How do I choose the cut-on or cut-off wavelength for edge filters (longpass and shortpass)?

Choosing the right edge filter—longpass or shortpass—for your optical system may sound technical, but once you understand the fundamental parameters, it becomes much more straightforward.

What Are Edge Filters?

Edge filters are optical elements with a steep transition between blocking and transmitting light, commonly called an 'edge.' Longpass filters allow wavelengths longer than a specified cut-on value to pass through, blocking shorter wavelengths. In contrast, shortpass filters transmit wavelengths shorter than a given cut-off wavelength and block those longer than this threshold. These filters are essential for separating or isolating spectral regions in light-based applications.

Key Applications

• Fluorescence Spectroscopy: Longpass filters allow observation of fluorescence by blocking excitation light and transmitting emission.
• Raman Spectroscopy: Both types are used to separate different light orders.
• Machine Vision & Photometry: Used as order-sorting or blocking filters to improve accuracy by removing unwanted wavelengths.

Cut-On and Cut-Off Wavelength: The Basics

The cut-on wavelength for a longpass filter or cut-off wavelength for a shortpass filter is the point where the filter’s transmission crosses 50%. In other words, at this wavelength, half the incoming light will pass through the filter while the remainder gets blocked. For example, a 500 nm longpass filter will transmit wavelengths longer than 500 nm and block those below, while a 600 nm shortpass filter will transmit wavelengths below 600 nm and block those above.

How to Choose the Right Wavelength

• Define Your Application: What are you trying to see or block? For fluorescence, block the excitation source but transmit emission.
• Know Your Source and Detector: Determine the spectrum of the source (laser, LED, etc.) and the sensitivity of your detector.
• Select the Key Wavelengths:
  For longpass, set the cut-on just above the excitation source and below the emission you want to detect.
  For shortpass, set the cut-off just below any unwanted higher wavelengths (such as detector saturation or unwanted IR).
• Consider Filter Slope/Edge Steepness: High-quality filters offer sharp edges (steep slope at transition), which means rapid transition from block to pass within a few nanometers—important for separating closely spaced wavelengths.
• Transmission and Blocking Ranges:
  • Transmission range is the passband for your signal.
  • Rejection range is the blockband for unwanted light.
• Account for Filter Tolerances: Manufacturing variance (often ±2 nm to ±5 nm) can affect exact performance, so factor this into your design.

Practical Example

Suppose your LED source peaks at 470 nm, and your fluorescent emission is at 525 nm. To isolate excitation, use a shortpass filter with a cut-off near 475 nm. For emission, use a longpass filter with a cut-on just below the emission at 515 nm, which blocks any residual excitation and passes the emission. If you need an especially narrow detection band, combining a shortpass and a longpass filter creates an effective bandpass filter for precisely the signal range you want.

Other Considerations

• Surface quality: Specified as 'scratch/dig' ratings; lower numbers mean higher optical quality.
• Optical Density (OD): Indicates blocking strength—higher OD means less background or leakage.

Summary Table: When to Use Which Wavelength

Bottom Line

• Start by knowing your light source and detection needs, then choose the cut-on/cut-off wavelength closely matched to the limits of what you want to block or transmit.
• For best results, use manufacturer data and consider slope, tolerances, and blocking range to achieve optimal system performance.