How to Measure Flatness - Technical Article

There are a number of ways to measure the flatness of a surface. The most common method within the Flat Lapping sector is by using a Monochromatic Sodium light unit and an Optical Flat. This gives extremely precise measurements, more accurate than most CMM measurements, in an economical way.

What are light bands?

Light Bands were discovered by Isaac Newton who first studied them in 1717. They are an interference pattern created by the reflection of light between two surfaces.

When using a monochromatic light source it is possible to use the phenomenon to calculate the flatness of a component, but the surface of the component must be reflective in order for the light bands to appear. The light bands are made up of a bright and dark fringe. Combined, these correspond to the wavelength of the monochromatic light which in the case of a Sodium light source is equal to 589nm. When checking parts for flatness, it is only the dark bands that are counted, so as this is half the total fringe, each dark band equals 294nm or 0.00029mm.

Diamond lapping processes are ideal for producing reflective surfaces, which can be measured for flatness using this method directly after the lapping operation.

Typical light band patterns which show flatness accuracy

Surface geometry 1 Light band
2 Light bands
3 Light bands
9 Light bands

Convex or Concave

Surface parallel to flat
Symmetrical Pattern
optical flat convex or concave convex or concave


With concave surface band will curve in opposite direction
Non-Symmetrical Pattern
optical flat convex convex


Convex or Concave
Symmetrical Pattern
optical flat cylindrical cylindrical

Saddle Shaped

Symmetrical Pattern
optical flat saddle shaped saddle shaped

How to read light bands with an Optical Flat

First clean the surfaces of the component and optical flat with a lens tissue or soft lint free cloth. Both faces must be absolutely clean. Place the optical flat carefully on top of the component. Do not slide it across. As the optical flat and component come together lines will appear through the flat. Manipulate it to obtain a line pattern, as illustrated. The lines are interference fringes or bands and are an indication of the level the component’s surface has risen or fallen in relation to the optical flat.

how to read light bands

Light band reading Showing perfect flatness

perfect flatness

Lapping plate flatness


Ring pattern moves towards finger pressure. If workpiece is convex the lapping plate is concave.

convex flatness

Lapping plate concave

The control rings must be moved to the outside of the plate to correct this condition


Ring pattern moves away from finger pressure. If workpiece is concave the lapping plate is convex.

concave flatness

Lapping plate convex

The control rings must be moved to the inside of the plate to correct this condition

The straight parallel bands, and not the width of light band indicates the flatness.

Surface finish chart

Surfaces are produced by a variety of material removal processes. The total geometry which results can best be considered to be split-up into three components - roughness, waviness and form.

Basic parameters

flatness parameters

Parameters - The various parameters Ra, and Rt are illustrated. It may be seen that the centre line is that line which divides the areas such that: A1 + A3 + ............ A7 = A2 + A4 + ............ A8 The two most common surface finish measurements are Ra and Rt These are described as follows:

Ra is universally recognised as the most used international parameter of roughness. It is the arithmetic mean of the departures of the roughness profile from the mean line.

Rt is the maximum peak to valley height of the profile over the measured length. Measurements are usually quoted in microns. 1 Micron = Approx 40 Micro Inch

Examples of Ra and Rt

example of ra and rt

Typical statements of surface finish or texture on drawing:

typical surface flatness

Symbol A How to specify maximum roughness value in Ra microns.
Symbol B How to specify maximum and minimum roughness values.
Symbol C How to specify maximum roughness and finishing process.

Kemet conversion tables

Imperial to Metric
    Millimetres (mm) Microns (μm) Angstroms (Å)
1 INCH (1.00”) = 25.4 25,400 254,000,000.
1 THOUS. (0.001”) = 0.0254 25.4 254,000
1 MICRO INCH (μin) = 0.0000254 0.0254 254
Metric to Imperial
    Inches Thousandths Micro-inches
1 MILLIMETRE (mm) = 0.039 37 39.37 39,370
1 MICRON (μm) = 0.000 039 37 0.039 37 39.37
1 ANGSTROM (Å) = 0.000 000 003 937 0.000 003 937 0.003 937

Equipment for measuring flatness

Kemet Optical Flats

optical flat

Produced from Quartz, Kemet Optical Flats are available in single and double sided, 1/4 light band or 1/10 light band accuracy. Standard sizes from 25mm up to 300mm diameter.

Kemet Monochromatic Light

monochromatic light

Produces clear flatness readings when used with Kemet Optical Flats. The compact designed Light is easily transportable and uses a sodium long-life sodium light source. Supplied with a flatness reading interpretation chart.

Kemet Flatness Gauges

flatness gauge

The Kemet Flatness Gauge is used to monitor a lapping plate’s flatness and to give an indication of the flatness the plate will produce on a given part size.