Light Emitting Diodes
Introduction
Light emitting diodes are used as light sources for most Light Box photometers. For many uses a white light LED is appropriate. On the other hand, for photometers that will be used repeatedly for common assays it is useful to have a dedicated light source. The table below lists several assays that are often performed in laboratories and that have dedicated photometers.
| Procedure |
Wavelength |
Type |
| Protein Assay |
595 nm, orange |
Absorbance |
| Beer color assay, for brewers |
430 nm, blue |
Absorbance |
| DNA Assay, Hoechst dye |
362 nm, UV |
Fluorescence |
| Turbidity |
860 or 940 nm, infrared |
Light scatter |
| E. coli |
420 nm |
Absorbance |
Table 1. Several procedures that require specific wavelengths of light. Absorbance assays have "straight through" geometry (LED and detector in straight line), whereas the fluorescence and turbidity assays have right angle geometry (detector at right angles to light source).
There are literally thousands of LEDs to choose from, in many colors and intensities. The wavelengths of light produced by LEDs include ultraviolet to the infrared and every color of the spectrum in between.
Figure 1. LEDs come in many colors. This selection contains red, orange, yellow, green, green-blue, blue, violet and ultraviolet. The ultraviolet LED also has some visible light.
The color of the LED light is chosen based on what is being measured. As mentioned in the page about complementary colors in the Science section, the light used to test for a substance in an absorbance assay should be near the peak absorbance wavelength. This maximizes the light absorption signal.
As an alternative to colored light from an LED, colored filters can be used with a white light source. Neutral density filters can be used to generate a standard curve with known optical densities.
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Terms and definitions
LED - Light emitting diode
nm - nanometer (1/1,000,000,000 meter), used to measure the wavelength of light.
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