ENLIGHT Formula Notation and Suffixes

Product Family: Luna Innovations Hyperion Platform / ENLIGHT Software

Overview

ENLIGHT software uses a shorthand notation system to simplify sensor formulas and expressions. This notation allows users to reference FBG wavelength values, zeroed references, normalized values, and filtered outputs without writing lengthy equations from scratch. Understanding these notations is essential for correctly configuring temperature sensors, strain sensors with temperature compensation, and dynamic sensors such as accelerometers.

This FAQ addresses the most common questions about ENLIGHT formula notation, including the suffix system and the built-in alpha filter.

Supplemental Material: Luna Optical Sensing Instrumentation and Software Manual

What do the _D, _N, _0, and _F suffixes mean in ENLIGHT formulas?

ENLIGHT uses shorthand suffixes appended to FBG or sensor names to represent commonly needed derived values. These suffixes simplify expressions so that you do not need to write out the full underlying equations each time.

The four primary suffixes are:

• X_0 — The reference (zeroed) value of an FBG wavelength at the time the sensor was last zeroed. This captures the baseline wavelength for a given sensor at a known starting condition.
• X_D — The delta value, calculated as X - X_0. This is the difference between the current wavelength and the reference wavelength. It represents the raw wavelength shift since the sensor was zeroed.
• X_N — The normalized value, calculated as X_D / X_0, which expands to (X - X_0) / X_0. This fractional wavelength shift is particularly useful for strain expressions, where the measurement is proportional to the relative change in wavelength rather than the absolute change.
• X_F — The alpha-filtered (low-pass filtered) value of the FBG. This is a smoothed version of the signal generated by ENLIGHT's built-in alpha filter. It is used primarily with dynamic sensors such as accelerometers to remove slow thermal drift and DC offsets. See the alpha filter section below for more detail.

In these definitions, “X” represents the FBG identifier as configured in ENLIGHT (for example, FBG_A1, Strain_FBG, or any user-defined name).

Example: If your strain FBG is named FBG_S1, then:

For further detail on these notations, refer to the Luna Optical Sensing Instrumentation and Software Manual starting on page 171.

What is the alpha filter in ENLIGHT and how does it work?

The alpha filter is a built-in ENLIGHT feature that provides a customized low-pass filtering function for FBG signals. It is accessed by appending the _F suffix to an FBG identifier in a sensor expression.

How it works

The alpha filter is a discrete low-pass filter implementation where the alpha factor is scaled inversely to the value of the signal's derivative. This means the filter adapts its behavior based on how quickly the signal is changing. Slow-moving components of the signal (such as thermal drift) are tracked by the filter, while fast-changing components (such as vibration) are not.

Primary use case — accelerometers and dynamic sensors

FBG-based accelerometers (such as single-FBG models) do not contain active electrical components or feedback mechanisms, so their readings can drift over time due to temperature changes. The alpha filter provides a way to remove this slow drift.

The recommended expression for a single-FBG accelerometer uses the alpha filter as follows:

Acceleration = (FBG1 - FBG1_F) / Sensitivity

Where:

• FBG1 is the current wavelength of the accelerometer FBG
• FBG1_F is the alpha-filtered (low-pass) value of that same FBG
• Sensitivity is the sensor sensitivity (e.g., in pm/g, from the calibration sheet)

By subtracting the filtered signal from the raw signal, you effectively create a high-pass filtered output that removes slow variations and DC offsets while preserving the dynamic vibration signal of interest.

Configuring the alpha value

The alpha parameter is configured in the Advanced section of the Edit FBG window in ENLIGHT. To access it, open the sensor tab, select the FBG in the top-left panel, and click Edit. The alpha value (in seconds) appears at the bottom of the dialog under “Advanced.”

Figure: ENLIGHT Edit FBG window showing the Alpha (sec) parameter under the Advanced section

A larger alpha value produces a slower filter response (longer time constant), meaning the filter will track slower changes. The appropriate alpha value depends on the lowest frequency of interest in your measurement. If the alpha value is too small, the filter may attenuate low-frequency signals of interest. If it is too large, drift removal will be sluggish.

When to use the alpha filter

• Single-FBG accelerometers that need thermal drift compensation
• Any dynamic measurement where slow offset drift needs to be removed
• Applications where you want to isolate AC (dynamic) signal content from DC (quasi-static) content

Note: Dual-FBG accelerometers (such as those using a push-pull configuration) use two FBGs that inherently compensate for temperature. These sensors typically do not require the alpha filter for thermal drift removal, though it may still be used if desired.

Where can I find the calibration coefficients for my sensor formulas?

All calibration coefficients needed for ENLIGHT formulas are provided on the sensor calibration sheet that ships with each sensor. Key values to look for include:

If you have misplaced your sensor calibration sheet, contact Luna technical support with your sensor serial number to obtain a replacement.

For additional technical guidance on ENLIGHT formulas and sensor configuration, refer to the Luna Optical Sensing Instrumentation and Software Manual or contact Luna Innovations technical support.