Filters
Filters are optically effective elements which allow selective transmission. Only part of the incident beam is transmitted; consequently, either coloured light is produced or invisible beam components (ultraviolet, infrared) are filtered out. Filter effects can be attained using selective absorption or using interference. The filters' permeability to light is known astransmittance.
Types of filters
Absorption filter
Absorption filters absorb certain spectral ranges and transmit the remaining radiation. The absorption process causes the filters to become hot. The separation of transmitted and reflected spectral components is not as exact as with interference filters and leads to a reduced edge steepness of the transmittance. Consequently, coloured glass filters create rather unsaturated colours. They have great longevity however.
Reflection filter
Interference filters (edge filters) are classed as reflection filters and give a high transmittance and an exact separation of transmitted and reflected spectral components. Glass filters coated with an interference coating can produce saturated colours. An accumulation of heat is avoided since reflection, and not absorption, takes place. The reflection spectrum is dependent on the angle of observation. Due to the vaporised coating, their scuff resistance is less than that of absorption filters.
Colour filters
Properties
Colour filters only transmit a certain part of the coloured, visible spectrum, whereby the remaining components of the radiation are filtered out. Colour filters made of plastic film are not heat resistant. Conversely, heat is not so critical for glass filters and, to an extent, they are resistant to temperature change. Absorption filters made of coloured glass attain lower colour saturation compared to interference filters. The colour-filtering property of interference colour filters is not immediately apparent - they do not look coloured.
Applications
In architectural lighting too, colours from the daylight spectrum are felt to be natural: magenta (conditions of light at sunset), amber (atmospheric light at sunrise), night-blue (clear night sky) and sky-blue (light of the sky by day). In scenic lighting, all colours of light come into play for highlighting and forming contrasts. In practice, when illuminating coloured surfaces, it is recommendable to perform lighting tests.
Corrective filters
Properties
Corrective filters designed as conversion filters will increase or reduce the colour temperature of the light source due to the spectral progression of the transmission. Skintone filters only correct the lamp's light spectrum in the green and yellow spectral range and thereby produce a very natural and pleasant effect on skin tones. Daylight-conversion filters transform the warm white colour temperature in the range of the neutral white colour of light, i.e. from 3000K to 4000K.
Applications
Skintone
Skintone filters are colour filters which improve the effect of natural warm colours, especially the colours of the skin. It is beneficial to use Skintone filters in communication areas, such as those of restaurants or cafés.
Daylight
Conversion filters are used to adapt the warm white light colour from halogen lamps to daylight lighting. Furthermore, by using daylight-conversion filters in warm white illuminated areas, it is also possible to create zones with neutral white light atmosphere.
Protective filters
Properties
UV filter
UV filters are suitable for completely blocking ultraviolet radiation while allowing optimal transmission of visible light. The separation between reflexion and transmission takes place at 400 nm. The steeper the edge of the transmission curve, the less the will be the colour distortion in the visible spectrum. UV filters are transparent (clear), the transmission is directional.
IR filter
Infrared filters absorb or reflect the thermal radiation above 800 nm while allowing optimal transmission of visible light spectrum. The thermal load on objects is reduced to a minimum. IR filters are transparent (clear), the transmission is directional. Adequate seperation between lamp and filter avoids a build-up of heat within the luminaire.
Applications
UV filter
Filtering out virtually all the ultraviolet radiation effectively delays the photochemical process of decay in textiles, watercolours, historic documents, artworks and other exhibits that are sensitive to light. This particularly applies to the bleaching of colours and to yellowing. In practice, since the UV component of high-pressure discharge lamps is already reduced by prescribed safety glasses, the highest ultraviolet loading is found from non-capsulated tungsten halogen lamps.
UV filters are suitable for use in:
- art museums
- art galleries
- natural-science museums
- antiquarian bookshops
IR filter
The use of infrared filters significantly reduces the thermal load and thus decreases the heat on an object or its surface. Materials sensitive to heat and humidity can thus be protected from drying out or distorting. High proportions of infrared radiation are emitted predominantly from light sources with low luminous efficacy, such as thermal radiators.
IR filters are suitable for use in:
- art museums
- art galleries
- natural-science museums
- antiquarian bookshops
- food shops
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