Mask Data for Better Modulation

Modulation is a measure of the range of intensity values in fringe data, versus the camera's entire range of readable values. Ideally fringes will have darkest values that are just above zero, and brightest values that are just below the camera's saturation point. Good modulation is required to obtain height values. Areas of low or zero modulation can lead to erroneous data that will manifest as phase ambiguities, spikes or noise in the measurement.

In typical measurement data areas of poor modulation will include:

• the regions beyond the edges of the test part
• regions of rolloff at the edges of an optic
• zones around dust or dirt particles
• areas of high slopes due to excessive waviness or roughness.

4D's 4Sight analysis software provides several mechanisms for masking invalid data:

• A Detector Mask is a set of shapes used to block regions of consistently invalid data, such as the areas beyond the pupil of a circular optic (and, typically, to block the edges of the part as well). It can also be used to isolate a region of interest.
• An Analysis Mask is a set of shapes that can be used to eliminate data around dust or debris, or to isolate a region of interest.
• A Modulation Threshold value can be applied to remove all pixels that do not change in intensity value sufficiently. This threshold can be used to eliminate areas outside of the pupil. It can also eliminate areas in regions of high slopes that fail to modulate sufficiently
• An Intensity Threshold will remove all pixels that fail to reach a brightness value when averaged over all frames (in a temporal measurement) or all phases (in a dynamic measurement). In a noisy environment some invalid pixels may still appear to modulate due to random spikes. In these cases an intensity value may block the pixels more effectively that a modulation threshold. 

A combination of these tools can be used to isolate valid pixels with reliable modulation for further analysis.