SOP: Selection of Filters for Colorimetric Measurements
GIFSQ — Global Institute for Food Safety & Quality
Principle
Optical filters are used in colorimeters (absorptiometers) for isolating a desired spectral region. They consist of coloured glass or coloured gelatin coated on glass and possess the property of selectively transmitting light from a specified region of the spectrum, the other regions being absorbed. The ideal way of selecting a filter for use with a coloured solution is to construct first, with a suitable spectrophotometer, the absorption curve for the visible spectrum, and to compare this curve with the spectral transmission curves of the set of filters supplied by the manufacturers; the best filter to use in a particular determination is the one which gives the maximum absorption, or minimum transmission, for a given concentration of the absorbing substance.
Apparatus required
- Spectrophotometer, for constructing the absorption curve of the coloured solution over the visible spectrum
- Photoelectric colorimeter, with the set of filters to be compared (alternative procedure)
- Set of coloured-glass or gelatin filters, such as the Kodak, Ilford or Corning series; interference filters (transmission type) where narrower transmitted bands are required
Procedure
Construct the absorption curve of the coloured solution over the visible spectrum by means of a suitable spectrophotometer
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Compare this curve with the spectral transmission curves of the set of filters supplied by the manufacturers (Ilford spectrum filters Nos. 601–609 and bright spectrum filters Nos. 621–626 are given below as reference)
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Choose the filter which gives the maximum absorption, or minimum transmission, for the given concentration of the absorbing substance; alternatively, construct optical-density (or transmittance) against concentration calibration curves with a photoelectric colorimeter, using each of the filters in turn, and apply the same criterion
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Where a spectrophotometer is not available, less satisfactory choices are a filter that gives the smallest transmission for a given concentration and depth of cell, or a filter whose colour is as close as possible to the complementary colour of the solution (see the complementary-colour guide below)
Reference tables
Choice of filter colour for a given colour of solution
| Colour of solution | Colour of filter |
|---|---|
| Purple | Green |
| Orange-red | Blue-green |
| Yellow | Blue |
| Violet-red | Purple |
| Blue | Red |
Ilford spectrum filters (Nos. 601–609)
| Number | Colour of filter | Transmission region (nm) | Peak wavelength (nm) |
|---|---|---|---|
| 601 | Spectrum violet | 380–470 | 405 |
| 602 | Spectrum blue | 440–490 | 425 |
| 603 | Spectrum blue-green | 470–520 | 47—* |
| 604 | Spectrum green | 500–540 | 490* |
| 605 | Spectrum yellow-green | 530–570 | 520* |
| 606 | Spectrum yellow | 560–610 | 586 |
| 607 | Spectrum orange | 570 with absorption increasing | 600 |
| 608 | Spectrum red | 620 into infrared | 660 |
| 609 | Spectrum deep red | ~650 into infrared | 690 |
* The printed peak wavelength is illegible or falls outside the stated transmission region in the available copy (“47°”, 490 against 500–540, 520 against 530–570); these values must be checked against a clean print.
Ilford bright spectrum filters (Nos. 621–626)
| Number | Colour of filter | Transmission region (nm) | Peak transmission (nm) |
|---|---|---|---|
| 621 | Bright spectrum violet | 340–515 | 450 |
| 622 | Bright spectrum blue | 375–530 | 470 |
| 623 | Bright spectrum blue-green | 460–545 | 490 |
| 624 | Bright spectrum green | 499–575 | 520 |
| 625 | Bright spectrum yellow-green | 510–590 | 540 |
| 626 | Bright spectrum yellow | 545–620 | 570 |
Note: the bright spectrum filters have a higher transmission (are considerably brighter) than the standard spectrum filters, but with a slightly wider transmission range.
Precautions
- Interference filters (transmission type) have somewhat narrower transmitted bands than coloured filters and are available commercially; they are composed of two highly reflecting but partially transmitting films of metal (usually silver) separated by a spacer film of transparent material, and the amount of separation of the metal films governs the wavelength position of the pass band.
- Light which is not transmitted by an interference filter is for the most part reflected.
- The wavelength region covered by interference filters is from 330 to 1,200 nm; the nominal peak transmittance is 45% (the available copy reads “459%”, which is garbled — check against a clean print) and the half-width is less than 20 nm. The half-width is the spectral width of the pass band in millimicrons (nm) at the level where the transmittance is one-half the peak transmittance.
- The transmission ranges in the tables above are those prescribed by the manufacturers and are given as printed in the book.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
Judd, D.B. & G. Wyszecki, Colour in Business, Science and Industry, 2nd edn., John Wiley & Sons, Inc., New York (1963) — cited in the book.