Where ‘θ’ is half of the angle of the cone of light which is collected by the front lens lens (i.e., the angular aperture).

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Numericalapertureunit

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Numerical aperture is proportional to refractive index. For example, air has a refractive index of 1.00, water has a refractive index of 1.33, whereas many of the immersion oils have refractive indexes around 1.52.

The NA of an objective is an important aspect as it relates to the final image formation seen when looking down through the eyepieces (which will be covered in full in a forthcoming blog article). Briefly, resolution relates to the amount of detail which can be seen in the final formation of an image. An objective with a high magnification would be unable to resolve detail in your sample without a similarly high NA.

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Eclipse provides custom shading products through a qualified network of local, independent dealers. They can provide you information on pricing, ordering and installation. Information provided by this search is for informational purposes only. Dealers are independently owned and operated; consequently, Eclipse assumes no responsibility for their actions.

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Numericalapertureformula with refractive index

Metaphorically, think of it this way: if you are standing in front of a door with a key hole which leads into another room, then when you are at a distance, you will only be able to see a little of the light and objects within the room. If you press your eye against the key hole, you will then see more of the detail and light in the room as you have, in theory, increased the angular aperture of your eye.

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Where ‘n’ is the refractive index of the medium between the cover glass and the objective front lens (e.g. air, water or oil).

Numericalapertureof lens

Refraction is described in a formula known as ‘Snell’s Law’. Refraction was first described in the year 984 by a Persian physicist and mathematician called Ibn Sahl. In 984, he presented a manuscript in which he described how mirrors and curved lenses focused and bent light. Snell’s Law is actually named after a Dutch mathematician and astronomer called Willebrord Snellius (1580-1626). Although he was credited for mathematically describing refraction, it is more accurate to say that he ‘rediscovered’ diffraction after the work of Ibn Sahl.

In summary, without a correspondingly high NA, a high magnification objective will have low resolution. Most microscope companies offer objectives which have high NA values for use with immersion medium. If you are in the lucky position of buying a custom microscope, or buying new objectives for your existing instrument, you should always consider buying objectives which offer the highest NA value which you can afford.

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To simplify this further, as light travels through one medium to another, it changes speed (e.g. when passing from air to water, light slows down). When light passes across the boundary of two different medium at an angle other than 900, this results in a change of direction. Although the frequency of light doesn’t change, the resultant wavelength will be determined by nature of the medium.

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Numericalapertureformula

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Numericalapertureof objective lens

Therefore, the refractive index of air is a limiting factor in achieving the highest possible NA of an objective. As a result, objectives with NA values greater than one are the immersion objectives where the air gap is replaced by a medium such as water or oil. An angular aperture of 1800 is physically unachievable- the widest angle of light which can be collected by an objective is around 1440. Consequently, the maximum achievable NA of a non-immersion objective is approximately 0.95 (which is equal to the sine of 72).

Snell’s Law describes the relationship between the angles of incidence and refraction of light as it travels through the boundary of two different medium (e.g. from air to glass). The Law states that the ratio of the sines of the angles of incident and refracted light are equivalent to the reciprocal of the ratio of the refractive indices through which the light passes.

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The NA of an objective is the simply the ability of the lenses to collect light at a fixed distance from the sample which you are viewing. When light passes through and leaves a specimen, it enters the front lens of an objective as an inverted cone. However, a percentage of this image-forming light is refracted and reflected. Objectives which have a high NA allow for increasingly oblique light waves to be collected by the front lens which will in turn form a final image which is not only relatively brighter, but contains more information and detail and is highly resolved.

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I recently purchased a 28×16-6 Eclipse Shading awning with Sunbrella Fabric. Every other awning dealer I checked could not supply me with an awning this size. Although this was not the least expensive awning i researched: as advertised it is one beefy, strong, bad ass awning. The lunar lighting package is awesome!!! If you want Quality when you invest this is the awning. I could not be more pleased with my decision.

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The Retractable Awning I purchased from Eclipse last year was one of the best investments I have made in my home. I shopped for two summers before finally pulling the trigger. The awning selection out there really varies in quality, from strong durable awnings like the Eclipse Semi-Cassette Awning I got, to other questionable models offered by other manufacturers. Be sure when you shop for an awning, you get a feel for the fabric and read reviews on which is best. Eclipse is by far the best awning available, and I would recommend it to anyone. Make sure you have it installed correctly, I used a local Eclipse retractable awning dealer from my town.

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Numericalapertureand resolution

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We purchased an Eclipse shading 4 years ago at the Jersey Shore and it has absolutely changed our summer lifestyle. We have a house on the bay that gets an intense Western exposure which means the Sun is hitting us hard until it goes down. The salesman suggested a drop shade off the front of our awning and it has just been a blessings. We have entertained and had so many good times n our back deck and it wouldn’t be possible without our beautiful Eclipse awning.

Our awning from Eclipse was installed 2 summers ago and has been wonderful for our back patio. Our guests are very impressed with the motorized aspect of our retractable unit and how whisper quiet it is! As a retired engineer, I was able to see that everything about this product scream QUALITY, that many of the other retractable awnings on the market lack. I would definitely buy from them again. My next move with them is one of their zipper solar screens. My backyard is a party now!

The light from the microscope source passes through the specimen/slide and continues through the air (or an immersion medium) as a cone of light between the cover glass and the objective front lens. The ‘angular aperture’ refers to the maximum angle of the edges of this image-forming cone of light which can be collected by the objective front lens when the specimen is in focus. In addition to an increasing NA, image brightness and image detail (resolution) are also related to the angular aperture.

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Etched onto the barrel of each objective on a microscope, you will find a variety of information. In addition to the magnification and the optical correction (see my article published entitled ‘Looking Down and Looking Through:  The Optics of a Microscope 2: The Objectives’ for more information on aberrations and corrections), you will find a number without units. This is the Numerical Aperture (or ‘NA’) of the objective.

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Numericalapertureof microscope

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I have had my Eclipse Shading for 5 years. It works as well today as it did the day we bought it. The fabric has kept its vibrant color and all the mechanisms seem to be still in great shape. The only thing I have had to do in 5 years is clean it. The Florida sun is brutal sometimes. Without my awning, my patio would be unbearable. The awning was worth every penny!

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The theoretical maximum angular aperture of light entering the front lens of an objective is 1800. This would give a θ value of 900 (half of the angle of the light cone). As a result, the theoretical maximum NA of an objective would be one (which is equal to the sine of 90). The refractive index of air is also one, therefore the maximum (theoretical) NA of an objective with an air gap between the front lens and the specimen would only equal one.

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I purchase two Eclipse Shading this past summer and I absolutely love them. They where installed on time and very clean. The awning also runs very smooth and quiet. The Lunar Light package and heating system are great! Now I can enjoy my deck on hot summer days and on cool spring and fall nights. It’s just one of the best investments /improvements I’ve made to my home. Eclipse Shading are great!

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To help to understand NA, it is useful to also have some understanding of refraction. In microscopy and optics, refraction refers to the change in direction of light waves which results from a change in the medium though which light passes (for example, glass, air, oil or water).

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After looking at several do-it-yourself websites or sites/brands that will arrange sub-contracted installation, I decided to use a local professional. Yes, I did send more but what I received was worth it! Fully custom made product from here in America and expert installation. Some things are worth getting a professional involved. The whole family loves the Eclipse Shading from Awning By Design. I give the entire experience and the product two thumbs up!

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My architect found Eclipse online and recommended one of their sun awnings. I considered them and Sunsetter sun awnings and noted that Eclipse’s products are far superior in quality. They have a much wider variety of fabric colors and engineering certifications on the frame (which means higher quality). Not to mention sun and rain sensors. Also, great service was provided prior to and after the sale. Thank you Eclipse!

There is an inverse relationship between the angular aperture and the working distance of an objective. I have covered working distance in my article entitled ‘Looking Down and Through: Microscope Optics 3: Oil Immersion Objectives’, but to briefly recap, the working distance is the actual distance (in millimetres or microns) between the objective front lens and the surface of the cover slip when the object is in sharp focus. Objectives with short working distances will consequently have a greater ability to gather more oblique light rays from a specimen compared to longer working distance objectives. Angular aperture is usually determined by the optics within the objective and each objective lens will have an optimal focal length and working distance- it can’t simply be increased by moving the objective closer to the slide!

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