The two primary settings for optimizing the light level are the camera's Exposure and Gain.
Exposure is the amount of time that the sensor receives light within one “frame” of the measurement. Increasing the Exposure time lets more light to reach the sensor. However, the longer duration also makes the measurement more susceptible to vibration and turbulence which can wash out, or smear, moving fringes.
Gain is a multiplier that digitally amplifies the camera’s received light levels. Increasing the Gain raises the intensity of the entire signal. This allows very low intensity values to be discerned and used in subsequent calculations. However, since Gain is a simple multiplier, the noise level will increase right along with the signal.
A good rule of thumb for setting these values is to start with the lowest Gain and increase the Exposure to just below the saturation point. If you cannot achieve sufficient intensity, then increase the Gain and re-adjust the Exposure.
Figure 1a shows low intensity fringes. It may be possible to make a measurement at these settings, but data may be lost due to insufficient modulation. Figure 1b shows good interference fringes, with excellent contrast and no saturation. which will produce excellent measurement results. In Figure 1c the intensity exceeds the camera’s dynamic range, saturating the pixels shown in red.
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| Figure 1. Interference fringes with, a) low intensity, b) good intensity, c) intensity exceeding the camera limit (saturated pixels in red). |
Use Exposure and Gain to strike a balance between maximizing the signal-to-noise ratio and reducing the integration time to limit environmental effects.
As an example, Figure 2 shows the interference fringes (left) and measured surface (right) of an optic in a high vibration environment. With Exposure = 12.9 ms and Gain = 1 the fringes are sufficiently bright, yet they are being smeared by the vibration. The resulting measured surface includes discontinuities due to phase unwrapping errors. Raising the Gain will allow for shorter Exposure time which will reduce the impact of the vibration—provided the Gain is not so high that it introduces excessive noise into the measurement.
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| Figure 2. A long Exposure time washes out the fringes (left), leading to unwrapping errors (right). Raising the Gain and lowering the Exposure solves the problem, though too much Gain can introduce noise. |
