Rich information regarding how darkfield works, condenser and objective selection, installation and adjustment.
"This video focuses on darkfield illumination. For this video I will be using a T490 microscope. Darkfield illumination is a lighting method used to increase contrast in low contrast samples. It's particularly useful for highlighting specific sample material while other methods such as page contrast are better for viewing interior detail. At the heart of darkfield illumination is a darkfield condenser. It works similarly to a brightfield condenser except it includes an opaque disk or stop which blocks direct light from entering the objective lens. Instead only oblique lighting which is refracted by the sample will enter. This results in certain objects being illuminated while the background remains dark. When choosing a darkfield condenser be sure the condensers numerical aperture is larger than the numerical aperture of the objective lens you plan to use. If the objective lenses aperture is larger than the condensers direct light will be transmitted and a dark field will not be achieved. At the dry condensers upper limit this image shows noticeable glare well above the contentious limit. The dark field is completely lost by reducing the aperture of the lens. The dark field may be restored there are two classes of condenser dry and immersion. a dry condenser is designed to work with lower magnification objectives with lower numerical apertures. This typically covers objectives up to 40 times with apertures of around 0.65 or less. An oil immersion condenser is needed for higher magnification lenses such as a 100 times. Due to the high numerical aperture of these lenses typically 1.25 or higher a condenser of correspondingly high aperture is needed especially to maintain high resolution while this combination will produce an image with improved contrast it may not be truly darkfield to the objective lenses high aperture as an option a variable aperture lens may be used to fine tune the dark field by closing a built-in iris the numerical aperture of the objective can be reduced until the field is adequately dark replacing the condenser is a simple process so condenser can be raised or lowered by using the adjustment knob while supporting the installed condenser loosen the locking screw until condenser can easily slip out next insert the darkfield condenser fully then securely tightened the locking screw a darkfield condenser should be centered for optimal results although the procedure is similar for dry and oil condensers. There are some minor differences for the dry condenser use a low powered objective preferably a 10 times by removing an eyepiece and image of the condenser lens can be seen. Adjust the height of the condenser and the stage if necessary until the opaque disk feels slightly less than the entire image circle leaving a ring of light. Now use the two centering screws to move the disk until it appears centered within the ring of light for the oil condenser use a high power objective preferably a 40 times by raising the condenser a dark circle should be visible. Now use the two centering screws to move the circle until it appears centered. For best results once the condenser is centered it should be raised until it makes contact with the slide and the lamp should be turned up to maximum brightness when working at high magnification you'll likely be using an oil objective lens and an oil condenser. Before using an oil condenser apply a drop of oil to the condenser lens. Once the condenser is installed raise it until the oil makes contact with the bottom of the slide. If using an oil objective lens apply a drop of oil to the coverslip then gently raise the stage until the oil makes contact with the lens now you can fine tune the focus. If using a variable aperture lens adjust the iris to fine tune the contrast."
Find our T490 Series Microscopes and Darkfield Condensers on Amazon:
T490B-DK (Dry Condenser): goo.gl/gYPzUa
T490B-DKO (Oil Condenser): goo.gl/Tsgt5T
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