Another significant source of error in length measurement is from the effect of temperature on the length of an object.  Materials have a physical property known as the Coefficient of Thermal Expansion. This property numerically describes the amount of length an object will change of a given change in temperature.

The Coefficient of Thermal Expansion value can be programmed into the 5508A either from the

• Manually typed in from the front keys on the 5508A
• Programmed in from the GPIB interface on the 5508A

The temperature of the object being measured is then input from either

• Manually typed in from the front keys on the 5508A
• Electronically input from sensors attached to dedicated rear connections
• Programmed in from the GPIB interface on the 5508A

The temperature input and the Coefficient of Thermal Expansion are then used in an equation to provide a measurement display of how long the measured object will be if it were at a standardized temperature of 20°C (or 68°F).  This consequently allows the operation of the interferometer at different temperatures.

The 5508A is originally intended to have up to three electronic sensors attached to the rear connections.The 5508A would then calculate the average of the three sensors.Apparently it is smart enough to know how many sensors are attached and takes the average value of the sensors.

Optionally the average value can be typed in from the front key pad or the average value of some external sensors can be programmed through the GPIB port.For this project I will be using a Zipit Z2 embedded minicomputer to control a set of i2c temperature sensors.The sensors eventually will be built into V-Block clamps that will be used to hold the object being measured in a consistent manner.The temperature will hopefully represent the real temperature of the object being measured.The i2c temperature sensors values will be averaged by a compiled program in the Zipit Z2, just as the wavelength compensation was described in my previous post.The average temperature value will then be transmitted to the HP 5508A through a USB to GPIB adapter which will provide communications between Zipit Z2 and the HP 5508A.Finally an HTML server in the Zipit will provide a way for a user to input the Coefficient of thermal Expansion value.