Köhler illumination

Köhler illumination is a type of specimen illumination used in transmitted-light microscopy. It was designed by August Köhler in 1893, and overcame limitations of previous types of sample illumination techniques (ie: critical illumination). Prior to the advent of Köhler illumination the filament of the bulb used to illuminate the sample could be visible in the sample plane. This created what is known as a filament image. Various techniques were used to overcome the filament image such as lowering the power of the light source, utilizing an opal bulb, or placing an opal glass diffuser in front of the light source. However, all these techniques, while being semi effective in reducing the filament image, had the effect of reducing the quality and uniformity of light reaching the sample. Reducing the power of the light source and utilizing an opal bulb both caused a narrowing of the spectrum of incident light. For transmitted-light microscopy wide spectrum white light is desired to realize the maximum amount of contrast. Further, adding an opal glass diffuser will cause the light reaching the sample to not be uniform. Uniformity of light is essential to avoid shadows, glare, and incorrect contrast when taking images.

Principals

Condenser Optical Components

• High intensity bulb
• Collector lens
• Field diaphragm
• Condenser diaphragm
• Condenser lens

Setting Up Köhler Illumination

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