Magnifiers are inspection instruments that enlarge the image of a sample. The level of magnification (generally less than 50 times) differentiates magnifiers and microscopes.

Operation

Magnifiers are used to increase the speed of assembly, improve inspections, enhance quality control, facilitate the manufacturing of small parts, and investigate or observe small objects. They are commonly used in electronics manufacturing and during medical procedures.

Magnifiers are evaluated on the parameters of task distance and task duration. Task distance is the space between the viewer and the object being viewed. Near tasks, such as reading, are usually done at distances of 16 inches or less. Intermediate tasks are those done at arm's length or slightly more—between 17 and 40 inches. Distance tasks are completed at approximately 5 feet or more.

The task duration refers to the time period during which the magnifier is utilized. Short-term activities are only a few minutes long, while extended viewing tasks are for longer periods of time. The grid below can be used to identify the best magnifier based on task distance and duration.

Magnifier selection grid

Table credit: eschenbach

Types

Mounting

• Floor stand: The device location is maintained via a stand which rests on the floor. The magnifier is mounted on a boom or extension for more ergonomic use.
• Desktop/tabletop: The magnifier includes a weighted stand or clamp for fixed positioning on a tabletop.
• Handheld/wearable: Personnel manipulate a magnifier's position with their hands, or the device is integrated into a garment for easy access and use. 
• Magnetic: A paramagnetic mechanism adheres the magnifier to attracted materials.
• Suspended: An articulated arm or boom magnifier extends from an overhead structure, providing a versatile range without interrupting floor space.

 Configurations

Specifications

Magnification is the ratio of the size of an image to its corresponding object, which is determined by linear measurement. This value is often represented by an "x," an indication that the number is a multiple. For example, a 2x magnifier doubles the apparent size of an object. This specification represents the entire magnification range, including both the eyepiece magnification and the objective magnification. If a high magnification is required, a microscope should be considered.

The magnification chart below shows how magnification, diopter, and focal length affect the object in view.

Chart credit: Dazor

Additional specifications include:

• Diopter: Diopter refers to the curvature of a lens. As the diopter increases, the lens becomes thicker and the curvature greater. As the curvature increases, light rays are redirected to fill a greater portion of the user's retina, which makes the object look bigger. The relationship between diopters and power is: the number of diopters divided by four equals power. For example, an eight-diopter lens would make an object look two times bigger, over and above what the unaided eye already sees (8 diopters ÷ 4 = 2x power).
• Illumination: Illumination describes the built-in lighting within the magnifier. If the application requires additional illumination there are several options including LED, halogen, and fluorescent lighting.
• Field of view: The extent of the visible image field that can be seen when the magnified image is in focus. As the power increases, the magnifier's field of view decreases.
• Focal length: Focal length, also known as working distance, is the distance between the front lens and specimen when the specimen is in focus. In most instances, the working distance of a magnifier decreases as magnification increases. For example, a 2x magnifier at a focal length of 5 in. means that the magnifier will provide double the magnification power and be clear in focus at 5 in. The shorter the focal length, the higher the magnification power. The image at right represents this concept. If the application uses low magnifications then a magnifier can give the user a longer focal length and also give more room to work side-to-side. Microscopes are great for focusing on a small point, but magnifiers provide a more comprehensive field of view. Working distance values are not included on all magnifiers and their presence varies depending upon the manufacturer. 
• Scale/reticle resolution: Scale or reticle resolution refers to the minimum scale or reticle that can be resolved with the magnifier. An optical device reveals the fineness of detail in an object. Objectively, resolution is specified as the minimum distance between two lines or points in the object that are perceived as separate by the human eye. Subjectively, the images of the two resolved points must fall on two receptors (rods or cones), which are separated by at least one other receptor on the retina of the eye. 
• Depth of field: The depth of field is the range of distance the magnifier can be moved from the object and maintain an acceptable degree of focus. The depth of field varies with the focal length and as the power increases, the depth of field shortens.

Features

Common features of magnifiers include:

• Bifocal lens: Magnifiers with a bifocal lens provide two or more, magnification levels. This style includes traditional bifocal lenses that have two different strength lenses melded together, as well as swing lens or drop alignment types.
• ESD safe: Electrostatic discharge (ESD) can damage sensitive electronic components. An ESD safe magnifier is designed so that random or static charge energy cannot be released.
• Scale/comparator reticle: The magnifier has an internal scale or reticle grid for comparative measurements of dimensional or orientation features. A reticle is a flat, transparent component inscribed or etched with a grid or graduations. Reticles can be placed with the magnifier, on top of, or along the side of the product being examined.
• Variable magnification: The magnifier provides several magnification levels. It may produce discrete or continuous changes in magnification using multiple, independent, or aligned lenses, or an optical or digital zoom system. The lenses themselves may swing or drop into alignment on some magnifiers.
• Work holders/manipulator: Magnifiers constructed with integral clamps, clips, or magnet holders for manipulating or holding the sample being viewed.

Applications

Applications that use magnifiers include assembly, document and photograph inspection, electronics manufacturing, forensics, industrial inspection, jewelry and gemology, medical and life science, vision aid, tool and die, machining, and textiles.

Standards

Magnifiers must adhere to certain standards and specifications to ensure proper design and function. 

• BS 5165: Characteristics of single-element, multi-element and twin-system magnifiers. Recommendations for their selection.

• BS 7522 P1: Specifies requirements, both optical and mechanical, for handheld and stand-mounted magnifying appliances for use as near-vision aids. Includes simple and compound magnifiers. 

References

Olympus Corp. - Working Distance Definition

Dazor Lighting Technology - How to Choose a Lighted Magnifier

Image Credit:

Molex | Moffatt Products | Carson Optical | Gemplers | Dazor | Bausch | Mitutoyo | LSS Products | Aven Inc.