An optical instrument can be defined as a device used for observing, measuring, and recording optical data or information. It is often the responsibility of the optical engineer to design the optical system to be incorporated into such a device. This chapter will describe several basic instruments and discuss their applications and some of the special aspects of these instruments as they affect their design and manufacture. The most common optical instruments are those intended to enhance visual capability. This chapter will deal primarily with instruments of this type.
A magnifying glass (or loupe, from Old French, meaning an imperfect gem), is the simplest of optical instruments intended for the enhancement of visual capability. It is a device frequently associated with jewelers, usually taking the form of a simple positive lens. In use, the magnifier is held close to the eye, while the object to be viewed is brought to the focal point of the lens.
The average young and healthy human eye is capable of focusing from infinity, down to a minimum distance of about 250 mm (10 in.). This same average eye is capable of resolving a repeating high-contrast target with equal width black and white lines when each line subtends an angle of 1 arcmin or more. Most often, when viewing an object it is our intent to distinguish as much detail on that object as is possible. To that end we first bring the object as close as possible to the eye. When that closest distance is 250 mm, the smallest resolved element on the object - a detail that subtends an angle of one minute (tanÎ¸ = 0.0003) - will have an actual size of 250 mm x 0.0003 mm = 0.075 mm. If this resolved element is a part of a repeating pattern of equal-thickness parallel black and white lines, then each cycle (one black plus one white line) will have a thickness of 0.150 mm. The frequency of this finest resolvable pattern will then be 1/0.150 mm = 6.67 cycles/mm.
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