1. الضوء والإشعاع

Spectral analysis: Colour glass filters    (1/4)

Photographers use colour glass filter for producing colour effects, or to avoid unwanted components of daylight such as ultraviolet which can cause excessive bluish cast in outdoor photography.

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Farbfilter
Colour glas filters for use with camera lenses.

Spectral measurements with colour glass filters are also commonly done in the laboratory. The image below shows a filter wheel equipped with several filters. By rotating the wheel, each filter can be positioned to the path of rays which is at the bottom of the wheel. The image on the right shows the same filter wheel in a backlight photo.

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filter wheel
A filter wheel with five optical filters for spectral measurements in the laboratory.

Colour glass filters allow for an easy selection of the desired spectral ranges in radiation measurements.

 

However, each measured wavelength requires a suitable filter, so this method is only practical for analysing not more than 10 spectral bands. Such instruments with more or less this number of spectral bands are said to be multispectral, while hyperspectral sensors measure data at 64, 128 or 256 different wavelengths. Instruments which measure luminosity in a broader spectral range and display their data as grey levels, e.g., in black-and-white images, are said to be panchromatic.

One disadvantage of using colour glass filters is that selected spectral bands can only be modified by changing the filters. Instruments used in satellites do not allow such a filter change. Much care is therefore taken during the planning phase of a satellite instrument in order to define the best suitable wavelengths for each particular mission.

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Filterrad
Backlighted photo of the filter wheel, with the following SCHOTT colour glass filters: RG1000: Infrared-transmitting black glass, transmits wavelengths of 1000 nm and above. KG 1: Heat absorbing glass, transmits in the visible and absorbs wavelengths above 750 nm. BG 12: Blue-violet glass, transmits wavelenghts between 400 (violet) and 480 nm (blue), and absorbs green, yellow and red light. VG 9: Green glass, transmits wavelengths of approx. 490 nm (light blue) through green to 560 nm (yellow). IF 430/10 is an interference filter, transmits at 430 nm (blue) with 10 nm bandwidth. Interference filters are discussed in a following section.

The table below indicates the spectral bands of the Thematic Mapper (TM) abord NASA's satellite Landsat 7 and of the SPOT 5 multispectral instrument of SPOT IMAGE. You may notice that the spectral bands of the two satellites which were both conceptualised for the remote sensing of land surfaces are very much alike.

TM SPOT 5 Spectral range Signal type
0.45-0.52 μm   blue-green reflective
0.52-0.60 μm 0.50-0.59 μm green-yellow reflective
0.63-0.69 μm 0.61-0.68 μm red reflective
0.76-0.90 μm 0.78-0.89 μm near IR reflective
1.55-1.75 μm 1.58-1.75 μm mid IR reflective
10.40-12.50 μm   thermal IR emissive
2.08-2.35 μm   mid IR reflective
0,59-0,90 μm 0,48-0,71 μm panchromatic reflective

Under signal type, reflective refers to reflected sun radiation and emissive refers to thermal emission of the Earth.