The white light interferometer relies on the concept of white light interference technology. The beam splitters split the light coming from the source into two separate beams after being expanded and collimated. One out of the two beams is sent (reflected) by the measured surface, whereas the reference mirror reflects the second beam. Eventually, the beams converge and interfere, enabling the microscope to transform the topographic features of the surface measured into interference fringe signals, thereby measuring three-dimensional topography. The white light interferometer is designed for non-contact instantaneous measurement. The surface roughness, microscopic topography, and size of crucial portions of precision parts may all be measured to nanoscale precision.
The composition of the white light interferometer system:
- The optical illumination system uses a halogen-based light source with a wavelength of 576 nanometers and a spectral range from 340 nanometers to 780 nanometers.
- The optical imaging system acquires an infinity optical imaging system based on an objective microscope lens and an imaging eyepiece. The vertical scanning control system consists of piezoelectric ceramics and a control driver. It can perfectly drive the microscope objective lens up and down with a movement range of 100mm using a closed-loop feedback control mechanism. The precision of the position movement is 0.1 nm.
- Raw image data is procured with the help of a computer which is an integral part, along with a digital signal co-processor of the signal processing system. A dedicated digital signal co-processor completes the data parsing job.
- The application software consists of operation control, result display, and post-processing parts. It’s primarily used for operating and controlling surface shape measurement instruments. Likewise, actual measurements are displayed as three and two-dimensional plane and cross-sectional distribution curves.
The measurement principle of the white light interferometer:
The principle of a white light interferometer is based on optical interference. It’s a highly-precise surface profile measuring instrument that divides a single beam into two parts using a transflective beam splitter. Furthermore, the split beams combine later, and the imaging system creates two superimposed images on the CCD camera’s photosensitive surface. The inference of two beams with each other leads to the display of light and dark fringes on the photosensitive surface. The brightness of the interference fringe is influenced by the optical path difference between the two beams. In contrast, the relative height of the sample is evaluated as per the brightness of the white light fringes. The origin position of the interference fringe is also important.
Pros of the white light interferometer:
- Cost-effective option for 3D measurement.
- The user interface and procedure make it simple and easy to use.
- It allows the measurement of non-contact non-flat samples. Optical profilometry allows non-planar surfaces to be measured with ease.
- Accessory replacement is not required. An LED light as a source is enough.
- The software can move the sample at any angle to measure 3D graphics and analyze sample images from numerous perspectives.
What is the difference between a white light interferometer and a laser interferometer?
A white light interferometer has a broadband light incident light used in low-coherence interference. The presence of a light source in the visible spectral region is not necessarily a prerequisite. The light source’s temporal coherence is quite low, while its spatial coherence is very strong. Light can be sent into a single-mode fibre to achieve high spatial coherence and bandwidth. However, the resultant power is not very strong. Often, the goal is to boost the brightness of the light, which is achieved super-radiant light sources such as super-luminescent diodes.
The laser interferometer employs a laser of known wavelength, and the Michelson interferometer system determines the general length measurement of displacement. Laser interferometers usually have high intensity, directivity, monochromaticity, spatial coherence, and narrow bandwidth. It’s used to determine liner position, speed, angle, true flatness, etc., with multiple refractors and mirrors. It can be used to calibrate precise machine machines and measuring instruments.
Differences in application areas:
White light interferometers are mostly used for three-dimensional microscopic topography, but a laser interferometer detects the motion of any equipment with accuracy.
Difference between Confocal Laser Microscope and White Light Interferometer
Industry inspections primarily use white light interferometers; however, the advancements have led to certain shortcomings. The last 3D confocal microscopes claim to complement one another.
Advantages of the white light interferometer:
- It offers a large range and rapid rate of measurements. The inclination must be set before measurement.
- Z-axis resolution can reach up to 0.1nm with 0.3 μm horizontal resolution.
3D confocal microscope:
The Z-axis resolution is relatively less than the white light interferometer. However, the accuracy of sub-micron and other dimension measurements is identical.
Disadvantages:
- a) The interferometer lacks the ability to detect and measure objects with steep angles. It’s difficult to collect information with accuracy due to the concentration of accuracy patterns in the area.
- b) A white light interferometer used for electromagnetic wave interference hinders curved surface measurement.
Advantages of 3D Confocal Microscopy:
- a) The ranging confocal system is equipped to detect angles as steep as 83°. It employs the reflected t white light intensity to determine shapes with large angle characteristics.
- b) A confocal microscope allows light to pass through the groove detection surface. The difference is that white light interferometers are limited by surface reflectivity. Poorly reflective surfaces make determination harder, thereby making the sample measurement impossible. The difference in reflected light between the reference surface and measurement surface also makes sample measurement difficult.
The 3D confocal microscope has the ability to absorb a wide variety of reflected lights without the need for critical prerequisites for the sample. It’s a total reflectance measurement instrument. When the reflectivity of the reference surface and measurement surface coincide at 100%, clear interference fringes are displayed. However, the contrast is blurred if the measurement surface reflectivity is between 0–1%.
FAQ:
Can white light interferometer measure slope roughness?
It depends on the range of the measuring slope of the workpiece. If the range is used to measure the slope roughness of a precision workpiece, then it’s possible to measure it with a white light interferometer.
Why is the measured value of the wavelength of the white interferometer too long?
Numerous reasons explain long measured wavelength values, such as:
- Excessive tightness of the pressure of the spring sheet on the measuring surface of the carriage body.
- Tightness of the baffle plate and guide rail.
- The thickness of the lubricating grease on the guide rale surface.
- The small size of the pressing force of the screw tailstock.
Maintenance method: It’s important to check the tightness of the spring sheet on the measuring surface of the carriage body. In case of tightness, deform the spring to minimize the pressure. Check the tightness of the baffle. Adjust and loosen the screws to boost contact between the baffle and guide rail if it’s tight. In case of thick tin pieces, remove the tin piece and fasten the screws by putting down the baffle. The addition of thick grease on the third guide rail will enhance the damping of the carriage. However, if the grease houses impurities, it will make the movement of the carriage on a linear path difficult. Lastly, check the spring in the tailstock. Small pressing forces can be overcome by unscrewing the knurled screw sleeve and stretching the spring. The slipping of worm gear should be solved by disassembling.
Why are white light interference fringes asymmetric?
The white light interferometer can be damaged during transportation or through excessive touch, damaging the beam splitter and compensation plate. The maintenance method involves the adjustment of both components. The absence of an autocollimator should be countered with two plates to observe the indoor target.
Why is the white light interferometer interference ring not round?
Non-circular interference might be due to:
- Reversal of a film layer of the beam splitter.
- Misalignment of exit pupils.
- The compressive stress of the beam splitter, compensation plate and mirrors.
The maintenance method includes the adjustment of the beam splitter to the second side of the incident light. Installation on the first side should be followed by loosening the three widths, and the equal-inclined interference should be a straight elliptical interference ring. Remove the head screw and the beam splitter plate, rotate it 180 degrees in the metal frame, and reinstall it with the three wide head screws at the original pressing force.
Why is the white light interferometer reading vacancy greater than 0.03mm?
A reading gap greater than 0.3mm might be due to:
- The matching clearance between the drive nut and screw rod is large.
- The baffle plate and guide rail are loose, or the top block clearance under the carriage is excessive.
The maintenance method includes adjustment of the top block. Loosening the block screw is the second option. With the left hand’s thumb, gently push the carriage body towards the reading head, press the top block with the middle finger and tighten the top block screw. If the distance between the baffle plate and the guide rail still doesn’t match the criteria, adjust the two screws on the transmission nut.