The spectral bandwidth of the OCT light source is indirectly proportional to the axial (depth) resolution of the imaging system. Therefore, broadband light sources are used to provide high axial resolution.

Thorlabs currently offers OCT systems that operate with a center wavelength of either 900 nm, 930 nm or 1300 nm. The center wavelength contributes to the actual imaging depth and resolution of the system. Shorter wavelength OCT systems, such as our 930 nm or 900 nm systems, are ideal for higher resolution imaging compared to systems with a center wavelength of 1300 nm. For imaging samples that have higher optical scattering properties, such as tissue, the longer wavelength systems are recommended. The longer center wavelength is not affected by scattering, and therefore, the light is able to penetrate deeper into the sample and return for detection.

Suppose that a stage micrometer scale has divisions that are equal to 0.1 mm, which is 100 micrometers (µm). Suppose that the scale is lined up with the ocular scale, and at 100x it is observed that each micrometer division covers the same distance as 10 ocular divisions. Then one ocular division (smallest increment on the scale) = 10 µm at 100 power. The conversion to other magnifications is accomplished by factoring in the difference in magnification. In the example, the calibration would be 25 µm at 40x, 2.5 µm at 400x, and 1 µm at 1000x.

The length (L) and width (W) of the FOV are limited by the scan lens properties. Most of our OCT systems have a 10 mm x 10 mm (L x W) FOV while the GANYMEDE-II-HR offers 6 mm x 6 mm (L x W) FOV in the default configuration. The maximum depth (D) attainable is set by the design of the OCT system. The graphic to the right shows variation in depth among all of our OCT systems. However, the actual imaging depth will typically depend on the optical properties of the sample. Our standard OCT systems are designed to provide an optimized balance between imaging depth and axial resolution. For applications requiring greater depth or higher resolution, we offer custom configurations.

The sensitivity of an OCT system describes the largest permissible signal attenuation within a sample that can still be distinguished from the noise. In practice, higher sensitivity OCT systems are capable of providing higher contrast images. Since the sensitivity of an OCT system can be increased by increasing the integration time, there is usually a tradeoff between A-scan rate and sensitivity.

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Your microscope may be equipped with a scale (called a reticule) that is built into one eyepiece. The reticule can be used to measure any planar dimension in a microscope field since the ocular can be turned in any direction and the object of interest can be repositioned with the stage manipulators. To measure the length of an object note the number of ocular divisions spanned by the object. Then multiply by the conversion factor for the magnification used. The conversion factor is different at each magnification. Therefore, when using a reticule for the first time, it is necessary to calibrate the scale by focusing on a second micrometer scale (a stage micrometer) placed directly on the stage.

OCT Cross-Sectional Image of a Human Finger. Layers of Skin: E-Epidermis; D-Dermis; BV-Blood Vessels. Image Size: 4.9 mm x 2.6 mm. Image Taken with a Telesto Series OCT System.

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For Spectral-Domain OCT systems, the A-Scan rate is determined by the speed of the camera in the detection spectrometer. For Swept-Source OCT systems, the A-Scan rate is determined by the sweep speed of the swept laser source. There is a tradeoff between A-Scan rate and the sensitivity of an OCT system: higher A-Scan rate results in lower sensitivity.

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Imec provides hyperspectral imaging with filters on chip. This eliminates the complex and delicate optical design of traditional hyperspectral systems.

Optical coherence tomography

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Thorlabs offers a wide variety of Optical Coherence Tomography (OCT) imaging systems. We recognize each imaging application has their specific needs. With the growing number of OCT systems available, it can be challenging to decide which system best meets your needs. Below we have put together a Selection Guide that outlines a few key technical specifications of each of our systems as well as some tips on how to choose the best OCT system for your application.

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A specimen such as Chaos (Pelomyxa) carolinensis represents a real challenge. Ameoboid organisms are irregularly shaped most of the time. Is it flat on the slide, or does it extend up toward the coverslip? Perhaps it is attached to both. What model do you use as a basis for volume estimation? Is it best to assume a particular shape and take measurements at different times? Is it best to estimate a maximum and minimum for each possible dimension and obtain a range of possible volumes? Remember, you are only asked to estimate. Sometimes the best estimates have a potential error of more than an order of magnitude.

As with general microscopy principles, the lateral resolution is dependent on the focusing objective in the imaging probe. All of Thorlabs’ OCT systems come with our specially designed OCT scan lens which provides telecentric scans across the entire field of view.

Identify the ocular micrometer. A typical scale consists of 50 - 100 divisions. You may have to adjust the focus of your eyepiece in order to make the scale as sharp as possible. If you do that, also adjust the other eyepiece to match the focus. Any ocular scale must be calibrated, using a device called a stage micrometer. A stage micrometer is simply a microscope slide with a scale etched on the surface. A typical micrometer scale is 2 mm long and at least part of it should be etched with divisions of 0.01 mm (10 µm).

Be aware that even under the best of circumstances the limit of resolution of your microscope is 1 or 2 µm (or worse) at any dry magnification, and 0.5 µm or so using oil immersion. No directly measured linear dimension or value that is calculated from a linear dimension should be reported with implied accuracy that is better than that. That includes means, surface areas, volumes, and any other derived values. For example, suppose you measure the length of a flagellum on a Chlamydomonas cell at 400x, and determine that it covered 3 1/2 ocular divisions. The length is directly calculated as 3.5 divisions times 2.5 µm per division, which comes out to 8.75 µm. You know, however, that at 400x the absolute best you can do is to estimate to the nearest µm, so before reporting this measurement round it to 9 micrometers (not 9.0, which would imply an accuracy to the nearest 0.1 µm). For more information on reporting uncertain quantities see our Resources section (analytical resources).

Some stage micrometers are finely divided only at one end. These are particularly useful for determining the diameter of a microscope field. One of the larger divisions is positioned at one edge of the field of view, so that the fine part of the scale ovelaps the opposite side. The field diameter can then be determined to the maximum available precision.

In many areas of experimental science, including biosciences, the ability to estimate and make reasonable assumptions is a valuable skill. In order to make some quantitative estimates, particularly of volumes, you will have to make assumptions regarding the shape of some organisms. For example, if a specimen appears round, you would likely make your volume calculation based on the assumption that the specimen is a perfect sphere. For something like a Paramecium you might assume a cylindrical shape in order to simplify your estimate, while remaining aware that you could be way off the mark.

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In OCT, the axial (depth) and lateral resolutions are dependent on different factors. The axial resolution of the OCT system is proportionally dependent on the center wavelength of the source and inversely proportional to the source bandwidth. In practice, the axial resolution is also improved by the index of refraction of the sample. For example, the axial resolution of the CALLISTO OCT system is 7 ?m in air or 5.3 ?m in water-rich samples such as tissue (n=1.35).

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Thorlabs offers a wide variety of OCT Imaging Systems. To assist in narrowing down which OCT system(s) is best suited for your application, we have provided the guide below. We always encourage all customers to contact us to discuss specific imaging requirements.

The calculation of a volume is subject to error propagation, namely the magnification of an error when deriving a figure from one or more measured variables. For example, suppose you measure the length and diameter of an object to be 65 and 30 micrometers, respectively, assuming a cylindrical shape. The volume is given by the formula v = ¼r2l, where r = radius and l = length. The formula gives a volume of 45, 946 µm3. The volume isn't accurate to the nearest cubic micrometer, however.

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Let's make the very optimistic assumption that the measurement of 65 micrometers is indeed accurate to the nearest 1 µm. Then the number 65 means "greater than 64.5 and less than 65.5." The number 30 really means "greater than or equal to 29.5 and less than or equal to 30.5." The smaller set of measurements yields a volume of 44,085 µm3, while the larger yields a volume of 47,855 µm3. False precision would be implied even if one reported a volume of 46,000 µm3, obtained by rounding the middle measurement. It would probably be better to report a range in this case, of 44,000 to 48,000 µm3. By the way, 46,000µm3 is 0.046 mm3, which probably represents a better choice of units in this case.

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What is the refractive index? · Mid Index = 1.54–1.63 · High Index = 1.64–1.73 · Ultra High Index = 1.74+.

A single depth profile (Intensity vs Depth) is called an A-Scan. A B-Scan, or two-dimensional cross-sectional image, is created by laterally scanning the OCT beam and collecting sequential A-scans. The speed with which a B-scan is collected depends on the A-Scan or Line rate.