Title: Visual acuity
Content:  in low light is achieved in near peripheral vision The maximum angular resolution of the human eye is 28 arc seconds or 0.47 arc minutes, this gives an angular resolution of 0.008 degrees, and at a distance of 1 km corresponds to 136 mm. This is equal to 0.94 arc minutes per line pair (one white and one black line), or 0.016 degrees. For a pixel pair (one white and one black pixel) this gives a pixel density of 128 pixels per degree (PPD). 6/6 vision is defined as the ability to resolve two points of light separated by a visual angle of one minute of 
Title: Naked eye
Content:  Observing a nearby small object without a magnifying glass or a microscope, the size of the object depends on the viewing distance. Under normal lighting conditions (light source ~ 1000 lumens at height 600–700 mm, viewing angle ~ 35 degrees) the angular size recognized by naked eye will be round 1 arc minute = 1/60 degrees = 0.0003 radians. At a viewing distance of 16" = ~ 400 mm, which is considered a normal reading distance in the US, the smallest object resolution will be ~ 0.116 mm. For inspection purposes laboratories use a viewing distance of 200–250 mm, which gives the smallest size of the object recognizable to the naked eye of ~0.058- 0.072 mm (~55-75 micrometers). The accuracy of a measurement ranges from 0.1 to 0.3 mm and depends on the experience of the observer. The latter figure is the usual positional accuracy of faint details in maps and technical plans.
Title: Eye
Content:  degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a grey block, is then the measurement of visual acuity of the eye. For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD. A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central fovea region. Spherical aberration limits the resolution of a 
Title: Optical resolution
Content:  The human eye is a limiting feature of many systems, when the goal of the system is to present data to humans for processing. For example, in a security or air traffic control function, the display and work station must be constructed so that average humans can detect problems and direct corrective measures. Other examples are when a human is using eyes to carry out a critical task such as flying (piloting by visual reference), driving a vehicle, and so forth. The best visual acuity of the human eye at its optical centre (the fovea) is less than 1 arc minute per line pair, reducing rapidly away from the fovea. The human brain requires more than just a line pair to understand what the eye is imaging. Johnson's criteria defines the number of line pairs of ocular resolution, or sensor resolution, needed to recognize or identify an item.
Title: Eye
Content:  mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair. A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an optotype, corresponds to 20/20 (normal vision) in humans. However, in the compound eye, the resolution is related to the size of individual ommatidia and the distance between neighbouring ommatidia. Physically these cannot be reduced in size to achieve the acuity seen with single lensed eyes as in mammals. Compound eyes have a much lower acuity than vertebrate eyes.
Title: Microscope
Content:  The most common type of microscope (and the first invented) is the optical microscope. This is an optical instrument containing one or more lenses producing an enlarged image of a sample placed in the focal plane. Optical microscopes have refractive glass (occasionally plastic or quartz), to focus light on the eye or on to another light detector. Mirror-based optical microscopes operate in the same manner. Typical magnification of a light microscope, assuming visible range light, is up to 1250x with a theoretical resolution limit of around 0.250 micrometres or 250 nanometres. This limits practical magnification to ~1500x. Specialized techniques (e.g., scanning confocal microscopy, Vertico SMI) may exceed this magnification but the resolution is diffraction limited. The use of shorter wavelengths 
Title: Near-field optics
Content:  The limit of optical resolution in a conventional microscope, the so-called diffraction limit, is in the order of half the wavelength of the light used to image. Thus, when imaging at visible wavelengths, the smallest resolvable features are several hundred nanometers in size (although point-like sources, such as quantum dots, can be resolved quite readily). Using near-field optical techniques, researchers currently resolve features in the order of tens of nanometers in size. While other imaging techniques (e.g. atomic force microscopy and electron microscopy) can resolve features of much smaller size, the many advantages of optical microscopy make near-field optics a field of considerable interest.
Title: Optical microscope
Content:  There is therefore a finite limit beyond which it is impossible to resolve separate points in the objective field, known as the diffraction limit. Assuming that optical aberrations in the whole optical set-up are negligible, the resolution d, can be stated as:  Usually a wavelength of 550 nm is assumed, which corresponds to green light. With air as the external medium, the highest practical NA is 0.95, and with oil, up to 1.5. In practice the lowest value of d obtainable with conventional lenses is about 200 nm. A new type of lens using multiple scattering of light allowed to improve the resolution to below 100 nm.
Title: Artificial structures visible from space
Content:  The human naked eye has an angular resolution of approximately 280 microradians (μrad) (approx 0.016° or 1 minute of arc), and the ISS targets an altitude of 400 km. Using basic trigonometric relations, this means that an astronaut on the ISS with 20/20 vision could potentially detect objects that are 112 m or greater in all dimensions. However, since this would be at the absolute limit of the resolution, objects on the order of 100 m would appear as unidentifiable specks, if not rendered invisible due to other factors, such as atmospheric conditions or poor contrast. For readability of text from the ISS, using the same trigonometric principles and a recommended character size of about 18 arcminutes, or about 5,000 μrad, each letter would need to be about 2.016 km in size for clear legibility in good conditions.
Title: Millimetre
Content:  On a metric ruler, the smallest measurements are normally millimetres. High-quality engineering rulers may be graduated in increments of 0.5 mm. Digital callipers are commonly capable of reading increments as small as 0.01 mm. Microwaves with a frequency of 300 GHz have a wavelength of 1 mm. Using wavelengths between 30 GHz and 300 GHz for data transmission, in contrast to the 300 MHz to 3 GHz normally used in mobile devices, has the potential to allow data transfer rates of 10 gigabits per second. The smallest distances the human eye can resolve is around 0.02 to 0.04 mm, approximately the width of a thin human hair. A sheet of paper is typically between 0.07 mm and 0.18 mm thick, with ordinary printer paper or copy paper approximately 0.1 mm thick.

Question: what is the limit of resolution of the human eye in micrometers?
Answer: ~55-75 micrometers.
~ 0.116 mm.