiance. 

More usually a bandwidth of 5-20 nm is needed.

A bandwidth of ZERO shows some of the subtle effects revealed by Mie calculations.  Change the wavelength by ~1 nm and the ring intensities of the glory or corona can alter significantly.  These rapid intensity fluctuations with small wavelength changes are not artefacts in the calculation - they are real.  They result from interference between different ray paths through (or reflected from) the droplet.


SIMULATION TYPE
--------------------------

Coronae, glories and fogbows are all aspects of the same phenomena.  IRIS only separates them so that the calculation can be optimised and speeded for specific angular ranges and to display the results appropriately.

The simulation type can be selected from the Control Panel or the main menu bar.


QUALITY
------------

Simulation of a full colour glory or fogbow requires, for each wavelength, Mie scattering calculations for a large number of angles. The calculation must then be repeated for a large number of wavelengths and then weighted and summed to produce the final colour and intensity distribution.  When cloud droplets with a finite size distribution are simulated the whole immense calculation must then be repeated for a number of different drop sizes!   All this can take time, a great deal of time. Acceptable run times are achieved by careful control of the integration conditions to achieve a balance between output quality and execution time. 

IRIS has four quality settings selected by clicking the "Qual" bars.  

The HIGHEST QUALITY (4 bars) has the integrations over angles, wavelengths and drop sizes optimised so that the visual appearance, colours and intensities of simulations show no perceptibly difference to those of calculations taking considerably longer.  

The HIGH QUALTY (3 bars) setting produces faster simulations with only a slight degradation of colours or intensities. This is not serious and is usually only noticeable when directly compared with the "4 bar settings".

The MEDIUM QUALITY (2 bar) and LOW QUALITY (1 bar) settings are faster still. They are useful for scouting simulations to set the parameters for a higher quality run.

SECTORS
--------------

The sectors in which to display the simulation are selected by clicking on the SECTORS display.  Up to four different simulations can be shown and compared.  

The last simulation can be redrawn into any sectors by selecting them and clicking the REDO button.  Only the plotting is repeated - the calculation remains stored until a new one is started.

Clicking in the "Clear" display clears individual sectors. They can be redrawn.

REDO
--------

This replots the untuned calculation into the sectors selected.  See display tuning.

TUNE
-------

Opens and closes the displays "Tune" panel.


||Save
Saving a simulation

Save a display (any combination of sectors) by clicking "File", "Save simulation image".  You will be prompted for a file name for a BMP image file. The Control and other panels open will NOT be visible.

||Tune
Tuning a simulation

The "Tune" panel is opened via the menu "Tools", "Tune last simulation" or the "Tune" button on the Control panel.

There are four sliders and text input boxes.  The upper three are "Brightness", "Contrast" and "Gamma" adjustments identical to those in image processing software like Photoshop.  To enter a specific value, type it into the adjacent text box.  

The fourth slider "Int"(ensity) displays the simulation multiplied in intensity by the factor entered.  This is useful for revealing faint outer corona rings or the secondary bow of a fogbow.

The settings are implemented by clicking "Display" NOT "Start"!

The original simulation values are always retained until another simulation is run.  Click "Restore original" to show them.

Coronae are automatically shown tuned. The raw computed intensities can be  shown by clicking "Restore original" or REDO.

SKY
-----
Glories may look unrealistic when first simulated because they are normally seen or photographed against a bright cloud background.  Add a grey background by clicking "Add sky background", adjusting the "Cloud" slider and then clicking "Display". It is removed by simply clicking "Remove sky background".

All image tuning actions are reversible.

||Zoom
Zoomed views

Click "Zoom" on the main menu to reveal view options.  

Click on the one of choice then starting simulation or click "Display" on the tuning panel.

The zoomed views are most useful for showing the detail of fogbows.

To redraw a simulation with a new zoom setting simply click REDO or DISPLAY on the tuning panel, there is no need to recalculate.

||Adv
Advanced settings

REFRACTIVE INDEX
----------------------------

Access via "Settings" on the main menu followed by "Refractive Index".

WATER 
IRIS uses real refractive indices from IAPWS (see acknowledgements).

The values are temperature dependent and the droplet temperature can be changed within the range 0 - 30 Celsius. 

The imaginary (absorption) component is from Pope and Fry (1977) and may be included by ticking the "Use imaginary component" box.  (See the note at the end of this section about intensity normalisation). 

OTHER SUBSTANCES

Ice, glass and other materials may be selected.  

Additional substances can be added.   These are entered into the data file "ris.xmt" which has instructions at its end.

SPECIFIC VALUES

Selection of "Enter a value" from the "Materials" drop down list allows specific real and imaginary components to be entered.  (See note below about normalisation when using non zero imaginary values).

Try simulation coronae and then glories for a range of real index values and then explain what is happening!

A NOTE ON NORMALISATION
IRIS currently automatically normalises all intensities before plotting so that the most intense red, green or blue component found has a value 255 (when using the tuning panel the slider "Int" actually changes this maximum to whatever is selected).  The regions searched are within the glory or within the corona but outside the intensely bright central aureole.

A side effect is that an overall reduction in intensity will not be seen when the absorptive (imaginary) component of the refractive index is made non zero.  This effect will be addressed in an upgrade.



COLOUR MODEL
------------------------
Neither monitors, TV screens, photographs nor the printed page accurately display all the possible colours.  In particular, NO pure spectral colour is properly displayed. This is because of limitations of phosphors, pigments and inks and the limited number used - only three on monitors.  The set of limited colours that ARE accurately displayed is the colour gamut of the display device. The problem is how best to approximate the non displayable colours, the out of gamut colours, using ones within the device gamut.  

Further challenges are how to deal with issues of white balance, individual perceptions and the circumstances in which real droplet scattering phenomena are viewed. In contrast to rigorous Mie calculations, colour representation involves empiricism and approximations and its perception is to a greater or lesser extent subjective.  

IRIS has two COLOUR MODELS.  Access them by clicking "settings" on the menu followed by "Colour Model".  The options are "CIE" and "Bruton"

CIE
----

IRIS takes empirical CIE (Commission Internationale Eclairage - International Lighting Commission) 1931 tristimulus values for pure spectral components.  In solar illumination simulations these are weighted by Raymond Lee's solar radiance function and used for integrating the Mie scattering functions over visible wavelengths. Out of gamut colour components are approximated to in-gamut values by projective techniques. A Rec709 D65 monitor gamut is used and a 6500K white point. 

BRUTON
--------------
Dan Bruton of Stephen F. Austin State University developed an empirical algorithm to generate in gamut RGB components which represent pure spectral components. The algorithm produces very realistic spectra and is available at www.physics.sfasu.edu/astro/color.html .  



||Tools
Tools

Access tools via the main menu.

SCALES

 "Scale on/off" adds or removes a temporary scale.

"Add permanent scale" stamps the scale permanently onto the simulation in a bright colour.  Use when saving the simulations.


SUN/MOON DISK

Draws or removes a 0.5deg diameter disk on the simulation.


TUNE LAST SIMULATION 
ADD SKY BACKGROUND

Opens the simulation tuning panel.


||Ack
Acknowledgements

The Mie algorithm in IRIS is based on "BHMIE" by Craig Bohren and Donald R. Huffman and contained in "Absorption and Scattering of Light by Small Particles", Wiley-Interscience (ISBN 0-471-29340-7).  

Professor Raymond Lee kindly provided the solar spectral radiance values in IRIS and also rainbow chromaticities against which parts of the program could be checked.  His paper "Mie theory, Airy theory, and the natural rainbow," Applied Optics 37, 1506-1519 (1998) available at http://www.usna.edu/Users/oceano/raylee/RLL_cv.html was invaluable for insight and validation.

Water refractive indices are from IAPWS (International Association for the Properties of Water and Steam) and in turn based on P. Schiebener, J. Straub, J. M. H. L. Sengers, J. S. Gallagher, "Refractive index of water and steam as function of wavelength, temperature and density," J. Phys. Ch. R.,19, 677-717, (1990).

Dan Bruton of Stephen F. Austin State University provided the basis of one of the colour models.  His algorithm is available at www.physics.sfasu.edu/astro/color.html .

Michael Schroeder helped spark off the construction of IRIS way back in June 2000 with Airy algorithms which will go into later versions.  I'm sorry it took so long!

Philip Laven translated BHMIE into VB and our dialogue has provided invaluable stimulus and impetus during the development.


||Con
Contact

Contact me via the Atmospheric Optics website at http://www.sundog.clara.co.uk/atoptics/phenom.htm accessed by clicking the lower left button.

Comments, bug reports and suggestions for upgrades are all very welcome!  I would like to hear about how you use IRIS and about extra features that would be useful.

Les Cowley

||Finish
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