Numerical Aperture (NA) is “… a critical value that indicates the light acceptance angle, which in turn determines the light gathering power, the resolving power, and depth of field of the objective.”(1) As light passes through a sample, the information describing the highest resolution information in the sample is diffracted at a very wide angle. Low magnification lenses typically have low NAs, meaning that they cannot capture the highest resolution information. To capture the widely diffracted information, high NA lenses move the front of the lens closer to the sample (increases the light acceptance angle). Dry lenses can only have NAs of up to 1.0. By using specially formulated oil and oil lenses, NAs of up to 1.4 can be achieved.

Optical Coherence Tomographyppt

High magnification without high NA does not give the resolving power that most people expect from a research grade microscope.  Using a high NA objective lens means that you are most likely sacrificing working distance (how deep into the sample that you can focus) for higher optical resolution. In most instances this is a very acceptable trade off.

What can anOCTscan detect

The OCT uses a light beam which is not harmful to the eye. The light measures the thickness of the macula, retina and optic nerves. The state-of-the-art software used at Wills Eye Hospital allows us to create topographical maps of the eye and compare the data results to a normative database. Our physicians can read the maps and see if areas of the retina and optic nerve may be thinner or thicker than normal.

NOTE: During OCT tests the patient places his or her chin in a chinrest while pressing their forehead to the forehead rest for several minutes each eye. Blinking is allowed but the patient must remain as still as possible for the best and quickest images to be captured.

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Scientists who study the physics of light are experts in optics. The field of optics includes light's physical attributes and the way our eyes detect it.

OCTeye test results

Many of us have looked though the eyepiece of a department store microscope and seen a fuzzy looking “something” with the highest magnification objective lens. It’s not completely surprising that an inexpensive lens would give a blurry image. There are many optical aberrations that need to be corrected to manufacture the expensive lenses that are used on research grade microscopes.

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The UArizona Microscopy Alliance is a volunteer and collaborative effort to bring information about shared microscopy facilities to the University of Arizona and the community.

Reviewed & updated 06/16/2017. Creation of this web page was originally supported as part of the Southwest Environmental Health Sciences Center at the University of Arizona, NIEHS P30 ES006694.

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We respectfully acknowledge the University of Arizona is on the land and territories of Indigenous peoples. Today, Arizona is home to 22 federally recognized tribes, with Tucson being home to the O’odham and the Yaqui. Committed to diversity and inclusion, the University strives to build sustainable relationships with sovereign Native Nations and Indigenous communities through education offerings, partnerships, and community service.

Optical coherence tomographymachine

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The “cost” of obtaining a higher NA is that the working distance (WD) of the lens becomes much shorter. Working distance is “… the distance between the objective front lens and the top of the cover glass when the specimen is in focus. In most instances, the working distance of an objective decreases as magnification increases.” (1) A smaller working distance can be a problem when you cannot see an object with a high magnification lens, even though you could see it with a low magnification lens. A 10x objective can have a WD of several millimeters (4-10mm, or 4000-10,000um). A well corrected, high NA 20x dry objective will have a WD of slightly less than 1mm (1000um). Most well corrected, high NA 40x and 60x oil objectives have working distances on the order of 0.1mm (100um).

The OCT testing system has the ability to track the blood vessels of the eye of each patient. This tracking enables us to not only obtain the highest quality of imagery for each unique eye but also allows us to scan each eye in the exact same area as previous scans, within a degree of one micron, to track for any changes that have occurred.

This is a non-invasive test most often performed while a patient is dilated, although new software allows for patients to be undilated as needed.

Light microscopes can, under the best conditions, resolve objects that are approximately equal to half the size of the wavelength used. In the real world this comes out to objects that are 250-300nm in size, if you are using a NA=1.4 objective lens (under optimal conditions). This means that you can make out two adjacent objects in this size range, assuming that you can see at least a 25% dip in intensity between them (Rayleigh criterion). Sample preparation is especially important when you want to resolve structures this small.

The OCT test uses a light beam, which is not harmful to the eye, in order to scan and image the front of the eye including the cornea, anterior chamber, the angles in the anterior chamber and the iris. Some scans are also able to measure the thickness of the cornea and topographical map of the cornea.

A volunteer and collaborative effort to bring information about shared microscopy facilities to the University of Arizona and the community.

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