Science students commonly observe microscope slides through a clear lens and are often unaware of the purpose of the blue filter that accompanies even the most inexpensive microscopes. This blue piece is a critical item to any scientist working in typical microscopy applications who needs to enhance microscope lighting for increased resolution and image clarity.
When attempting to increase resolution, decreasing the wavelength of light used in microscopic observation has the greatest effect on resolution, compared to other common methods of changing the angle of incidence or refractive index. Since the shortest visible light wavelength is blue, all microscopes now include a blue filter in their basic design. The least expensive microscopes are accompanied by a thicker and darker filter, while higher quality lenses enhance the quality of the light to compensate for lens aberration due to color filtration.
In low light levels, it is advisable to insert a blue filter into the brightfield condenser for observing samples such as dry layer of blood. Typically, in these conditions, the lamp creates a yellowing effect on the slide, which can impair observations and data interpretation. Due to their short wavelength, blue filters alter the effect of the lamp, bringing the light closer to a natural white and projecting a true image of the sample.
Simple magnification without increasing detail is useless in revealing the structure of small objects. Filtering light through a single wavelength increases the numerical aperture, which produces an image that is both magnified and distinct. Blue filters achieve a resolving power of 200 nanometers, which is the maximum resolution at which two objects can be observed simultaneously without appearing fuzzy or as a single object.
Blue filters are typically used for brightfield microscopy; however, one darkfield illumination technique employs the use of blue filters inserted before the condenser in order to color light rays of high aperture with a different tint than light rays at low aperture. This increases contrast between a specimen and its background. It is important to remember that this method is affected by low apparent resolution and is often afflicted by low light intensity in the final image of certain samples.