Optical Glass Metrology – Techniques / Limitations - Owens Design

Limitations of Optical Measurement Techniques
: Contrast
- Differential intensity in measured light is improved by optical contrast (due to color or reflectivity);
: Light Spectrum (Reflection / Absorption)
- Light must be reflected by object to be measured; Absorption of light by object can be overcome
by changes in light source / wavelength
: Wavelength
- Direct distance measurement is limited by the resolution of the measuring instrument or
reference source; but interpolation is possible at less than one wavelength through interference
techniques
- Visible light has a wavelength of 380nm (Blue) to 750nm (Red); therefore optical techniques are
limited to resolution of about 0.5 micron in the visible spectrum
: Ambient Light “Noise”
- Measured light must differ in intensity from ambient light sources; signal-to-noise ratio should
exceed 10:1
- Selecting frequency / wavelength of light different than highest intensity ambient wavelengths
(probably in visible red spectrum) can also improve signal-to-noise ratio in the specific frequency
measured
: Optical Distortion
- Due primarily to lens quality limitations; atmospheric limitations such as air contaminants
: SICK AG: : BU02 side session: high end displacement
Patrick Nutz 19.03.2009 3