Lambda Plate Method for measuring of stress in glass

Home Methods Compensator Senarmont Fringes

Set-up with Lambda plate
First polariser
at +45 degrees
Sample with shift = +100 nm
Sample with shift = 0 nm
Sample with shift = -100 nm
Wave plate and Analyser
at -45 degrees with the
main stress axis
The Lambda plate (or "full wave plate") adds a constant retardation of 550 nm (this is a full wave for green light) to the retardation Ls of the sample. This gives colored light dependent of the stress level. As given before, the intensity after the crossed Analyser is Ic = 1/2 Io ( 1 - cos( 2 p (L/l) ) ) , with
L = total shift in [nm] = 550 + Ls
l = wavelength of the light in [nm]
f = 2 p (Ls/ l )

Sample without retardation, Ls = 0:
For green light with l = 550 nm, the intensity is zero while the intensities for other wavelengths are not:
l = 550 nm gives Ic = 1/2 Io ( 1 - cos(2 p * 550 / 550 ) ) = 0
l = 650 nm gives Ic = 1/2 Io ( 1 - cos(2 p * 550 / 650 ) )
l = 450 nm gives Ic = 1/2 Io ( 1 - cos(2 p * 550 / 450 ) )

Sample with retardation Ls = 100 nm
For red light with l = 650 nm, the intensity is zero while the intensities for other wavelengths are not:
l = 650 nm gives Ic = 1/2 Io ( 1 - cos(2 p * 650 / 650 ) ) = 0
l = 550 nm gives Ic = 1/2 Io ( 1 - cos(2 p * 650 / 550 ) )
l = 450 nm gives Ic = 1/2 Io ( 1 - cos(2 p * 650 / 450 ) )

Sample with retardation Ls = -100 nm
For blue light with l = 450 nm, the intensity is zero while the intensities for other wavelengths are not:
l = 450 nm gives Ic = 1/2 Io ( 1 - cos(2 p * 450 / 450 ) ) = 0
l = 550 nm gives Ic = 1/2 Io ( 1 - cos(2 p * 450 / 550 ) )
l = 650 nm gives Ic = 1/2 Io ( 1 - cos(2 p * 450 / 650 ) )


So the colors give an indication of the stress level. Because colors are difficult to remember, most of the time a color is compared with the color in various reference samples.