Cut-on Wavelength

|Team Syronoptics

Cut-On Wavelength is a primary optical specification used to characterize Longpass Filters. It defines the specific wavelength at which the filter's transmission increases to 50% of its peak transmission as it transitions from a blocking state (stopband) to a transmitting state (passband).

In optical coating terminology, the Cut-On wavelength (λcut-on) indicates the spectral position of the filter's edge.

  • 50% Point: The industry standard definition for Cut-On is the wavelength where absolute transmission reaches 50%.
  • Transition: It marks the midpoint of the "slope," where the filter switches from attenuating light (blocking) to allowing light to pass (transmitting).

Cut-On vs. Cut-Off

It is often confused with "Cut-Off" wavelength, but the distinction is simple based on the slope:

  • Cut-On: Used for Longpass filters - Transmission goes up as wavelength increases (Low -> High).
  • Cut-Off: Used for Shortpass filters - Transmission goes down as wavelength increases (High -> Low).

Note: Bandpass filters typically have both a Cut-On wavelength on the short-wavelength side and a Cut-Off wavelength on the long-wavelength side.

Example: The 550 nm Longpass Filter (LP550)

A common real-world example is the LP550 Longpass Filter. For this optic, the Cut-On Wavelength is specified at 550 nm.

Spectral Behavior

  1. The Cut-On Point (550 nm): At exactly 550 nm, the filter transmits 50% of the incident light.
  2. The Stopband (< 550 nm): Wavelengths shorter than the cut-on (e.g., blue or green light at 450–530 nm) are blocked, with transmission dropping rapidly to near 0% (high Optical Density).
  3. The Passband (> 550 nm): Wavelengths longer than the cut-on (e.g., orange, red, or NIR light at 570–700 nm) are transmitted, with transmission rising to 90–95% or higher.

Application in Fluorescence Microscopy

In a fluorescence system using a dye like TRITC, an LP550 filter serves as an effective emission filter because of its specific Cut-On placement:

  • Excitation Block: It blocks the intense green excitation light (approx. 530 nm), which falls below the 550 nm Cut-On.
  • Signal Transmission: It passes the orange/red fluorescence emission (approx. 575 nm), which falls above the 550 nm Cut-On.