The light gathering ability of a microscope objective is quantitatively expressed in terms of the numerical aperture (NA). The objective’s NA is a measure of its ability to capture image-forming light rays: Higher NA values allow increasingly oblique rays (representing finer object structures) to enter the front lens of the objective, producing a higher-resolution image with greater specimen detail. This interactive tutorial demonstrates the change in numerical aperture light cones displayed by a microscope objective with corresponding changes in numerical aperture. The angular aperture value corresponding to a given NA-value is also depicted here.

Norland OpticalAdhesive 60

Please note that in some cases, you may be prompted to enter your login credentials again to access certain areas or resources within our training platform.

n is the refractive index of the media in the object space (between the cover glass and the objective’s front lens) and θ is half the full angular aperture. The value of n varies between 1.0 for air and 1.58 for most immersion media used in optical microscopy. The angular aperture, which varies with the objective focal length, is the maximum angle of image-forming light rays diffracted by the specimen that the front lens of the objective can capture when the specimen is in focus. As the objective focal length decreases, the maximum angle between the specimen and the outer diameter of the objective front lens increases, causing a proportional increase in the angular aperture. From the above equation, it is obvious that the NA increases with both the angular aperture and the refractive index of the imaging medium.

Norland OpticalAdhesive 61

NOA 61 meets Federal Specification MIL-A-3920 for optical adhesives and is approved for use on all government contracts specifying such adhesives. The adhesive is designed to give the best possible optical bond to glass surfaces, metals, fiberglass and glass filled plastics. NOA61 is recommended for bonding lenses, prisms and mirrors for military, aerospace and commercial optics as well as for terminating and splicing optical fibers.

Norland OpticalAdhesive 81

The brightness and resolution of an image formed by an objective at a given magnification increases with its NA value, respectively the diameter of the angular aperture (the angle of the light cone collected by the objective). Light rays emanating from the specimen pass through air (or a liquid-based immersion medium) located between the cover glass and the objective’s front lens. The angular aperture is expressed as the angle between the microscope’s optical axis and the direction of the most oblique light rays captured by the objective (see the tutorial figure). Mathematically, the NA is expressed as:

Are you curious about how microscope objectives capture finer object structures to produce higher-resolution images? This foundational knowledge article on Numerical Aperture and Light Cone Geometry will give you a sound understanding of the light gathering ability of microscope objectives and how it is expressed through the numerical aperture (NA). An interactive tutorial allows you to visualize changes in the illumination cone as you vary NA values. You will also learn about the role of the refractive index and the limitations of the maximum achievable NA values.

NorlandAdhesives

NOA 61 also has excellent clarity, low shrinkage and as light flexibility that make it superior to other materials for optical bonding. These characteristics are important in order for the user to produce high quality optics and achieve long term performance under changing environments.

Theoretically, the maximum angular aperture achievable with a dry (air) microscope objective would be 180 degrees, resulting in a value of 90 degrees for the half angle used in the NA equation. The sine of 90 degrees is one, indicating that the numerical aperture is limited not only by the angular aperture but also by the refractive index of the imaging medium. Most microscope objectives are designed to operate with air (refractive index= 1.0) as the imaging medium between the cover glass and the front lens of the objective. This yields a theoretical maximum NA of 1.00. For practical reasons (available working distance), the highest desirable value for the NA of a dry objective is 0.95 (the half angle of the aperture is approximately 72 degrees). Immersion objectives achieve much higher NAs at the expense of free working distance and spherical aberration sensitivity.

Norland OpticalAdhesive 68

The tutorial displays a schematic drawing of a microscope objective. The actual angular aperture of the light cone and the corresponding NA value are indicated in the tutorial window. To operate the tutorial, use the Numerical Aperture slider to change the NA value from low (left) to high (right). As you vary the numerical aperture value with the slider, the size and shape of the illumination cone entering the objective’s front lens is altered. The adjustable NA for this tutorial is 0.03 to 0.95. The approximate objective magnification has also been assigned to each NA value.

NOA 61 is cured by ultraviolet light with maximum absorption within the range of 320-380 nanometers with peak sensitivity around 365nm.  The recommended energy required for full cure is 3 Joules/sq. cm in these wavelengths. The cure is not inhibited by oxygen, hence any areas in contact with air will cure to a non-tacky state when exposed to ultraviolet light.

Join the ranks of world-leading microscopists with our expert training courses. Whether you're in academia or industry, a biologist, materials scientist, or somewhere in between, our training courses will help you unlock the full potential of your microscopy skills.