Gemlab.UK Gemmological Spectroscopy COLOUR FILTERSand Education
The use of colour filters in gem testing can be a simple and quick method to give an indication of possible identity or to eliminate any possibilities according to the reaction of the gemstone to a particular filter. Colour filters do not provide an immediate identification, only information which can help towards identification. Observations must be practiced over many samples and the results interpreted with care until a proficient method has been mastered.
A colour filter may be composed of coloured glass which, apart from being expensive, is rather limited in the selection of colours. Gelatine sheet or a gelatine film between glass cover plates has been used, but found to be subject to deterioration with colour loss and melting in excessive heat. Modern polycarbonate filters are available in many colours and more reliable with high intensity lighting as they often incorporate an additional heat shield lamination . A colour filter may also be a liquid in a glass container but they all function in a similar way in that they either absorb or reflect certain wavelengths of visible light and allow the residual wavelengths to be transmitted. This is what the human eye detects as colour. One thing of paramount importance is to use a light source which has a continuous spectrum. such as tungsten light. Standard fluorescent or tri-phosphor coated tubes and bulbs (Energy saving lamps) are not suitable, and neither are most "White" LED lamps. For constant results a standard light source must be maintained during observation as slight variations in the appearance of samples can occur according to the colour temperature. The illumination used when working with these filters may be an incandescent light bulb with a colour temperature rating between 2,700'K and 3,300`K such as a simple 100watt bulb.
Basic colour FILTERS
A coloured gemstone also acts as a colour filter as described above. Therefore when viewed in conjunction with another colour filer, the resulting body colour perceived by the eye is a combination of those wavelengths which have been TRANSMITTED by both FILTER AND GEMSTONE. This explains the changes seen in the basic body colour of some gemstones such as emerald, jadeite, ruby, spinel, aquamarine, and tanzanite when viewed through certain colour filters. To understand this we must compare the spectrum of the gemstone to that of the filter. As an example see the following illustrations concerning emerald below.
EMERALD AND "EMERALD FILTERS"
In the example shown above it can e seen that the Chelsea Filter will only allow the deep red to pass to the observer and so both emeralds will appear red or pink, depending on the depth of colour, when viewed through the filter. In the case of the Hanneman - Hodgkinson Synthetic Emerald Filter it allows two areas of the spectrum to pass, blue - green and orange - red. The balance of these colours is so critical that it will swing in favour of which emerald transmits more in either one or other of these wave bands. As most synthetic emeralds transmit red light strongly due to the high levels of chromium in their composition the balance shifts and they appear pink whereas the natural emerald remains green. However there are several exceptions to this rule in certain Russian hydrothermally grown synthetic emeralds which remain green. This can be resolved with the use of the Hanneman - Hodgkinson Synthetic Emerald Support Filter details of which are shown in the Colour Filter Section of the Database
Another "Emerald Filter" which provides useful body colour observations with the gemstones referred to above is the Hanneman PMCF (Poor Man's Colour Filter) The spectrum of this filter is illustrated and discussed in the Colour Filter section of the Spectra Database
Special COLOUR FILTERS
There is a group of colour filters which can be adapted to gem testing in other aspects apart from observing their body colour under standard tungsten illumination. These have various classifications such as Complementary Filters which contrast in colour to another filter or to the colour of a gemstone. They can be used to define Colour Distribution in gemstones or to improve observations in certain areas of the visible spectrum during Spectra Observation. Other filters may provide a pattern in their spectrum as a convenient means of comparison to gem spectra during such observations. Another use is to detect Fluorescence by the method often referred to as the "Crossed Filters Test" Conversion Filters are used in photography for colour correction between daylight and tungsten light and may be adapted to detect Colour Shift and Colour Change in gemstones such as the "Alexandrite Effect" A conversion filter with a small transmission band, often referred to as a Monochromatic Filter, is produced to pass a very limited band of wavelengths to try to provide a single colour. Some are almost monochromatic but others have too broad a transmission band to be considered as monochromatic. A yellow or orange type is available to simulate sodium light for use with Refractometer Readings.
colour distribution in gemstones can often be uneven in the form of straight or curved zones which may help to distinguish their natural or artificial origin. Where the colour is pale or of low saturation these zones may be difficult to detect, particularly when immersed in a liquid of similar refractive index as the stone, in order to cancel surface reflections and enhance internal features. To improve the observation of these colour zones a Complementary Filter may be used. For example, certain pale blue or purple filters will help to resolve the colour banding in yellow sapphire and pale citrine. In the case of red stones, such as a ruby or pink sapphire, a green or cyan filter can improve observations. This is of great benefit in capturing uneven colour distribution on a photographic image.
In order to resolve absorption lines in the deep red or deep blue - violet areas of the visible spectrum a strong light source may be necessary. Usually this can be difficult due to the unwanted transmission of the opposite end and the intense glare from the central yellow - green area. A Complementary Filter of an appropriate colour to enhance the area of the spectrum being examined will subdue the excessive transmission from the remainder of the spectrum and resolve the absorption lines more clearly. For example a suitable blue filter can enhance the line at 415nm. in diamond. A suitable red or orange filter will enhance the line at 691nm. in green jadeite where the necessity of an intense light source transmits a great deal of light from yellow to blue. This must be subdued if the line in the red is to be seen. I refer to a "Suitable filter" to remind readers that some filters transmit a much wider range of wavelengths than is suggested by their colour. Some colour filters can also be applied to spectra observation as an aid to calibrating the approximate position of some absorption lines in the absence of a built in scale within the spectroscope. An example here is to compare the spectrum of potassium permanganate to that of almandine garnet.
Certain gemstones will fluoresce in visible light usually due the presence of chromium in their composition. This can be detected with the use of two complimentary colour filters of a suitable colour which will absorb or transmit the wavelengths necessary to exhibit this fluorescence. This is often referred to as "The Crossed Filters Technique" but the term may be confused with using crossed polarising filters for the detection of double refraction and may be better referred to as "The Complementary Filters Technique"
The technique is to transmit a concentrated beam of white light, such as a tungsten spot light through a suitable blue filter an on to the gemstone. The light TRANSMITTED and INTERNALLY REFLECTED from the gemstone is then viewed through a red filter which may show a red response. This indicates that the blue light entering the gemstone has undergone a loss in energy and is emitted at a longer wavelength which is passed by the red filter to be seen as fluorescence. If the gemstone is inert the red filter simply absorbs the blue light from the first filter and the stone remains dark.
The choice of the blue filter is of paramount importance and should pass only a narrow wave band in the violet - blue area. Some may also pass a little green, which is permissible, but cobalt blue filters pass a considerable amount of red light and should not be used. Until the last few years the most suitable blue filter was a solution of copper sulphate. This is still useful if contained in a small round flat bottomed flask such as a pycnometer which will also act as a condensing lens as well as preventing the stone from overheating. Filters are now available which are more simple to handle such as the blue "Hanneman Chromium Fluorescence Filter" and the "Hanneman Red Filter for Crossed Filter Test" Also available are the blue "Exciter Filter" and the red "High Barrier Filter" produced by the lateDr. J. B Nelson.
Colour SHIFT AND COLOUR CHANGE
When daylight and tungsten light are available at the time of examining a gemstone while searching for any colour shift between these two types of illumination this is sufficient to detect any difference. However when only one is present, such as at night or in a room without a window, or in daylight with no light shield or tungsten lamp the situation may be resolved by using a pair of Conversion Filters to correct the colour temperature of the illumination to the required degree. Filter manufacturers produce a wide range of photographic colour correction filters to convert various types of lighting. The two which concern us here are a pale orange filter (Mired Shift = +131) to convert daylight illumination to 3200'K and a pale blue filter (Mired Shift = -131) to convert tungsten illumination to 5500'K. A filter containing these two separate components and also with both overlapped provides a centre section with a uniform transmission of 50% of all visible wavelengths (Mired Shift = 0) This was designed and produced by the late Dr. W. Hanneman as the "Alexandrite and Colour Shift Filter" which is described in the Colour Filter Section of the Database
If a sodium lamp with a very narrow transmission at 589nm. is not available, orange and yellow filters may be used to give an improved reading at the shadow edge on a refractometer scale by subduing the colours caused by dispersion when using white light. Although referred to as "Sodium filters" the transmission band may be quite broad and the effect is more like the yellow LED lamps available for this use. These filters may be fitted between the illumination source and the light inlet aperture at the back of the instrument or over the eyepiece. Some correction factor may be necessary depending on the depth of colour of the filter as readings may be .002 to .005 higher if the filter becomes a more reddish orange. Birefringence calculations remain unaffected.
It must be remembered that a tungsten spot light must not be placed too close to a filter, especially if is one of the earlier gelatine type, otherwise the filter may be damaged.
COLOUR FILTER SECTION OF DATABASE
The purpose of this is to make filter spectra available for comparison to the gemstone spectra. In that way the viewer may see which colours are transmitted to the eye and understand which areas in the combined spectra form the final colour. However three very important points must be made here regarding any spectrum.
1) Commercially produced filters may occasionally incur slight modification by the manufacturers, therefore the spectra shown on the database should be considered as "Typical" of those filters at the time of observation.
2) Some filters used in gem testing may consist of several overlapping components. You may be surprised therefore to discover those colours which are absorbed and those that are transmitted in the spectrum of these filters.
3) The human eye and brain cannot predetermine what exact colour will result from a visible spectrum or vice versa.
To view the filters and their spectra go to COLOUR FILTERS in the right hand column in the DATABASE
Each filter and it's spectrum is described in detail. For information on their use and the colour responses of particular gemstones consult the instructions which usually accompanies the filters from the supplier.
Many filters listed in the COLOUR FILTER DATABASE and others used for gemmological purposes were supplied by Hanneman Gemological Instruments in the U.S.A.
I am grateful for the collaboration with the late Dr. W. Hanneman and Alan Hodgkinson and their permission to photograph the filters and their absorption spectra.
Copyright John S. Harris - from the year 2000 to the current date.