Science behind the art
Light
Light is energy that travels in wave form. The human eye responds to certain wavelengths and these make up the visible spectrum. Wavelengths outside this spectrum are invisible to us, such as infra red, ultra violet and X-ray.
Isaac Newton showed that by shining white light through a glass prism it could be separated back into its different wavelengths.
Each wavelength within the visible spectrum is recognised by our eyes as providing a particular colour sensation, the diagram below clearly indicates the visible colours and their corresponding wavelengths. White light consists of all of the visible wavelengths, present in equal amounts.
By using filters to selectively reduce the level of light at certain wavelengths we can create coloured light to meet our individual requirements, whether technical or aesthetic.
Most artificial light sources do not actually produce white light. For example, incandescent sources such as tungsten generate light which has more energy at the red end of the spectrum, whereas a fluorescent source often has spikes of energy mainly in the blue and green region. Filters can be used to correct these differences and make one light source appear like another.
In order to record and communicate colour accurately, you either need to create a physical example of that colour that will never fade or become damaged, or use a mathematical model. A model uses numbers to describe different attributes of a certain colour, these being HUE, SATURATION and LIGHTNESS. The HUE describes the physical colour - red, yellow, green etc. SATURATION is a perception of how strong the hue of the colour is represented in the sample. The lightness (or darkness) of a colour is perceived, when a comparison made to a similar area that is not coloured, but lit with the same strength of illumination.
As there are three attributes to a colour, the numbers associated with them in a mathematical model can be thought of as a position in a three dimensional shape, this shape is called a colour space.
The particular colour space used by LEE Filters technicians was devised in 1931 by the Commision International Eclairage (CIE ) and is one of the many internationally recognised standard colour spaces.
The HUE and SATURATION of any colour can be represented by its position on a chromaticity diagram, as seen below. The diagram contains all visible colours, and all possible densities of these colours, in a two dimensional configuration. Pale colours in the centre and saturated versions of those same colours at the edges. A colour’s position on this diagram will be represented by its Chromaticity Co-ordinates.
How to use this website
The technical information contained on this website is designed to help you choose the correct colour for your requirements in a number of different ways.
The spectral power distribution ( SPD ) curves illustrated, show the percentage of light at each wavelength across the visible spectrum that is passed when light is shone through the filter. From this data you can tell which constituent parts of the source will be transmitted, and which will be reduced.
The Y% figure is representative of overall average transmission of that filter, as perceived by the human eye. The Y value is actually one of the TRISTIMULUS VALUES, a set of values unique to each colour, that are calculated mathematically from the data contained in the SPD graph.
The absorption (abs) of a filter is calculated from the Y% value, and is another way of expressing the light stopping properties of that filter. Abs is a linear scale, so values can be added or subtracted more easily than using Y%.
The Chromaticity co-ordinates published for each colour are measured and calculated using a theoretical standard light source, and can be plotted on the chromaticity diagram to establish that particular colour’s characteristics in relation to all other colours.
Choosing filter materials
Since subtractive filters achieve their purpose by absorbing energy, knowing the expected spectral performance of a particular filter and in particular, its overall Transmission Efficiency Y, can help the user to select the materials used, whether being polyester, high temperature polymer or glass. Each material has recommended temperature limits, and our staff are always happy to advise on the best material for a particular job, and on its durability. The lifetime that may be expected from a particular filter in a particular application can often be difficult to predict, because it depends upon many different factors. We have many years of experience in lots of different areas, and our staff will readily offer the practical knowledge that they have gained as to how to prolong the lifetime of any particular filter.