Surface varies with the materials, forming, processing, engineering function, and size of an object and the features are in general very complicated. Surface texture is traditionally greatly characterized by form, waviness and roughness among lots of factors, and the latter two are its key components.

Means having irregularities that surface forming instruments leave behind on the surface due to the peculiar properties during production process.

Waviness means having curves on the surface which appear at the larger interval than that of roughness, marked by characteristics of individual machines, such as unbalanced grinding wheel, uneven feed screw, vibrating machine, and uneven heat treatment of materials.

Form refers to outward appearance of a surface which roughness and waviness are not considered.

All engineering surfaces has their peculiar engineering function. Among these, surface roughness has effects on precision assembly of machine components and quality of optical parts, so more emphasis is being put on it than other factors. Steel industry weighs the roughness determining the polishing, coating, and plating of steel sheet and wear resistance of rolls which shall be subject to quality control.

As attention is increasingly paid to surface roughness, consumers also get interested in it. So it is necessary to manage on-line surface roughness management, coping with this trend.

Usually, the smoothness or roughness over surface is a relative notion, perceived subjectively. It can be inferred that it is not considered as the properties of an object as it varies with the evaluators. However, the surface has its peculiar value representing itself, thus enabling to be quantified.

We can impose its peculiar designed function on engineering surface, thus predicting the degree of satisfaction through measurement.

Surface texture is related to area. Surface roughness also involves the area. However, we has traditionally dealt with the first dimension only. Surface texture is regarded as the notion measured and formed along the lines and naturally accepted by many lines. The reason is that two dimension factors of surface are deemed as being in the continuum of one dimension roughness similar to them.

One good example is that stylus-using measurement, traditionally used for surface roughness measurement, is made along the lines over surface.

In roughness profile, measure the length l in the direction of the center line, let the center line be X axis, and determine the roughness profile ratio y = f(x) in unit of micrometer (㎛).

Ra does not include information about convexity/concavity and shape of roughness profile.

This means that surfaces of different shapes can have the same Ra value.

Ra is the common method that is used internationally to indicate surface roughness. Measure the sum of areas that are enclosed by the curve of convexity/concavity and the center line, and divide the sum by the length of the center line. Then this corresponds to the arithmetic mean deviation about the center line.

Ra, that is also indicated as CLA (center line average) or AA (arithmetic average), is defined in KS B 0161 together with Rmax and Rz.

The principal six cutoff lengths are 0.08, 0.25, 0.8, 2.5, 8 and 25 and the standard is 0.8 mm.

For indication of center line surface roughness, use a pair of the center line surface roughness in micrometer (㎛) and the cutoff length or Ra c in millimeter. For a cutoff of 0.8 mm, this value can be omitted.

For calculation of representative value, it is also possible to use root-mean-square method (RMS) for statistical purpose.

Ra does not include information about convexity/concavity and shape of roughness profile.

This means that surfaces of different shapes can have the same Ra value.

Ra is the common method that is used internationally to indicate surface roughness. Measure the sum of areas that are enclosed by the curve of convexity/concavity and the center line, and divide the sum by the length of the center line. Then this corresponds to the arithmetic mean deviation about the center line.

Ra, that is also indicated as CLA (center line average) or AA (arithmetic average), is defined in KS B 0161 together with Rmax and Rz.

The principal six cutoff lengths are 0.08, 0.25, 0.8, 2.5, 8 and 25 and the standard is 0.8 mm.

For indication of center line surface roughness, use a pair of the center line surface roughness in micrometer (㎛) and the cutoff length or Ra c in millimeter. For a cutoff of 0.8 mm, this value can be omitted.

In a roughness profile, measure the highest vertical distance between two lines parallel to the mean line in the unit of micrometer (㎛), which is the same as the highest peak to the lowest valley distance within a sample length.

As it is possibile to measure without determining center line, it is widely used. For Rmax, it varies with the definition or signs depending on specifications. It is used by KS B 0161-1988 in Korea, ANSI B 46.1-1978 in U. S and JIS B 0601-1982 in Japan. DIN 4768-1974 adopts the highest value among Rmaxs measured and ISO 4287/1-1984 uses Ry.

In KS, measure the distance between parallel lines that pass through the third peak and valley, respectively from the upper and lower part in parallel with mean line of a profile in the unit of micrometer (㎛).

In ISO, measure the distance between the fifth mean height from the highest peak and the fifth valley mean depth from the lowest valley within the sampling length in the unit of micrometer.

Rmax	Rz	Ra	Triangular sign
(0.05S) 0.1S 0.2S 0.4S 0.8S	(0.05Z) 0.1Z 0.2Z 0.4Z 0.8Z	(0.012a) 0.025a 0.05a 0.1a 0.2a	▽▽▽▽
1.6S 3.2S 6.3S	1.6Z 3.2Z 6.3Z	0.4a 0.8a 1.6a	▽▽▽
12.5S (18S) 25S	12.5Z (18Z) 25Z	3.2a   6.3a	▽▽
(35S) 50S (70S) 100S	(35Z) 50Z (70Z) 100Z	12.5a   25a	▽
Not specified.			~

Measurement method	Measuring apparatus	Illustration
Sensory comparison	Roughness sample
	Standard method for comparison
Oblique cut method	Cutting tools, Microscope
Stylus-using method	Stylus-using measuring system (Recording gauge or indicator)
Optical cut method	Optical microscope
Optical reflectance	Reflective roughness gauge
Optical interference method	Micro-interferometer