Transition Width and Edge Steepness
Figure 1: Transition width and edge steepness illustrated.
Semrock edge filters for application-specific Raman systems and fluorescence imaging are specified with a guaranteed “Transition Width.” These include our steepest Verona™ and RazorEdge® Raman filters as well as our EdgeBasic™ filters.
Transition Width: the maximum allowed spectral width between the laser line (where OD > 6) and the 50% transmission wavelength
An edge filter can also be described by its “Edge Steepness,” the actual steepness of the filter, regardless of the precise wavelength placement of the edge.
Edge Steepness: the actual steepness of a filter, measured from the highest wavelength with OD 6 to the 50% transmission wavelength
Figure 1 illustrates Transition Width and Edge Steepness for an edge filter designed to block the 785 nm laser line (example shows a “U-grade” RazorEdge filter). Table 1 below lists the guaranteed Transition Width and typical Edge Steepness (for 25 mm diameter parts) for Semrock edge filters.
Edge Filter Type | Guaranteed Transition Width | Typical Edge Steepness (% of laser wavelength) |
| Verona | < 0.2% (< 39 cm-1 for 532 nm) | |
| RazorEdge "E-grade" | < 0.5% (< 90 cm-1 for 532 nm) | 0.2% (1.1 nm for 532) |
| RazorEdge "U-grade" | < 1.0% (< 186 cm-1 for 532 nm) | 0.5% (2.7 nm for 532) |
| RazorEdge "S-grade"* | < 2.0% (< 369 cm-1 for 532 nm) | 0.5% (2.7 nm for 532) |
| EdgeBasic | < 2.5% (< 458 cm-1 for 532 nm) | 1.5% (8 nm for 532) |
*RazorEdge S-Grade LWP and SWP have been discontinued.
All Verona and RazorEdge filters provide exceptional steepness to allow measurement of signals very close to the blocked laser line with high signal-to-noise ratio. However, the state-of-the-art Verona filters take closeness to a new level.
Figure 2: Transition widths and edge steepness for LP02-785RE and LP02-785RU
The graph on the left illustrates that “U-grade” RazorEdge filters have a transition width that is 1% of the laser wavelength – thus a 785 nm filter is guaranteed to have < 50% transmission by 792.9 nm, corresponding to a maximum wavenumber shift of 126 cm–1. “E-grade” filters have a Transition Width that is twice as narrow, or 0.5% of the laser line. So a 785 nm filter is guaranteed to have < 50% transmission by 788.9 nm, corresponding to a maximum wavenumber shift of 63 cm–1.
“Edge steepness” is the actual steepness of the filter, regardless of the precise wavelength placement of the edge. “U-grade” RazorEdge filters are designed to have a steepness of 0.5% of the laser wavelength, or 3.9 nm (63cm –1) for a 785 nm filter. The “E-grade” filters are designed to have an edge steepness 2.5x smaller – only 0.2% of the laser wavelength, or 1.6 nm (25 cm–1) for a 785 nm filter.